Lennox 51M37 Thermostat User Manual

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OPERATION

MANUAL

E2005 Lennox Industries Inc.

Dallas, Texas, USA

51M37 Merit^®Series

5/2 Day Programmable Thermostat

CONTROLS

505,050M

05/05

Supersedes 03/05

Litho U.S.A.

Table of Contents

51M37 Series Thermostat . . . . . . . . . . . . . . . . . . . . . .

General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Initial Thermostat Power-up . . . . . . . . . . . . . . . . . . . . .

Buttons, Backlight, Timers & Settings . . . . . . . . . . . .

DAY/TIME − Set Day and Time . . . . . . . . . . . . . . . . . .

HEAT − Heat Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . .

COOL − Cool Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . .

HOLD − Temperature Hold Mode . . . . . . . . . . . . . . . .

PROG − Thermostat Programming . . . . . . . . . . . . . . .

FAN − Controlling Fan Operation . . . . . . . . . . . . . . . . .

SETTINGS − Filter and Maintenance Reminders . . .

SETTINGS − Balance Point . . . . . . . . . . . . . . . . . . . . .

Service Indicator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Thermostat RESET . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Removing/Installing Thermostat . . . . . . . . . . . . . . . . .

Default Thermostat Settings . . . . . . . . . . . . . . . . . . . .

Technical Specifications . . . . . . . . . . . . . . . . . . . . . . . .

RETAIN THESE INSTRUCTIONS

FOR FUTURE REFERENCE

51M37 Merit^®Series Thermostat

The Lennox Merit Series 5/2 day programmable

electronic thermostat 51M37 provides excellent

temperature control and a large, easy-to-read display. It

includes a programmable filter change reminder, an

equipment maintenance reminder, and a system check

indicator to notify the user when the equipment requires

service.

The 51M37 is suitable for heat pump, 3-stage heat/2-stage

cool applications using a gas or electric auxiliary heat

source. An optional outdoor temperature sensor provides

auxiliary heat lockout, balance point operation, and

dual−fuel compatibility.

Thermostat Output Table . . . . . . . . . . . . . . . . . . . . . . . 10

General

Initial Power−Up Display

MAINT

FILTER

MO TU WE TH FR SA SU

These instructions are intended as a general guide and do

not supersede local codes in any way. Consult authorities

having jurisdiction before installation.

SERVICE

_PMI8:88

Check equipment for shipping damage. If you find any

damage, immediately contact the last carrier.

FAN

AUTO

PROG A | B | C | D HOLD

Introduction

SYSTEM

COOL

OFF

EMG.

HEAT

ROOM

SETPOINT

This document describes the operation of Lennox thermostat

51M37. Refer to the installation manual for instructions re-

garding installation and wiring of the thermostat.

88^°

Initial Thermostat Power-up

Figure 1

When power is initially applied to the thermostat, the dis-

play will appear as shown in figure 1.

All display segments are momentarily activated. This oc-

curs as a normal part of thermostat initialization.

05/05

505,050M

Page 1

*2P0505*

*P505050M*

Within a few seconds, the Home Screen appears (see fig-

ure 2) with default settings as shown. After about 1 minute

of initialization time, the actual room temperature will be

displayed.

Thermostat Buttons

Heat Cool

Prog Day/Time

Fan Hold

NOTE − Tables 2 and 3 on page 8 show all system and

programming defaults.

Home Screen

MAINT

(DOOR

OPEN)

MO TU WE TH FR SA SU

FILTER

Settings Enter

SERVICE

_PMI2:00

FAN

AUTO

PROG A B C D HOLD

Figure 3

ROOM

70^°

SYSTEM

COOL

OFF

When an Arrow, HOLD, HEAT, or COOL button is pressed,

SETPOINT and the temperature setting appears for 15

seconds. If desired, start making changes within 15 sec-

onds or the HOME screen will return. The backlight will

turn off 15 seconds after the HOME screen reappears.

HEAT

Figure 2

At this point, the thermostat will be fully functional; its de-

fault temperature setpoint (not shown) is 70°F. At this

point, if the equipment has been fully powered and if a heat

demand were present, the system would begin operating.

DAY/TIME − Setting the Day and Time

Press the DAY/TIME button and set the CURRENT hour,

minute, and day of week as follows:

NOTE − Temperature scale default is Fahrenheit units but

may be reset to show Celsius, if desired. See page 8.

1. ꢀAM12" will flash on the screen. Press the Up/Down ar-

rows buttons to change the hour. (ꢀAM" or ꢀPM" must

correspond to time of day.) Press DAY/TIME OR, if ad-

justing for daylight savings time, pressing ENTER

stores the single change and exits to the HOME

screen, bypassing minutes and day of week.

Buttons, Backlight, Timers & Settings

Buttons are located behind the small door on the right−

hand side of the thermostat (see figure 3).

IMPORTANT

Do NOT begin pressing buttons until after you read

the following section describing each button.

2. Minutes will flash. Use the Up/Down arrow buttons to

display the minutes past the hour. Press DAY/TIME.

3. Day ꢀMO" (Monday) will flash. Use Up/Down arrow

buttons to display the current day. Day selections are

abbreviated as ꢀMOꢀ, ꢀTU", ꢀWE", ꢀTH", ꢀFR", ꢀSA",

and ꢀSU". Press DAY/TIME.

A pale blue display backlight illuminates for 30 seconds

each time any button is pressed.

When PROG or DAY/TIME is pressed, a field begins flash-

ing, expecting another input. Start making changes within

15 seconds or the HOME screen will return.

4. The HOME screen reappears; confirm day and time

are correct. This completes day and time setting.

Page 2

505050M 5/1/05

HEAT − Using the Heat Mode

Heat Mode Disabled

SYSTEM

COOL

OFF

ROOM

75^°

Normal Heat Mode

SETPOINT

In normal heat mode, both the heat pump and the backup

heat source are used to provide heat. If the thermostat de-

tects that the heat pump is not able to provide enough heat

(as may be the case in very cold weather), then the backup

heat source is activated.

75^°

HEAT

Figure 6

Heating Demand

Emergency Heat Mode

The thermostat must be in either normal or emergency

heat mode in order to properly control the heating equip-

ment. In either heat mode, when the actual temperature is

lower than the temperature setpoint (as shown in figure 7),

the thermostat detects a heating demand and activates the

heating equipment to satisfy the demand. Heating opera-

tion is indicated by a flame icon in the SYSTEM box.

In emergency heat mode, only the backup heat source

provides heat ꢁthe heat pump is disabled. The backup

heat source is activated only when there is a heat demand.

Enabling Normal Heat Mode

Use the HEAT button to select normal heat mode, emer-

gency heat mode, or to disable heat modes as desired. If

the thermostat is in OFF or COOL mode, pressing the Heat

button enables Heat mode. This is indicated by HEAT in

the SYSTEM box as shown in figure 4.

Heating Demand

SYSTEM

ROOM

SETPOINT

COOL

OFF

Normal Heat Mode Enabled

72^°

75^°

Figure 7

Heat Cool

HEAT

_PM5:30

FAN

Prog Day/Time

HOLD

AUTO

SYSTEM

When the actual temperature rises above the temperature

setpoint, the flame icon will disappear. This indicates that

the heating demand has been satisfied and that the heat-

ing equipment has been turned off.

ROOM

SETPOINT

66^°

Fan Hold

HEAT

Settings Enter

Heat pump operation is locked out for 5 minutes after a de-

mand has been satisfied. If another heat pump demand

occurs during this 5−minute interval, the flame icon will

flash; however, the heat pump will not run until the 5−min-

ute delay has elapsed.

Figure 4

NOTE − The heat pump is activated for at least 4 minutes if

no buttons are pressed during the demand interval. The

backup heat source is activated for at least 3 minutes if no

buttons are pressed during the demand interval.

Enabling Emergency Heat Mode

If the thermostat is in heat mode, pressing the Heat button

enables EMG. HEAT mode (see figure 5). If the thermostat

is in normal heat mode when the HEAT button is pressed,

then emergency heat mode is enabled.

If your system supports 3−stage heating (as does the

51M37 thermostat), you may notice various heating levels

being delivered during a demand.

Emergency Heat Mode Enabled

SYSTEM

ROOM

If a small heat pump heating demand is present, "Lo" is dis-

played in the SYSTEM box; if a large heat pump heating

demand is present, "Hi" is displayed (see figure 8).

SETPOINT

COOL

OFF

EMG.

HEAT

62^°

Disabling Heat Mode

75^°

Large Heat Demand

Figure 5

SYSTEM

COOL

OFF

ROOM

65^°

SETPOINT

If the thermostat is in emergency heat mode when the

HEAT button is pressed, then heat modes are disabled.

This is indicated by OFF in the SYSTEM box as shown in

figure 6.

75^°

HEAT

Figure 8

Page 3

51M37 SERIES 5/2

Cooling Demand

COOL − Using the Cool Mode

Enabling and Disabling Cool Mode

Set the thermostat to cool mode to control the cooling

equipment. Then, if the room temperature is higher than

the temperature setpoint, as shown in figure 1 1, the ther-

mostat detects a cooling demand and will activate the cool-

ing equipment to satisfy the demand.

Use the COOL button to enable or disable cool mode as

desired. If the thermostat is in HEAT or OFF mode, cool

mode is enabled when the COOL button is pressed. This is

indicated by COOL in the SYSTEM box (see figure 9).

Cooling Demand

MO TU WE TH FR SA SU

_PMi0:00

ROOM

FAN

AUTO

Turn Cool ON/OFF

PROG A B C D HOLD

Heat Cool

SYSTEM

COOL

OFF

FAN

I0:30

Prog Day/Time

SETPOINT

75^°

HOLD

AUTO

70^°

SYSTEM

COOL

ROOM

SETPOINT

^°70^°

Fan Hold

HEAT

Figure 11

Settings Enter

Cooling operation is indicated by flashing ꢀsnowflake"

icons in the SYSTEM box. When the actual temperature

drops below the temperature setpoint, the snowflake icons

will disappear. This indicates that the cooling demand has

been satisfied and that the cooling equipment has been

turned off.

Figure 9

If your system supports 2−stage cooling (as does the

51M37 thermostat), you may notice various cooling levels

being delivered during a demand. Also, if a small cooling

demand is present, "Lo" will be displayed in the SYSTEM

box. However, if a large cooling demand is present, "Hi" will

be displayed in the SYSTEM box (shown in figure 1 1).

If the thermostat is in cool mode, pressing the Cool button

disables COOL mode (indicated by OFF in the SYSTEM

box − see figure 10).

Cool Mode Disabled

NOTE − If no buttons are pressed during a demand for

cooling, the equipment must operate for at least 4 minutes.

After a demand has been satisfied, cooling equipment op-

eration is locked out for 5 minutes. If another cooling de-

mand occurs during this 5−minute interval, ꢀCOOL" and the

snowflakes will flash; however, the cooling equipment will

not operate until the 5−minute delay has elapsed.

SYSTEM

ROOM

SETPOINT

COOL

75^°

OFF

75^°

HEAT

Figure 10

Page 4

505050M 5/1/05

PROG − Thermostat Programming

HOLD − Using Temperature Hold Modes

The 51M37 thermostat can be programmed to perform a

set of either heating or cooling events (but not a

combination of heat and cool) for 5 consecutive days using

a set of 4 unique events per day. The remaining 2 days can

then be set for a different set of 4 unique events per day.

Both the consecutive days and the events/temperature are

set by the homeowner.

When HOLD is displayed at the HOME screen, the ther-

mostat is in a temperature hold condition. This means that

the temperature program data is ignored and the thermo-

stat functions much like a non−programmable thermostat.

Adjusting Temperature Setpoint in Hold Mode

The temperature setpoint represents the desired tempera-

ture of the space around the thermostat. The default tem-

perature setpoint in Hold mode is 70°F.

To Change Consecutive Days...

To alter the 5 consecutive days, press and hold the

PROG button for 5 seconds. The 5 consecutive day period

is then displayed (default is MOnday thru FRiday). To

change to a different 5-consecutive days, use the Up/

Down arrow buttons. Any 5 consecutive−day span may be

selected, for example, in figure 14, Saturday through

Wednesday is defined as the 5−day programming (Thurs-

day and Friday would constitute the 2−day programming).

Press the PROG button when finished.

To adjust the setpoint, press the UP or DOWN (YB) arrow

buttons (see figure 12); the existing setpoint is displayed to

the right of the actual room temperature. Each button

press adjusts the setpoint up or down by 1 degree.

Hold Temperature Mode

Heat Cool

FAN

Day/Time

Prog

I0:36

HOLD

AUTO

SYSTEM

COOL

Change Consecutive days

ROOM

SETPOINT

^°68^°

Fan Hold

MO TU WE TH FR SA SU

FILTER

Settings Enter

Figure 14

To Set Program Events and Temperatures...

Figure 15 gives an example of how the two sets of pro-

grams can be set for a normal workweek and weekend.

Figure 12

After the desired setpoint is reached, the HOME screen

will reappear after about 15 seconds.

In the 5−day bar graph, note how programs A and C reflect

the desired warmth while the home IS occupied (72°); B

allows less heating while the home is NOT occupied; D re-

flects a cooler sleeping temperature. The 2−day bar graph

would support day−long occupancy and, because the first

program begins later, a less−demanding time schedule.

Permanent Hold Mode

At any time the program is running, from the HOME

screen, set a permanent hold (program override) by press-

ing the HOLD button (see figure 12). The thermostat now

functions much like a non−programmable thermostat. Use

the Up/Down arrow buttons to adjust the hold setpoint. To

return to the program, press HOLD again.

5/2 Program Example

5−DAY PROGRAM: | A

72°

68°

64°

6AM

Temporary (2-Hour) Hold

At any time the program is running, from the HOME

screen, set a temporary 2-hour hold by pressing the Up/

Down arrow buttons until the desired setpoint is displayed;

ꢀHOLD" flashes (see figure 13). This overrides the pro-

gram for 2 hours from the last button press, then returns to

the program.

NOON

6PM

MID

NIGHT

6AM

2−DAY PROGRAM:

| A |

72°

68°

64°

6AM

NOON

6PM

MID

NIGHT

6AM

Temporary Hold Temperature Mode

Figure 15

Heat Cool

NOTE − Pressing ENTER during the following program-

ming steps, saves and exits to the HOME screen.

FAN

Day/Time

Prog

I0:36

AUTO

PROG

HOLD

SYSTEM

COOL

To program events and temperatures, perform the follow-

ing steps, once with Cool selected and once with Heat se-

lected.

ROOM

SETPOINT

^°68^°

Fan Hold

Settings Enter

1. Press and release PROG. ꢀAM 6:00", period ꢀA", and

the 5 consecutive days are displayed; ꢀAM 6" flashes.

2. Use the Up/Down arrow buttons to select the desired

hour; press PROG when the desired hour is reached.

Figure 13

3. Use the Up/Down arrow buttons to select the desired

minute; press PROG.

While in Temporary Hold, press HOLD once to switch to

Permanent Hold (HOLD displays solid; PROG not dis-

played); press HOLD again to return to the program

(PROG displays; HOLD not displayed).

4. Use the Up/Down arrow buttons to select the desired

temperature setpoint; press PROG.

Page 5

51M37 SERIES 5/2

5. Repeat steps 2 through 4 for periods B, C, and D.

6. Repeat steps 1 through 5 for the 2−day program.

Reminder Settings Display

For MAINT,

press Settings

button 2 times

MAINT

FILTER

NOTE − This thermostat will NOT automatically switch

from heating to cooling, or vice versa; operator involve-

ment is required. At the change of seasons, or to accomo-

date abnormal seasonal temperature swings, you must

manually select to the opposite conditioning (Heat or Cool)

program.

For FILTER,

press Settings

button 1 time

OFF

Figure 18

FAN − Controlling the Fan Operation

The default setting for the reminders is OFF (disabled).

Press Up/Down arrow buttons to select the desired re-

minder intervals.

Use the FAN button to select either continuous fan mode or

auto fan mode.

To change from continuous to auto fan mode (or vice ver-

sa), press the FAN button. Note whether a fan icon in the

FAN box is present (indicating that the fan is running) or not

(fan not running).

Table 1

Filter and Maintenance Reminders

Buttons to

Use

Available Settings and

How to Use

If continuous fan mode is enabled (ON displayed in FAN

box − see figure 16), the fan will run continuously regard-

less of whether the heating or cooling equipment is run-

ning.

Reminder

FILTER

Settings

(1st press)

then Arrows

to scroll

Total fan run time expressed in

months (Off, 1, 3, 6, 12); for

example, if fan runs 12 hours a

day, 1 month reminder displays

in 2 calendar months.

selections

Using Fan ON

Heat Cool

Settings

(2nd press)

then Arrows

to scroll

Elapsed chronological time in

months (Off, 6, 12). Use this,

for example, to remind yourself

when to perform routine checks

or when to call a technician for

perodic preventive

MAINT

_PMI0:I0

FAN

Day/Time

Prog

PROG

SYSTEM

HEAT

ROOM

67^°

selections

Fan Hold

maintenance.

Settings Enter

Enter

Stores settings.

NOTE − The HOME screen will reappear about 15 seconds

after the final arrow button press. OR, press ENTER at any

time to store any changes and exit to the HOME screen.

Figure 16

If auto fan mode is selected (AUTO displayed in FAN box −

see figure 17), the fan will only run when the heating or

cooling equipment is running.

After either programmed interval has elapsed, the remind-

er will be displayed as shown in figure 19.

Using Fan Auto

Heat Cool

Reminders

MAINT

FAN

MO TU WE TH FR SA SU

FILTER

Day/Time

Prog

I0:36

PROG

AUTO

SERVICE

SYSTEM

COOL

ROOM

Fan Hold

72^°

FAN

AUTO

I0:i2

Settings Enter

PROG A B C D HOLD

ROOM

SYSTEM

COOL

OFF

Figure 17

78^°

SETTINGS − Filter/Maintenance Reminders

HEAT

The 51M37 thermostat is designed to remind you when the

filter needs changing or when routine maintenance is re-

quired, as (and if) defined, by you. These optional remind-

ers are not enabled until you activate them. To do so, press

the Settings button (shown below the Fan button in figure

17) once or twice for the desired reminder as shown in fig-

ure 18 and as described in table 1.

Figure 19

After the filter has been changed or maintenance per-

formed, reset the reminder by pressing the SETTINGS

button for 4 seconds. The screen will blink for a few mo-

ments to indicate that the timer has been reset.

Page 6

505050M 5/1/05

Service Indicator Flashing

SETTINGS − Balance Point

MO TU WE TH FR SA SU

FILTER

SERVICE

NOTE − The balance point is only available with the out-

door sensor.

_PMI0:i4

If the optional outdoor sensor (X2658) is connected to the

thermostat, balance point adjustment is available. The bal-

ance point feature allows the measured outdoor tempera-

ture to govern operation of the heat pump and backup heat

source.

FAN

AUTO

PROG A B C D HOLD

ROOM

SYSTEM

COOL

OFF

79^°

Use the SETTINGS button to access the balance point

setting. If the outdoor sensor is attached to the thermostat,

press the SETTINGS button three times to access the Bal-

ance Point Settings screen (see figure 20).

HEAT

Figure 21

Balance Point Setting

ROOM

Thermostat RESET

SETPOINT

Under some abnormal conditions, it may be necessary to

ꢀreset" the thermostat to its default condition. Such a RE-

SET would delete all programming and settings and there-

fore should only be used on rare occasions when the the

thermostat fails to function as designd and/or as pro-

grammed. Such an instance can occur as a result of a pow-

er surge or similar electrical disturbance (e.g. after an elec-

trical storm or power outage). The RESET button can be

used to recover from this situation.

55^°

Figure 20

The balance point can be set at any temperature between

1°F and 55°F (−17°C to 13°C). A balance point setting of

zero disables the balance point function (in this case, the

thermostat behaves as if no outdoor sensor is attached).

The default balance point setting is 55°F.

To adjust the balance point up or down, press the Up/Down

arrow buttons to the right of the display screen. The bal-

ance point setting increases by 1°F each time the Up but-

ton is pressed and decreases by 1°F each time the Down

button is pressed.

CAUTION

When the RESET button is pressed, ALL settings re-

vert back to the defaults, including the default pro-

gram (see tables 2 and 3).

After the desired balance point is reached, the HOME

screen will reappear after about 15 seconds.

The RESET button is an unlabeled, recessed button lo-

cated behind the door, on the right−hand side of the ther-

mostat, below the SETTINGS button (see figure 3). Use a

paper clip or small pencil to press the RESET button; ALL

thermostat settings will be reset to the defaults listed in the

Default Thermostat Settings section.

NOTE − The balance point feature allows the outdoor tem-

perature to rise or fall 3°F above or below the balance

point. This prevents excessive cycling of the equipment

when the outdoor temperature is near the balance point.

For example, if the balance point is 40°F and the actual

outdoor temperature is 35°F, the outdoor temperature

must rise to 43°F before equipment adjustment occurs.

Conversely, if the balance point is 40°F and the actual out-

door temperature is 45°F, the outdoor temperature must

drop to 37°F before equipment adjustment occurs.

Removing/Installing Thermostat

The thermostat hinges on tabs on the top of the subbase;

no tool is needed to remove the thermostat from the sub-

base. Pivot the bottom of the thermostat outward (releas-

ing the snaps), then lift up to remove.

Service Indicator

When abnormal equipment operation is detected, the

SERVICE indicator will flash on the screen (see figure 21).

This indicates that the equipment requires service from a

qualified service technician.

To replace it, first position the top tilted toward the wall

bracket and align it until you feel the tabs and slots engage;

then, while the top is in place, pivot the bottom toward the

wall until the thermostat snaps into place.

Page 7

51M37 SERIES 5/2

Default Thermostat Settings

CAUTION

24VAC is present on the terminals of the thermostat

bracket. If removing the thermostat from the wall,

use caution and avoid touching any of the connec-

tor terminals on the wall bracket.

Default thermostat settings are shown in table 2 and the

default program is shown in table 3.

Table 2

Also, when working with the thermostat dip

switches, use a non−conductive tool and take cau-

tion to avoid making any contact with the circuit

board, its imprinted circuitry and its connector

prongs.

Default Thermostat Settings

Mode

Heat (Permanent Hold

Mode)

Setpoint

70°F (or 21°C)

Fan

Auto

Filter Reminder

Maintenance Reminder

OFF

Temperature Display

Display Scale: Fahrenheit or Celsius user selectable (via

DIP switch; see figure 22)

Display range: 35°F (2°C) to 99°F (37°C)

Display resolution: 1°F (1°C)

Display Accuracy: +/−1°F

OFF

Equipment Protection

Timers

Reset Back to Zero

Table 3

Default Program Settings

If the Fahrenheit/Celsius display must be changed, use a

plastic, non−conductive tool to push the dip switch to the

right position (see figure 22).

Temperature Temperature

(Heat) (Cool)

Programs

Time

Changing Fahrenheit/Celsius Setting

Weekday − A 6:00am 70_F / 21°C 78_F / 26°C

Weekday − B 8:00am 62_F / 17°C 85_F / 29°C

Weekday − C 6:00pm 70_F / 21°C 78_F / 26°C

Weekday − D 10:00pm 62_F / 17°C 82_F / 28°C

Weekend − A 6:00am 70_F / 21°C 78_F / 26°C

Weekend − B 8:00am 62_F / 17°C 85_F / 29°C

Weekend − C 6:00pm 70_F / 21°C 78_F / 26°C

Weekend − D 10:00pm 62_F / 17°C 82_F / 28°C

PLUG IN (Use Care To Avoid Bending Prongs.)

Weekday

Monday, Tuesday, Wednesday,

Thursday, Friday

F/C (Fahrenheit/Celsius) switch

shown in Celsius position

Weekend

Saturday, Sunday

Figure 22

Temperature Measurement Range

Measurement Scale: Fahrenheit

Measurement Range: 35°F to 99°F

Measurement Resolution: 0.5°F

Measurement Accuracy: +/−1°F

Field Offset: via DIP switches to +/−3°F

Sampling Method: temperature measurements sampled

every 15 seconds. Displayed temperature is the average

of the last four measurements.

Technical Specifications

Thermostat Type

Electronic programmable thermostat for heat pump,

3-stage heat/2-stage cool.

NOTE − 2 stages of heat pump heat and one stage of auxil-

iary (gas or electric) backup heat are supported.

Temperature Setpoint Range

Setting range: 50°F (10°C) to 90°F (32°C)

Setting resolution: 1°F (1°C)

Power Supply Range

18VAC − 30VAC (24VAC nominal), 60Hz

Page 8

505050M 5/1/05

Smart Setback Recovery (via DIP switch #6)

Smart Setback Recovery (SSR) affects the way the ther-

mostat responds to program events. If SSR is disabled, the

thermostat will react to a program event at the time the

event occurs. However, if SSR is enabled, the thermostat

will react to a program event before the event occurs such

that the desired temperature is reached at the time of the

event, not after.

Equipment Protection Override

Both the minimum compressor OFF timer and the mini-

mum equipment cycle timer can be over−ridden by press-

ing and holding either the HEAT or COOL button down for 4

seconds.

Over−Temperature Protection

Thermal-mechanical switch opens W1, Y1, Y2, and E at

93°F+/−6°F.

Fan Control

AUTO or ON modes.

Filter Reminder

Settings of Off, 1, 3, 6 or 12 (months of fan run time) are

available. When programmed time has elapsed, a FILTER

indicator is displayed.

I/O Relays

All thermostat relays are latching type to minimize power

consumption.

Maintenance Reminder

Table 4

51M37 Terminal Designations

Term. Description

Settings of Off, 6 or 12 (months of chronological time) are

available. When programmed time has elapsed, a mainte-

nance indicator ꢀMAINT" is displayed.

Reversing valve, heat active

Reversing valve, cool active

24VAC

Service Reminder

The SERVICE indicator is displayed only under the follow-

ing conditions:

First-stage cooling/heating, compressor

energized

if the thermostat Y1 terminal has been activated with

24VAC for at least 5 minutes, AND the L terminal is

shorted to the R terminal;

W1*

Auxiliary heating, furnace energized

Second-stage cooling/heating, compressor

energized

if the thermostat Y1 terminal has been activated with

24VAC for at least 5 minutes, AND the L terminal is

shorted to the C terminal.

Emergency heat

Fan control

Service Indicator

Power Loss/Recovery

24VAC common

Thermostat memory is retained for a minimum of 24 hours

during a power loss (includes retention of program infor-

mation, HOLD status, programmed temperature setpoint,

heat/cool and fan mode settings, filter reminder status,

maintenance reminder status, and equipment protection

timers). After 24 hours of power loss, programmed settings

will be lost and replaced with default settings.

Outdoor temperature sensor connection 1

Outdoor temperature sensor connection 2

* For most applications, E will be jumpered to W1. If sep-

arate wires are not provided for both E and W1, jumper

the E terminal to the W1 terminal on the thermostat sub-

base. For applications involving the use of a balance

point (whereby the outdoor temperature is to be used to

restrict either heat pump operation or backup heat

source operation), the optional outdoor sensor (part

number X2658) MUST be installed.

IMPORTANT

Power must be applied for at least six consecutive

hours prior to a power loss in order for memory to

be retained for the specified time.

Equipment Protection Timers

Minimum Compressor OFF time: 5 minutes

Minimum Compressor ON time: 4 minutes

Minimum Furnace ON time: 3 minutes

LCD Backlight

Activated for 30 seconds when any button is pressed.

Minimum furnace cycle time (elapsed time between any

furnace activation and the next furnace activation): 6 min-

utes.

NOTE − During an electrical storm or similar disturbance,

the backlight may activate for a few seconds. This is nor-

mal and will no longer occur after the electrical disturbance

has passed.

Minimum elapsed time between any compressor activa-

tion and the next compressor activation: 6 minutes.

Thermostat Operating Conditions

NOTE − All protection timers (except the compressor OFF

timer) can be over−ridden if a heating or cooling demand is

initiated or terminated using the UP, DOWN, HEAT, or

COOL buttons.

35°F to 105°F, 5% to 90% RH

Thermostat Storage Conditions

−40°F to 185°F, 5% to 95% RH

Page 9

51M37 SERIES 5/2

Table 5

Thermostat Output Table

THERMOSTAT OUTPUTS

Table5 depicts the 51M37 thermostat output states for var-

ious input conditions. The following notes described terms

used in the table.

Y1 Y2

Demand Condition

Cooling Demands

SMALL

NOTES:

LARGE

S X = output is activated with 24VAC.

S BBP: outdoor temperature is below balance point

S ABP: outdoor temperature is above balance point

No Demand

Heating Demands

w/Electric Backup (no outdoor sensor)

S In all cases, the state of the B terminal is opposite that

SMALL

LARGE

of the O terminal

Very LARGE

Emergency

No Demand

S Data are tabulated for AUTO fan setting. If the fan set-

ting is ON, the G output is activated in all cases

S Upstage timers:

w/Gas Backup (dual fuel; no outdoor sensor)

30 minutes (when upstaging from small to large

demand)

SMALL

LARGE

15 minutes (HEAT MODE ONLY − when upstaging

from large to very large demand)

Very LARGE

Emergency

No Demand

S The temperature ranges expressed in the following

definitions of ꢀSMALL/LARGE/VERY LARGE" de-

mands are for guidance only; actual temperatures

may vary:

w/Electric Backup (outdoor sensor attached)

SMALL (Above Balance Point)

LARGE (ABP)

With a small HEAT demand, temperature is:

below setpoint –0.5F but above setpoint –1.5F

AND 30-minute upstage timer HAS NOT expired.

Very LARGE (ABP)

SMALL (Below Balance Point)

LARGE (BBP)

With a large HEAT demand, temperature is:

less than setpoint –1.5F

OR 30-minute upstage timer HAS expired.

Very LARGE (BBP)

Emergency

With a very large heat demand, temperature is:

below setpoint –2.5F

OR 15−minute 2 upstage timer HAS expired.

No Demand

w/Gas Backup (dual fuel; outdoor sensor attached)

SMALL (ABP)

LARGE (ABP)

Very LARGE (ABP)

SMALL (BBP)

LARGE (BBP)

Very LARGE (BBP)

Emergency

Emergency heat demand (only if emergency heat is

enabled): temperature is below setpoint –0.5F.

With a small COOL demand, temperature is:

above setpoint +0.5F but below setpoint +1.5F

AND 30-minute upstage timer HAS NOT expired.

With a large COOL demand, temperature is:

above setpoint +1.5F

OR 30-minute upstage timer HAS expired.

No Demand

Page 10

505050M 5/1/05

OPERATION

MANUAL

E2005 Lennox Industries Inc.

Dallas, Texas, USA

51M35 Merit^®Series

5/2 Day Programmable Thermostat

CONTROLS

505,049M

05/05

Supersedes 03/05

Litho U.S.A.

Table of Contents

51M35 Series Thermostat . . . . . . . . . . . . . . . . . . . . . .

General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Initial Thermostat Power-up . . . . . . . . . . . . . . . . . . . . .

Buttons, Backlight, Timers & Settings . . . . . . . . . . . .

DAY/TIME − Set Day and Time . . . . . . . . . . . . . . . . . .

HEAT − Heat Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . .

COOL − Cool Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . .

HOLD − Temperature Hold Mode . . . . . . . . . . . . . . . .

PROG − Thermostat Programming . . . . . . . . . . . . . . .

FAN − Controlling Fan Operation . . . . . . . . . . . . . . . . .

SETTINGS − Filter /Maintenance Reminder . . . . . . .

Service Indicator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Thermostat RESET . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Removing/Installing Thermostat . . . . . . . . . . . . . . . . .

Default Thermostat Settings . . . . . . . . . . . . . . . . . . . .

Technical Specifications . . . . . . . . . . . . . . . . . . . . . . . .

Thermostat Output Table . . . . . . . . . . . . . . . . . . . . . . .

RETAIN THESE INSTRUCTIONS

FOR FUTURE REFERENCE

51M35 Merit^®Series Thermostat

The Lennox Merit Series 5/2 day programmable

electronic thermostat 51M35 provides excellent

temperature control and a large, easy-to-read display. This

product includes a programmable filter change reminder,

an equipment maintenance reminder, and a system check

indicator to notify the user when the equipment requires

service.

Thermostat 51M35 is suitable for non-heat pump, two-

stage heat/two-stage cool applications using a gas or elec-

tric furnace.

General

Initial Power−Up Display

MAINT

FILTER

MO TU WE TH FR SA SU

These instructions are intended as a general guide and do

not supersede local codes in any way. Consult authorities

having jurisdiction before installation.

SERVICE

_PMI8:88

Check equipment for shipping damage. If you find any

damage, immediately contact the last carrier.

FAN

AUTO

PROG A | B | C | D HOLD

Introduction

SYSTEM

COOL

OFF

EMG.

HEAT

ROOM

SETPOINT

This document describes the operation of Lennox thermostat

51M35. Refer to the installation manual for instructions re-

garding installation and wiring of the thermostat.

88^°

Initial Thermostat Power-up

Figure 1

When power is initially applied to the thermostat, the dis-

play will appear as shown in figure 1.

All display segments are momentarily activated. This oc-

curs as a normal part of thermostat initialization.

05/05

505,049M

Page 1

*2P0505*

*P505049M*

Within a few seconds, the Home Screen appears (see fig-

ure 2) with default settings as shown. After about 1 minute

of initialization time, the actual room temperature will be

displayed.

Thermostat Buttons

Heat Cool

Prog Day/Time

Fan Hold

NOTE − Tables 2 and 3 on page 6 show all system and

programming defaults.

MAINT

Home Screen

MO TU WE TH FR SA SU

FILTER

(DOOR

OPEN)

Settings Enter

SERVICE

_PMI2:00

FAN

AUTO

PROG A B C D HOLD

Figure 3

ROOM

70^°

SYSTEM

COOL

OFF

When an Arrow, HOLD, HEAT, or COOL button is pressed,

SETPOINT and the temperature setting appears for 15

seconds. If desired, start making changes within 15 sec-

onds or the HOME screen will return. The backlight will

turn off 15 seconds after the HOME screen reappears.

HEAT

Figure 2

At this point, the thermostat will be fully functional; its de-

fault temperature setpoint (not shown) is 70°F. At this

point, if the equipment has been fully powered and if a heat

demand were present, the system would begin operating.

DAY/TIME − Setting the Day and Time

Press the DAY/TIME button and set the CURRENT hour,

minute, and day of week as follows:

NOTE − Temperature scale default is Fahrenheit units but

may be reset to show Celsius, if desired. See page 7.

1. ꢀAM12" will flash on the screen. Press the UP/DOWN

arrows to change the hour. (ꢀAM" or ꢀPM" must corre-

spond to time of day.) Press DAY/TIME OR, if adjust-

ing for daylight savings time, pressing ENTER stores

the single change and exits to the HOME screen, by-

passing minutes and day of week.

Buttons, Backlight, Timers & Settings

Buttons are located behind the small door on the right−

hand side of the thermostat (see figure 3).

IMPORTANT

Do NOT begin pressing buttons until after you read

the following section describing each button.

2. Minutes will flash. Use the UP/DOWN arrow button to

display the minutes past the hour. Press DAY/TIME.

3. Day ꢀMO" (Monday) will flash. Use UP or DOWN arrow

buttons to display the current day. Day selections are

abbreviated as ꢀMOꢀ, ꢀTU", ꢀWE", ꢀTH", ꢀFR", ꢀSA",

and ꢀSU". Press DAY/TIME.

A pale blue display backlight illuminates for 30 seconds

each time any button is pressed.

When PROG or DAY/TIME is pressed, a field begins flash-

ing, expecting another input. Start making changes within

15 seconds or the HOME screen will return.

4. The HOME screen reappears; confirm day and time

are correct. This completes day and time setting.

Page 2

505049M 5/1/05

HEAT − Using the Heat Mode

COOL − Using the Cool Mode

Enabling and Disabling Cool Mode

Enabling and Disabling Heat Mode

Use the COOL button to enable or disable cool mode as

desired. If the thermostat is in HEAT or OFF mode, cool

mode is enabled when the COOL button is pressed. This is

indicated by COOL in the SYSTEM box (see figure 8).

The thermostat must be in the HEAT mode to control the

heating equipment. Press the Heat button to enable or dis-

able HEAT mode. If the thermostat is in COOL or OFF

mode, pressing the Heat button enables the HEAT mode

(indicated by HEAT in the SYSTEM box − see figure 4).

Turn Cool ON/OFF

Turn Heat ON/OFF

Heat Cool

FAN

I0:30

Prog Day/Time

_PM5:30

FAN

HOLD

AUTO

SYSTEM

COOL

Prog Day/Time

ROOM

SETPOINT

^°70^°

HOLD

AUTO

SYSTEM

Fan Hold

ROOM

SETPOINT

66^°

Fan Hold

HEAT

Settings Enter

Figure 7

Figure 4

If the thermostat is in cool mode, pressing the Cool button

disables COOL mode (indicated by OFF in the SYSTEM

box − see figure 5).

If the thermostat is in heat mode, pressing the Heat button

disables HEAT mode (indicated by OFF in the SYSTEM

box − see figure 5).

Cooling Demand

Set the thermostat to cool mode to control the cooling

equipment. Then, if the room temperature is higher than

the temperature setpoint, as shown in figure 8, the thermo-

stat detects a cooling demand and will activate the cooling

equipment to satisfy the demand.

Heat/Cool Mode Disabled

SYSTEM

COOL

OFF

ROOM

75^°

Heating Demand

SETPOINT

75^°

HEAT

Cooling Demand

Figure 5

SYSTEM

COOL

OFF

ROOM

SETPOINT

75^°

Set the thermostat to heat mode to control the heating

equipment. Then, if the room temperature is lower than the

temperature setpoint, as shown in figure 6, the thermostat

detects a heating demand and will activate the heating

equipment to satisfy the demand.

70^°

HEAT

Figure 8

Cooling operation is indicated by flashing ꢀsnowflake"

icons in the SYSTEM box. When the actual temperature

drops below the temperature setpoint, the snowflake icons

will disappear. This indicates that the cooling demand has

been satisfied and that the cooling equipment has been

turned off.

Heating Demand

SYSTEM

ROOM

SETPOINT

COOL

72^°

OFF

75^°

If your system supports 2−stage cooling (as does the

51M35 thermostat), you may notice various cooling levels

being delivered during a demand. Also, if a small cooling

demand is present, "Lo" will be displayed in the SYSTEM

box. However, if a large cooling demand is present, "Hi" will

be displayed in the SYSTEM box (shown in figure 8).

HEAT

Figure 6

Heating operation is indicated by a flame icon in the SYS-

TEM box. When the actual temperature rises above the

temperature setpoint, the flame icon will disappear. This

indicates that the heating demand has been satisfied and

that the heating equipment has been turned off.

NOTE − If no buttons are pressed during a demand for

cooling, the equipment must operate for at least 4 minutes.

After a demand has been satisfied, cooling equipment op-

eration is locked out for 5 minutes. If another cooling de-

mand occurs during this 5−minute interval, ꢀCOOL" and the

snowflakes will flash; however, the cooling equipment will

not operate until the 5−minute delay has elapsed.

If your system supports 2−stage heating (as does the

51M35 thermostat), you may notice various heating levels

being delivered during a demand.

NOTE − Heating equipment is activated for at least 3 min-

utes if no buttons are pressed during the demand interval.

Page 3

51M35 SERIES 5/2

HOLD − Using Temperature Hold Modes

PROG − Thermostat Programming

The 51M35 thermostat can be programmed to perform a

set of either heating or cooling events (but not a

combination of heat and cool) for 5 consecutive days using

a set of 4 unique events per day. The remaining 2 days can

then be set for a different set of 4 unique events per day.

Both the consecutive days and the events/temperature are

set by the homeowner.

When HOLD is displayed at the HOME screen, the ther-

mostat is in a temperature hold condition. This means that

the temperature program data is ignored and the thermo-

stat functions much like a non−programmable thermostat.

Adjusting Temperature Setpoint in Hold Mode

The temperature setpoint represents the desired tempera-

ture of the space around the thermostat. The default tem-

perature setpoint in Hold mode is 70°F.

To Change Consecutive Days...

To alter the 5 consecutive days, press and hold the PROG

button for 5 seconds. The existing 5 consecutive day peri-

od is then displayed (default is MOnday thru FRiday). To

change to a different 5-consecutive days, use the Up/

Down arrow buttons. Any 5 consecutive−day span may be

selected, for example, in figure 11, Saturday through

Wednesday is defined as the 5−day programming (Thurs-

day and Friday would constitute the 2−day programming).

Press the PROG button when finished.

To adjust the setpoint, press the UP or DOWN (YB) arrow

buttons (see figure 9); the existing setpoint is displayed to

the right of the actual room temperature. Each button

press adjusts the setpoint up or down by 1 degree.

Hold Temperature Mode

Heat Cool

Change Consecutive days

FAN

Day/Time

Prog

I0:36

HOLD

AUTO

MO TU WE TH FR SA SU

SYSTEM

COOL

FILTER

ROOM

SETPOINT

^°68^°

Fan Hold

Figure 11

Settings Enter

To Set Program Events and Temperatures...

Figure 12 gives an example of how the two sets of pro-

grams can be set for a normal workweek and weekend.

In the 5−day bar graph, note how programs A and C reflect

the desired warmth while the home IS occupied (72°); B

allows less heating while the home is NOT occupied; D re-

flects a cool sleeping temperature. The 2−day bar graph

would support day−long occupancy and, because the first

program begins later, a less−demanding time schedule.

Figure 9

After the desired setpoint is reached, the HOME screen

will reappear after about 15 seconds.

Permanent Hold Mode

At any time the program is running, from the HOME

screen, set a permanent hold (program override) by press-

ing the HOLD button (see figure 9). The thermostat now

functions much like a non−programmable thermostat. Use

the Up/Down arrow buttons to adjust the hold setpoint. To

return to the program, press HOLD again.

5/2 Program Example

5−DAY PROGRAM: | A

72°

68°

64°

6AM

Temporary (2-Hour) Hold

NOON

6PM

MID

NIGHT

6AM

At any time the program is running, from the HOME

screen, set a temporary 2-hour hold by pressing the Up/

Down arrow buttons until the desired setpoint is displayed;

ꢀHOLD" flashes (see figure 10). This overrides the pro-

gram for 2 hours from the last button press, then returns to

the program.

2−DAY PROGRAM:

| A |

72°

68°

64°

6AM

NOON

6PM

MID

NIGHT

6AM

Figure 12

Temporary Hold Temperature Mode

NOTE − Pressing ENTER during the following program-

ming steps, saves and exits to the HOME screen.

Heat Cool

FAN

To program events and temperatures, perform the follow-

ing steps, once with Cool selected and once with Heat se-

lected.

Day/Time

Prog

I0:36

AUTO

PROG

HOLD

SYSTEM

COOL

ROOM

SETPOINT

^°68^°

Fan Hold

1. Press and release PROG. ꢀAM 6:00", period ꢀA", and

the 5 consecutive days are displayed; ꢀAM 6" flashes.

2. Use the Up/Down arrow buttons to select the desired

hour; press PROG when the desired hour is reached.

3. Use the Up/Down arrow buttons to select the desired

minute; press PROG.

Settings Enter

Figure 10

4. Use the Up/Down arrow buttons to select the desired

temperature setpoint; press PROG.

While in Temporary Hold, press HOLD once to switch to

Permanent Hold (HOLD displays solid; PROG not dis-

played); press HOLD again to return to the program

(PROG displays; HOLD not displayed).

5. Repeat steps 2 through 4 for periods B, C, and D.

6. Repeat steps 1 through 5 for the 2−day program.

Page 4

505049M 5/1/05

NOTE − This thermostat will NOT automatically switch

from heating to cooling, or vice versa; operator involve-

ment is required. At the change of seasons, or to accomo-

date abnormal seasonal temperature swings, you must

manually select to the opposite conditioning (Heat or Cool)

program.

Reminder Settings Display

For MAINT,

press Settings

button 2 times

MAINT

FILTER

For FILTER,

press Settings

button 1 time

OFF

FAN − Controlling the Fan Operation

Use the FAN button to select either continuous fan mode or

auto fan mode.

Figure 15

The default setting for the reminders is OFF (disabled).

Press Up/Down arrow buttons to select the desired re-

minder intervals.

To change from continuous to auto fan mode (or vice ver-

sa), press the FAN button. Note whether a fan icon in the

FAN box is present (indicating that the fan is running) or not

(fan not running).

Table 1

Filter and Maintenance Reminders

If continuous fan mode is enabled (ON displayed in FAN

box − see figure 13), the fan will run continuously regard-

less of whether the heating or cooling equipment is run-

ning.

Buttons to

Use

Available Settings and

How to Use

Reminder

FILTER

Settings

(1st press)

then Arrows

to scroll

Total fan run time expressed in

months (Off, 1, 3, 6, 12); for

example, if fan runs 12 hours a

day, 1 month reminder displays

in 2 calendar months.

Using Fan ON

Heat Cool

_PMI0:I0

FAN

selections

Day/Time

Prog

PROG

SYSTEM

HEAT

Settings

(2nd press)

then Arrows

to scroll

Elapsed chronological time in

months (Off, 6, 12). Use this,

for example, to remind yourself

when to perform routine checks

or when to call a technician for

perodic preventive

MAINT

ROOM

67^°

Fan Hold

Settings Enter

selections

maintenance.

Figure 13

Enter

Stores settings.

If auto fan mode is selected (AUTO displayed in FAN box −

see figure 14), the fan will only run when the heating or

cooling equipment is running.

NOTE − The HOME screen will reappear about 15 seconds

after the final arrow button press. OR, press ENTER at any

time to store any changes and exit to the HOME screen.

After either programmed interval has elapsed, the remind-

er will be displayed as shown in figure 16.

Using Fan Auto

Heat Cool

Reminders

MAINT

FAN

Day/Time

Prog

I0:36

MO TU WE TH FR SA SU

FILTER

PROG

AUTO

SYSTEM

COOL

ROOM

SERVICE

Fan Hold

72^°

FAN

AUTO

I0:i2

Settings Enter

PROG A B C D HOLD

ROOM

SYSTEM

COOL

OFF

Figure 14

78^°

SETTINGS − Filter/Maintenance Reminders

HEAT

The 51M35 thermostat is designed to remind you when the

filter needs changing or when routine maintenance is re-

quired, as (and if) defined, by you. These optional remind-

ers are not enabled until you activate them. To do so, press

the Settings button (shown below the Fan button in figure

14) once or twice for the desired reminder as shown in fig-

ure 15 and as described in table 1.

Figure 16

After the filter has been changed or maintenance per-

formed, reset the reminder by pressing the SETTINGS

button for 4 seconds. The screen will blink for a few mo-

ments to indicate that the timer has been reset.

Page 5

51M35 SERIES 5/2

Service Indicator

Removing and Installing Thermostat

The thermostat hinges on tabs on the top of the subbase;

no tool is needed to remove the thermostat from the sub-

base. Pivot the bottom of the thermostat outward (releas-

ing the snaps), then lift up to remove.

When abnormal equipment operation is detected, the

SERVICE indicator will flash on the screen (see figure 17).

This indicates that the equipment requires service from a

qualified service technician.

To install it, first position the top tilted toward the wall brack-

et and align it until you feel the tabs and slots engage; then,

while the top is in place, pivot the bottom toward the wall

until the thermostat snaps into place.

Service Indicator Flashing

MO TU WE TH FR SA SU

FILTER

SERVICE

Default Thermostat Settings

FAN

_PMI0:i4

Default thermostat settings are in table 2 and the default

program is shown in table 3.

AUTO

PROG A B C D HOLD

ROOM

Table 2

SYSTEM

COOL

OFF

Default Thermostat Settings

79^°

Mode

Heat (Permanent Hold

Mode)

HEAT

Setpoint

70°F (or 21°C)

Figure 17

Fan

Auto

Filter Reminder

Maintenance Reminder

OFF

Thermostat RESET

OFF

Under some abnormal conditions, it may be necessary to

ꢀreset" the thermostat to its default condition. Such a RE-

SET would delete all programming and settings and there-

fore should only be used on rare occasions when the the

thermostat fails to function as designd and/or as pro-

grammed. Such an instance can occur as a result of a pow-

er surge or similar electrical disturbance (e.g. after an elec-

trical storm or power outage). The RESET button can be

used to recover from this situation.

Equipment Protection

Timers

Reset Back to Zero

Table 3

Default Program Settings

Temperature Temperature

(Heat) (Cool)

Programs

Time

Weekday − A 6:00am 70_F / 21°C 78_F / 26°C

Weekday − B 8:00am 62_F / 17°C 85_F / 29°C

Weekday − C 6:00pm 70_F / 21°C 78_F / 26°C

Weekday − D 10:00pm 62_F / 17°C 82_F / 28°C

Weekend − A 6:00am 70_F / 21°C 78_F / 26°C

Weekend − B 8:00am 62_F / 17°C 85_F / 29°C

Weekend − C 6:00pm 70_F / 21°C 78_F / 26°C

Weekend − D 10:00pm 62_F / 17°C 82_F / 28°C

CAUTION

When the RESET button is pressed, ALL settings re-

vert back to the defaults, including the default pro-

gram (see tables 2 and 3).

The RESET button is an unlabeled, recessed button lo-

cated behind the door on the right−hand side of the thermo-

stat, below the SETTINGS button (see figure 3). Use a pa-

per clip or small pencil to press the RESET button; ALL

thermostat settings will be reset to the defaults listed in the

Default Thermostat Settings section.

Weekday

Monday, Tuesday, Wednesday,

Thursday, Friday

Weekend

Saturday, Sunday

Page 6

505049M 5/1/05

Temperature Setpoint Range

Setting range: 50°F (10°C) to 90°F (32°C)

Setting resolution: 1°F (1°C)

Technical Specifications

Thermostat Type

Electronic programmable thermostat for 2−Stage (gas or

electric) Heat/2-Stage Cool, non−heat pump, non−power

robbing applications.

Smart Setback Recovery (via DIP switch #6)

Smart Setback Recovery (SSR) affects the way the ther-

mostat responds to program events. If SSR is disabled, the

thermostat will react to a program event at the time the

event occurs. However, if SSR is enabled, the thermostat

will react to a program event before the event occurs such

that the desired temperature is reached at the time of the

event, not after.

Power Supply Range

18VAC − 30VAC (24VAC nominal), 60Hz

CAUTION

Fan Control

AUTO or ON modes, gas or electric heat compatible via

DIP switches (also see Thermostat Output section).

24VAC is present on the terminals of the thermostat

bracket. If removing the thermostat from the wall,

use caution and avoid touching any of the connec-

tor terminals on the wall bracket.

I/O Relays

All thermostat relays are latching type to minimize power

consumption.

Also, when working with the thermostat dip

switches, use a non−conductive tool and take cau-

tion to avoid making any contact with the circuit

board, its imprinted circuitry and its connector

prongs.

Table 4

51M35 Terminal Designations

Terminal

Description

24VAC

Temperature Display

Display Scale: Fahrenheit or Celsius user selectable (via

DIP switch; see figure 18)

Display range: 35°F (2°C) to 99°F (37°C)

Display resolution: 1°F (1°C)

Display Accuracy: +/−1°F

First-stage cooling

First-stage heating

Second-stage cooling

Second-stage heating

Fan control

If the Fahrenheit/Celsius display must be changed, use a

plastic, non−conductive tool to push the dip switch to the

right position (see figure 18).

Service Indicator

24VAC common

Changing Fahrenheit/Celsius Setting

Equipment Protection Timers

Minimum Compressor OFF time: 5 minutes

Minimum Compressor ON time: 4 minutes

Minimum Furnace ON time: 3 minutes

Minimum furnace cycle time (elapsed time between any

furnace activation and the next furnace activation): 6 min-

utes.

Minimum elapsed time between any compressor activa-

tion and the next compressor activation: 6 minutes.

NOTE − All protection timers (except the compressor OFF

timer) can be over−ridden if a heating or cooling demand is

initiated or terminated using the UP, DOWN, HEAT, or

COOL buttons.

PLUG IN (Use Care To Avoid Bending Prongs.)

F/C (Fahrenheit/Celsius) switch

shown in Celsius position

Equipment Protection Override

Both the minimum compressor OFF timer and the mini-

mum equipment cycle timer can be over−ridden by press-

ing and holding either the HEAT or COOL button down for 4

seconds.

Figure 18

Temperature Measurement Range

Measurement Scale: Fahrenheit

Measurement Range: 35°F to 99°F

Measurement Resolution: 0.5°F

Measurement Accuracy: +/−1°F

Over−Temperature Protection

Thermal-mechanical switch opens W1 and W2 at

93°F+/−6°F.

Field Offset: via DIP switches to +/−3°F

Sampling Method: temperature measurements sampled

every 15 seconds. Displayed temperature is the average

of the last four measurements.

Filter Reminder

Settings of Off, 1, 3, 6 or 12 (months of fan run time) are

available. When programmed time has elapsed, a FILTER

indicator is displayed.

Page 7

51M35 SERIES 5/2

NOTE − The temperature ranges expressed in the follow-

ing definitions of ꢀSMALL/LARGE" demands are for guid-

ance only; actual temperatures may vary.

Maintenance Reminder

Settings of Off, 6 or 12 (months of chronological time) are

available. When programmed time has elapsed, a mainte-

nance indicator ꢀMAINT" is displayed.

With a small HEAT demand, temperature is:

below setpoint –0.5F but above setpoint –1.5F

AND 30-minute upstage timer HAS NOT expired.

Service Reminder

The SERVICE indicator is displayed only under the follow-

ing conditions:

With a large HEAT demand, temperature is:

less than setpoint –1.5F

OR 30-minute upstage timer HAS expired.

if the thermostat Y1 terminal has been activated with

24VAC for at least 5 minutes, AND the L terminal is

shorted to the R terminal;

With a small COOL demand, temperature is:

above setpoint +0.5F but below setpoint +1.5F

AND 30-minute upstage timer HAS NOT expired.

if the thermostat Y1 terminal has been activated with

24VAC for at least 5 minutes, AND the L terminal is

shorted to the C terminal.

With a large COOL demand, temperature is:

above setpoint +1.5F

OR 30-minute upstage timer HAS expired.

Power Loss/Recovery

Table 5

Thermostat memory is retained for a minimum of 24 hours

during a power loss (includes retention of program infor-

mation, HOLD status, programmed temperature setpoint,

heat/cool and fan mode settings, filter reminder status,

maintenance reminder status, and equipment protection

timers). After 24 hours of power loss, programmed settings

will be lost and replaced with default settings.

Thermostat Outputs

Condition

W1 W2 Y1 Y2

Gas Heat, Auto Fan

HEAT Demand, SMALL

HEAT Demand, LARGE

COOL Demand, SMALL

COOL Demand, LARGE

No Demand

IMPORTANT

Power must be applied for at least six consecutive

hours prior to a power loss in order for memory to

be retained for the specified time.

Gas Heat, Continuous Fan

HEAT Demand, SMALL

HEAT Demand, LARGE

COOL Demand, SMALL

COOL Demand, LARGE

No Demand

LCD Backlight

Activated for 30 seconds when any button is pressed.

NOTE − During an electrical storm or similar disturbance,

the backlight may activate for a few seconds. This is nor-

mal and will no longer occur after the electrical disturbance

has passed.

Electric Heat, Auto Fan

HEAT Demand, SMALL

HEAT Demand, LARGE

COOL Demand, SMALL

COOL Demand, LARGE

No Demand

Thermostat Operating Conditions

35°F to 105°F, 5% to 90% RH

Thermostat Storage Conditions

−40°F to 185°F, 5% to 95% RH

Electric Heat, Continuous Fan

Thermostat Output Table

HEAT Demand, SMALL

HEAT Demand, LARGE

COOL Demand, SMALL

COOL Demand, LARGE

No Demand

Table5 depicts the 51M35 thermostat output states for var-

ious input conditions. The following notes described terms

used in the table.

NOTE − X = output is activated with 24VAC.

Page 8

505049M 5/1/05

OPERATION

MANUAL

E2005 Lennox Industries Inc.

Dallas, Texas, USA

51M34 Merit^®Series

5/2 Day Programmable Thermostat

CONTROLS

505,048M

05/05

Supersedes 03/05

Litho U.S.A.

Table of Contents

51M34 Series Thermostat . . . . . . . . . . . . . . . . . . . . . .

General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Initial Thermostat Power-up . . . . . . . . . . . . . . . . . . . . .

Buttons, Backlight, Timers & Settings . . . . . . . . . . . .

DAY/TIME − Set Day and Time . . . . . . . . . . . . . . . . . .

HEAT − Heat Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . .

COOL − Cool Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . .

HOLD − Temperature Hold Mode . . . . . . . . . . . . . . . .

PROG − Thermostat Programming . . . . . . . . . . . . . . .

FAN − Controlling Fan Operation . . . . . . . . . . . . . . . . .

SETTINGS − Filter /Maintenance Reminder . . . . . . .

Service Indicator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Thermostat RESET . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Removing/Installing Thermostat . . . . . . . . . . . . . . . . .

Default Thermostat Settings . . . . . . . . . . . . . . . . . . . .

Technical Specifications . . . . . . . . . . . . . . . . . . . . . . . .

Thermostat Output Table . . . . . . . . . . . . . . . . . . . . . . .

RETAIN THESE INSTRUCTIONS

FOR FUTURE REFERENCE

51M34 Merit^®Series Thermostat

The Lennox Merit Series 5/2 day programmable

electronic thermostat 51M34 provides excellent

temperature control and a large, easy-to-read display. This

product includes a programmable filter change reminder,

an equipment maintenance reminder, and a system check

indicator to notify the user when the equipment requires

service.

Thermostat 51M34 is suitable for non−heat pump, single-

stage heat/single-stage cool applications with either gas or

electric furnace.

General

Initial Power−Up Display

MAINT

FILTER

MO TU WE TH FR SA SU

These instructions are intended as a general guide and do

not supersede local codes in any way. Consult authorities

having jurisdiction before installation.

SERVICE

_PMI8:88

Check equipment for shipping damage. If you find any

damage, immediately contact the last carrier.

FAN

AUTO

PROG A | B | C | D HOLD

Introduction

SYSTEM

COOL

OFF

EMG.

HEAT

ROOM

SETPOINT

This document describes the operation of Lennox thermostat

51M34. Refer to the installation manual for instructions re-

garding installation and wiring of the thermostat.

88^°

Initial Thermostat Power-up

Figure 1

When power is initially applied to the thermostat, the dis-

play will appear as shown in figure 1.

All display segments are momentarily activated. This oc-

curs as a normal part of thermostat initialization.

05/05

505,048M

Page 1

*2P0505*

*P505048M*

Within a few seconds, the Home Screen appears (see fig-

ure 2) with default settings as shown. After about 1 minute

of initialization time, the actual room temperature will be

displayed.

Thermostat Buttons

Heat Cool

Prog Day/Time

Fan Hold

MAINT

Home Screen

MO TU WE TH FR SA SU

FILTER

SERVICE

(DOOR

OPEN)

_PMI2:00

Settings Enter

FAN

AUTO

PROG A B C D HOLD

ROOM

70^°

SYSTEM

COOL

OFF

Figure 3

HEAT

When an Arrow, HOLD, HEAT, or COOL button is pressed,

SETPOINT and the temperature setting appears for 15

seconds. If desired, start making changes within 15 sec-

onds or the HOME screen will return. The backlight will

turn off 15 seconds after the HOME screen reappears.

Figure 2

At this point, the thermostat will be fully functional; its de-

fault temperature setpoint (not shown) is 70°F. At this

point, if the equipment has been fully powered and if a heat

demand were present, the system would begin operating.

DAY/TIME − Setting the Day and Time

NOTE − Temperature scale default is Fahrenheit units but

may be reset to show Celsius, if desired. See page 7.

Press the DAY/TIME button and set the CURRENT hour,

minute, and day of week as follows:

1. ꢀAM12" will flash on the screen. Press the UP/DOWN

arrows to change the hour. (ꢀAM" or ꢀPM" must corre-

spond to time of day.) Press DAY/TIME OR, if adjust-

ing for daylight savings time, pressing ENTER stores

the single change and exits to the HOME screen, by-

passing minutes and day of week.

Buttons, Backlight, Timers & Settings

Buttons are located behind the small door on the right−

hand side of the thermostat (see figure 3).

IMPORTANT

Do NOT begin pressing buttons until after you read

the following section describing each button.

2. Minutes will flash. Use the UP/DOWN arrow button to

display the minutes past the hour. Press DAY/TIME.

3. Day ꢀMO" (Monday) will flash. Use UP or DOWN arrow

buttons to display the current day. Day selections are

abbreviated as ꢀMOꢀ, ꢀTU", ꢀWE", ꢀTH", ꢀFR", ꢀSA",

and ꢀSU". Press DAY/TIME.

A pale blue display backlight illuminates for 30 seconds

each time any button is pressed.

When PROG or DAY/TIME is pressed, a field begins flash-

ing, expecting another input. Start making changes within

15 seconds or the HOME screen will return.

4. The HOME screen reappears; confirm day and time

are correct. This completes day and time setting.

Page 2

505048M 5/1/05

HEAT − Using the Heat Mode

COOL − Using the Cool Mode

Enabling and Disabling Cool Mode

Enabling and Disabling Heat Mode

The thermostat must be in the HEAT mode to control the

heating equipment. Press the Heat button to enable or dis-

able HEAT mode. If the thermostat is in COOL or OFF

mode, pressing the Heat button enables the HEAT mode

(indicated by HEAT in the SYSTEM box − see figure 4).

Use the COOL button to enable or disable cool mode as

desired. If the thermostat is in HEAT or OFF mode, cool

mode is enabled when the COOL button is pressed. This is

indicated by COOL in the SYSTEM box (see figure 7).

Turn Heat ON/OFF

Turn Cool ON/OFF

Heat Cool

_PM5:30

FAN

I0:30

Prog Day/Time

Fan Hold

HOLD

AUTO

HOLD

AUTO

SYSTEM

COOL

ROOM

SETPOINT

^°70^°

SETPOINT

66^°

Fan Hold

HEAT

Settings Enter

Figure 4

Figure 7

If the thermostat is in heat mode, heat mode is disabled

when the HEAT button is pressed. This is indicated by OFF

in the SYSTEM box as shown in figure 5.

If the thermostat is in cool mode, pressing the Cool button

disables COOL mode (indicated by OFF in the SYSTEM

box − see figure 5).

Heat/Cool Mode Disabled

Cooling Demand

SYSTEM

COOL

OFF

ROOM

75^°

Heating Demand

Set the thermostat to cool mode to control the cooling

equipment. Then, if the room temperature is higher than

the temperature setpoint, as shown in figure 8, the thermo-

stat detects a cooling demand and will activate the cooling

equipment to satisfy the demand.

SETPOINT

75^°

HEAT

Figure 5

Cooling Demand

Set the thermostat to heat mode to control the heating

equipment. Then, if the room temperature is lower than the

temperature setpoint, as shown in figure 6, the thermostat

detects a heating demand and will activate the heating

equipment to satisfy the demand.

SYSTEM

COOL

OFF

ROOM

75^°

SETPOINT

70^°

HEAT

Figure 8

Heating Demand

SYSTEM

ROOM

Cooling operation is indicated by flashing ꢀsnowflake"

icons in the SYSTEM box. When the actual temperature

drops below the temperature setpoint, the snowflake icons

will disappear. This indicates that the cooling demand has

been satisfied and that the cooling equipment has been

turned off.

SETPOINT

COOL

72^°

OFF

75^°

HEAT

Figure 6

NOTE − If no buttons are pressed during a demand for

cooling, the equipment must operate for at least 4 minutes.

After a demand has been satisfied, cooling equipment op-

eration is locked out for 5 minutes. If another cooling de-

mand occurs during this 5−minute interval, ꢀCOOL" and the

snowflakes will flash; however, the cooling equipment will

not operate until the 5−minute delay has elapsed.

Heating operation is indicated by a flame icon in the SYS-

TEM box. When the actual temperature rises above the

temperature setpoint, the flame icon will disappear. This

indicates that the heating demand has been satisfied and

that the heating equipment has been turned off.

NOTE − Heating equipment is activated for at least 3 min-

utes if no buttons are pressed during the demand interval.

Page 3

51M34 SERIES 5/2

HOLD − Using Temperature Hold Modes

PROG − Thermostat Programming

The 51M34 thermostat can be programmed to perform a

set of either heating or cooling events (but not a

combination of heat and cool) for 5 consecutive days using

a set of 4 unique events per day. The remaining 2 days can

then be set for a different set of 4 unique events per day.

Both the consecutive days and the events/temperature are

set by the homeowner.

When HOLD is displayed at the HOME screen, the ther-

mostat is in a temperature hold condition. This means that

the temperature program data is ignored and the thermo-

stat functions much like a non−programmable thermostat.

Adjusting Temperature Setpoint in Hold Mode

The temperature setpoint represents the desired tempera-

ture of the space around the thermostat. The default tem-

perature setpoint in Hold mode is 70°F.

To Change Consecutive Days...

To alter the 5 consecutive days, press and hold the PROG

button for 5 seconds. The existing 5 consecutive day peri-

od is then displayed (default is MOnday thru FRiday). To

change to a different 5-consecutive days, use the Up/

Down arrow buttons. Any 5 consecutive−day span may be

selected, for example, in figure 11, Saturday through

Wednesday is defined as the 5−day programming (Thurs-

day and Friday would constitute the 2−day programming).

Press the PROG button when finished.

To adjust the setpoint, press the UP or DOWN (YB) arrow

buttons (see figure 9); the existing setpoint is displayed to

the right of the actual room temperature. Each button

press adjusts the setpoint up or down by 1 degree.

Hold Temperature Mode

Heat Cool

Change Consecutive days

FAN

Day/Time

Prog

I0:36

HOLD

AUTO

MO TU WE TH FR SA SU

SYSTEM

COOL

FILTER

ROOM

SETPOINT

^°68^°

Fan Hold

Figure 11

Settings Enter

To Set Program Events and Temperatures...

Figure 12 gives an example of how the two sets of pro-

grams can be set for a normal workweek and weekend.

In the 5−day bar graph, note how programs A and C reflect

the desired warmth while the home IS occupied (72°); B

allows less heating while the home is NOT occupied; D re-

flects a cool sleeping temperature. The 2−day bar graph

would support day−long occupancy and, because the first

program begins later, a less−demanding time schedule.

Figure 9

After the desired setpoint is reached, the HOME screen

will reappear after about 15 seconds.

Permanent Hold Mode

At any time the program is running, from the HOME

screen, set a permanent hold (program override) by press-

ing the HOLD button (see figure 9). The thermostat now

functions much like a non−programmable thermostat. Use

the Up/Down arrow buttons to adjust the hold setpoint. To

return to the program, press HOLD again.

5/2 Program Example

5−DAY PROGRAM: | A

72°

68°

64°

6AM

Temporary (2-Hour) Hold

NOON

6PM

MID

NIGHT

6AM

At any time the program is running, from the HOME

screen, set a temporary 2-hour hold by pressing the Up/

Down arrow buttons until the desired setpoint is displayed;

ꢀHOLD" flashes (see figure 10). This overrides the pro-

gram for 2 hours from the last button press, then returns to

the program.

2−DAY PROGRAM:

| A |

72°

68°

64°

6AM

NOON

6PM

MID

NIGHT

6AM

Figure 12

Temporary Hold Temperature Mode

NOTE − Pressing ENTER during the following program-

ming steps, saves and exits to the HOME screen.

Heat Cool

FAN

To program events and temperatures, perform the follow-

ing steps, once with Cool selected and once with Heat se-

lected.

Day/Time

Prog

I0:36

AUTO

PROG

HOLD

SYSTEM

COOL

ROOM

SETPOINT

^°68^°

Fan Hold

1. Press and release PROG. ꢀAM 6:00", period ꢀA", and

the 5 consecutive days are displayed; ꢀAM 6" flashes.

2. Use the Up/Down arrow buttons to select the desired

hour; press PROG when the desired hour is reached.

3. Use the Up/Down arrow buttons to select the desired

minute; press PROG.

Settings Enter

Figure 10

4. Use the Up/Down arrow buttons to select the desired

temperature setpoint; press PROG.

While in Temporary Hold, press HOLD once to switch to

Permanent Hold (HOLD displays solid; PROG not dis-

played); press HOLD again to return to the program

(PROG displays; HOLD not displayed).

5. Repeat steps 2 through 4 for periods B, C, and D.

6. Repeat steps 1 through 5 for the 2−day program.

Page 4

505048M 5/1/05

NOTE − This thermostat will NOT automatically switch

from heating to cooling, or vice versa; operator involve-

ment is required. At the change of seasons, or to accomo-

date abnormal seasonal temperature swings, you must

manually select to the opposite conditioning (Heat or Cool)

program.

Reminder Settings Display

For MAINT,

press Settings

button 2 times

MAINT

FILTER

For FILTER,

press Settings

button 1 time

OFF

FAN − Controlling the Fan Operation

Figure 15

Use the FAN button to select either continuous fan mode or

auto fan mode.

The default setting for the reminders is OFF (disabled).

Press Up/Down arrow buttons to select the desired re-

minder intervals.

To change from continuous to auto fan mode (or vice ver-

sa), press the FAN button. Note whether a fan icon in the

FAN box is present (indicating that the fan is running) or not

(fan not running).

Table 1

Filter and Maintenance Reminders

If continuous fan mode is enabled (ON displayed in FAN

box − see figure 13), the fan will run continuously regard-

less of whether the heating or cooling equipment is run-

ning.

Buttons to

Use

Available Settings and

How to Use

Reminder

FILTER

Settings

(1st press)

then Arrows

to scroll

Total fan run time expressed in

months (Off, 1, 3, 6, 12); for

example, if fan runs 12 hours a

day, 1 month reminder displays

in 2 calendar months.

Using Fan ON

Heat Cool

_PMI0:I0

FAN

selections

Day/Time

Prog

PROG

Settings

(2nd press)

then Arrows

to scroll

Elapsed chronological time in

months (Off, 6, 12). Use this,

for example, to remind yourself

when to perform routine checks

or when to call a technician for

perodic preventive

MAINT

SYSTEM

HEAT

ROOM

67^°

Fan Hold

selections

Settings Enter

maintenance.

Enter

Stores settings.

Figure 13

NOTE − The HOME screen will reappear about 15 seconds

after the final arrow button press. OR, press ENTER at any

time to store any changes and exit to the HOME screen.

If auto fan mode is selected (AUTO displayed in FAN box −

see figure 14), the fan will only run when the heating or

cooling equipment is running.

After either programmed interval has elapsed, the remind-

er will be displayed as shown in figure 16.

Using Fan Auto

Heat Cool

Reminders

MAINT

FAN

Day/Time

Prog

MO TU WE TH FR SA SU

I0:36

FILTER

PROG

AUTO

SYSTEM

COOL

ROOM

SERVICE

Fan Hold

72^°

FAN

AUTO

I0:i2

Settings Enter

PROG A B C D HOLD

ROOM

SYSTEM

COOL

OFF

Figure 14

78^°

SETTINGS − Filter/Maintenance Reminders

HEAT

The 51M34 thermostat is designed to remind you when the

filter needs changing or when routine maintenance is re-

quired, as (and if) defined, by you. These optional remind-

ers are not enabled until you activate them. To do so, press

the Settings button (shown below the Fan button in figure

14) once or twice for the desired reminder as shown in fig-

ure 15 and as described in table 1.

Figure 16

After the filter has been changed, reset the filter reminder

by pressing the SETTINGS button for 4 seconds. The

screen will blink for a few moments to indicate that the tim-

er has been reset.

Page 5

51M34 SERIES 5/2

Service Indicator

Removing/Installing Thermostat

The thermostat hinges on tabs on the top of the subbase;

no tool is needed to remove the thermostat from the sub-

base. Pivot the bottom of the thermostat outward (releas-

ing the snaps), then lift up to remove.

When abnormal equipment operation is detected, the

SERVICE indicator will flash on the screen (see figure 17).

This indicates that the equipment requires service from a

qualified service technician.

To replace it, first position the top tilted toward the wall

bracket and align it until you feel the tabs and slots engage;

then, while the top is in place, pivot the bottom toward the

wall until the thermostat snaps into place.

Service Indicator Flashing

MO TU WE TH FR SA SU

FILTER

SERVICE

Default Thermostat Settings

FAN

_PMI0:i4

Default thermostat settings are in table 2 and the default

program is shown in table 3.

AUTO

PROG A B C D HOLD

ROOM

Table 2

SYSTEM

COOL

OFF

Default Thermostat Settings

79^°

Mode

Heat (Permanent Hold

Mode)

HEAT

Setpoint

70°F (or 21°C)

Figure 17

Fan

Auto

Filter Reminder

Maintenance Reminder

OFF

Thermostat RESET

OFF

Under some abnormal conditions, it may be necessary to

ꢀreset" the thermostat to its default condition. Such a RE-

SET would delete all programming and settings and there-

fore should only be used on rare occasions when the the

thermostat fails to function as designd and/or as pro-

grammed. Such an instance can occur as a result of a pow-

er surge or similar electrical disturbance (e.g. after an elec-

trical storm or power outage). The RESET button can be

used to recover from this situation.

Equipment Protection

Timers

Reset Back to Zero

Table 3

Default Program Settings

Temperature Temperature

(Heat) (Cool)

Programs

Time

Weekday − A 6:00am 70_F / 21°C 78_F / 26°C

Weekday − B 8:00am 62_F / 17°C 85_F / 29°C

Weekday − C 6:00pm 70_F / 21°C 78_F / 26°C

Weekday − D 10:00pm 62_F / 17°C 82_F / 28°C

Weekend − A 6:00am 70_F / 21°C 78_F / 26°C

Weekend − B 8:00am 62_F / 17°C 85_F / 29°C

Weekend − C 6:00pm 70_F / 21°C 78_F / 26°C

Weekend − D 10:00pm 62_F / 17°C 82_F / 28°C

CAUTION

When the RESET button is pressed, ALL settings re-

vert back to the defaults, including the default pro-

gram (see tables 2 and 3).

The RESET button is an unlabeled, recessed button lo-

cated behind the door on the right−hand side of the thermo-

stat, below the SETTINGS button (see figure 3). Use a pa-

per clip or small pencil to press the RESET button; ALL

thermostat settings will be reset to the defaults listed in the

Default Thermostat Settings section.

Weekday

Monday, Tuesday, Wednesday,

Thursday, Friday

Weekend

Saturday, Sunday

Page 6

505048M 5/1/05

Field Offset: via DIP switches to +/−3°F

Technical Specifications

Thermostat Type

Sampling Method: temperature measurements sampled

every 15 seconds. Displayed temperature is the average

of the last four measurements.

Electronic programmable thermostat for 1-Stage-

Heat/1-Stage Cool, gas or electric heat, non−heat pump,

non−power robbing applications.

Temperature Setpoint Range

Setting range: 50°F (10°C) to 90°F (32°C)

Setting resolution: 1°F (1°C)

Power Supply Range

Smart Setback Recovery (via DIP switch #6)

Smart Setback Recovery (SSR) affects the way the ther-

mostat responds to program events. If SSR is disabled, the

thermostat will react to a program event at the time the

event occurs. However, if SSR is enabled, the thermostat

will react to a program event before the event occurs such

that the desired temperature is reached at the time of the

event, not after.

18VAC − 30VAC (24VAC nominal), 60Hz

CAUTION

24VAC is present on the terminals of the thermostat

bracket. If removing the thermostat from the wall,

use caution and avoid touching any of the connec-

tor terminals on the wall bracket.

Fan Control

AUTO or ON modes, gas or electric heat compatible via

DIP switches (also see Thermostat Output section).

Also, when working with the thermostat dip

switches, use a non−conductive tool and take cau-

tion to avoid making any contact with the circuit

board, its imprinted circuitry and its connector

prongs.

I/O Relays

All thermostat relays are latching type to minimize power

consumption.

Table 4

51M34 Terminal Designations

Temperature Display

Terminal

Description

Display Scale: Fahrenheit or Celsius user selectable (via

DIP switch; see figure 18)

Display range: 35°F (2°C) to 99°F (37°C)

Display resolution: 1°F (1°C)

Display Accuracy: +/−1°F

24VAC

First stage cooling

First stage heating

Fan control

Service Indicator

24VAC common

If the Fahrenheit/Celsius display must be changed, use a

plastic, non−conductive tool to push the dip switch to the

right position (see figure 18).

Equipment Protection Timers

Minimum Compressor OFF time: 5 minutes

Minimum Compressor ON time: 4 minutes

Minimum Furnace ON time: 3 minutes

Changing Fahrenheit/Celsius Setting

Minimum furnace cycle time (elapsed time between any

furnace activation and the next furnace activation): 6 min-

utes.

Minimum elapsed time between any compressor activa-

tion and the next compressor activation: 6 minutes.

NOTE − All protection timers (except the compressor OFF

timer) can be over−ridden if a heating or cooling demand is

initiated or terminated using the UP, DOWN, HEAT, or

COOL buttons.

PLUG IN (Use Care To Avoid Bending Prongs.)

Equipment Protection Override

Both the minimum compressor OFF timer and the mini-

mum equipment cycle timer can be over−ridden by press-

ing and holding either the HEAT or COOL button down for 4

seconds.

F/C (Fahrenheit/Celsius) switch

shown in Celsius position

Figure 18

Over−Temperature Protection

Thermal-mechanical switch opens W1 at 93°F+/−6°F.

Temperature Measurement Range

Measurement Scale: Fahrenheit

Measurement Range: 35°F to 99°F

Measurement Resolution: 0.5°F

Measurement Accuracy: +/−1°F

Filter Reminder

Settings of Off, 1, 3, 6 or 12 (months of fan run time) are

available. When programmed time has elapsed, a FILTER

indicator is displayed.

Page 7

51M34 SERIES 5/2

Maintenance Reminder

Thermostat Operating Conditions

Settings of Off, 6 or 12 (months of chronological time) are

available. When programmed time has elapsed, a mainte-

nance indicator ꢀMAINT" is displayed.

35°F to 105°F, 5% to 90% RH

Thermostat Storage Conditions

Service Reminder

The SERVICE indicator is displayed only under the follow-

ing conditions:

−40°F to 185°F, 5% to 95% RH

Thermostat Output Table

if the thermostat Y1 terminal has been activated with

24VAC for at least 5 minutes, AND the L terminal is

shorted to the R terminal;

Table5 depicts the 51M34 thermostat output states for var-

ious input conditions.

Table 5

if the thermostat Y1 terminal has been activated with

24VAC for at least 5 minutes, AND the L terminal is

shorted to the C terminal.

Thermostat Outputs

Condition

Power Loss/Recovery

Gas Heat, Auto Fan

HEAT Demand

Thermostat memory is retained for a minimum of 24 hours

during a power loss (includes retention of program infor-

mation, HOLD status, programmed temperature setpoint,

heat/cool and fan mode settings, filter reminder status,

maintenance reminder status, and equipment protection

timers). After 24 hours of power loss, programmed settings

will be lost and replaced with default settings.

COOL Demand

No Demand

Gas Heat, Continuous Fan

HEAT Demand

COOL Demand

No Demand

IMPORTANT

Power must be applied for at least six consecutive

hours prior to a power loss in order for memory to

be retained for the specified time.

Electric Heat, Auto Fan

HEAT Demand

COOL Demand

No Demand

LCD Backlight

Activated for 30 seconds when any button is pressed.

Electric Heat, Continuous Fan

HEAT Demand

COOL Demand

No Demand

NOTE − During an electrical storm or similar disturbance,

the backlight may activate for a few seconds. This is nor-

mal and will no longer occur after the electrical disturbance

has passed.

NOTE − X = output is activated with 24VAC.

Page 8

505048M 5/1/05

OPERATION

MANUAL

E2005 Lennox Industries Inc.

Dallas, Texas, USA

51M39 Merit^®Series

7 Day Programmable Thermostat

CONTROLS

505,068M

07/05

Litho U.S.A.

Table of Contents

51M39 Series Thermostat . . . . . . . . . . . . . . . . . . . . . .

General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Initial Thermostat Power-up . . . . . . . . . . . . . . . . . . . . .

Buttons, Backlight, Timers & Settings . . . . . . . . . . . .

DAY/TIME − Set Day and Time . . . . . . . . . . . . . . . . . .

HEAT − Heat Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . .

COOL − Cool Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . .

AUTO − Using the Autochangeover Mode . . . . . . . . .

HOLD − Temperature Hold Mode . . . . . . . . . . . . . . . .

PROG − Thermostat Programming . . . . . . . . . . . . . . .

COPY − Copy a Program . . . . . . . . . . . . . . . . . . . . . . .

FAN − Controlling Fan Operation . . . . . . . . . . . . . . . . .

Displaying Outside Temperature . . . . . . . . . . . . . . . . .

SETTINGS − Filter /Maintenance Reminder . . . . . . .

Service Indicator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Thermostat RESET . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Removing/Installing Thermostat . . . . . . . . . . . . . . . . .

Default Thermostat Settings . . . . . . . . . . . . . . . . . . . .

Thermostat Output Table . . . . . . . . . . . . . . . . . . . . . . .

Technical Specifications . . . . . . . . . . . . . . . . . . . . . . . .

RETAIN THESE INSTRUCTIONS

FOR FUTURE REFERENCE

51M39 Merit^®Series Thermostat

The Lennox Merit Series 7 day programmable electronic

thermostat 51M39 provides excellent temperature control

and a large, easy-to-read display. This product includes a

programmable filter change reminder, an equipment

maintenance reminder, and a system check indicator to

notify the user when the equipment requires service.

Thermostat 51M39 is suitable for non-heat pump, two-

stage heat/two-stage cool applications using a gas or elec-

tric furnace. Also, the thermostat provides autochange-

over capability.

General

Initial Power−Up Display

Copy tofrom MO TU WE TH FR SA SU

These instructions are intended as a general guide and do

not supersede local codes in any way. Consult authorities

having jurisdiction before installation.

PROG

A B C D

I8:88

HOLD

Check equipment for shipping damage. If you find any

damage, immediately contact the last carrier.

FAN ON

AUTO

COOL SETPOINT

FILTER

SERVICE

COOL

OFF

MAINTENANCE

Introduction

AUTO

EMG.

HEAT

OUTROOM

i88^°

HEAT SETPOINT

This document describes the operation of Lennox thermo-

stat 51M39. Refer to the installation manual for instructions

regarding installation and wiring of the thermostat.

Initial Thermostat Power-up

Figure 1

When power is initially applied to the thermostat, the dis-

play will appear as shown in figure 1.

All display segments are momentarily activated. This oc-

curs as a normal part of thermostat initialization.

07/05

505,068M

Page 1

*2P0705*

*P505068M*

Within a few seconds, the HOME screen appears (see fig-

ure 2) with default settings as shown. After about 1 minute

of initialization time, the actual room temperature will be

displayed.

Thermostat Buttons

Heat Cool

Prog Auto

Copy Day/Time

Fan Hold

NOTE − Tables 2 and 3 on page 7 show all system and

programming defaults.

HOME Screen

(DOOR

OPEN)

PROG

A B C D

HOLD

Settings Enter

(Outside)

PM I0:26

FAN ON

AUTO

COOL

Figure 3

OFF

ROOM

AUTO

EMG.

70^°

HEAT

When an Arrow, HOLD, HEAT, or COOL button is pressed,

SETPOINT and the temperature setting appears for 15

seconds. If desired, start making changes within 15 sec-

onds or the HOME screen will return. The backlight will

turn off 15 seconds after the HOME screen reappears.

Figure 2

At this point, the thermostat will be fully functional; its de-

fault temperature setpoint (not shown) is 70°F. At this

point, if the equipment has been fully powered and if a heat

demand were present, the system would begin operating.

DAY/TIME − Setting the Day and Time

NOTE − Temperature scale default is Fahrenheit units but

may be reset to show Celsius, if desired. See page 8.

Press the DAY/TIME button and set the CURRENT hour,

minute, and day of week as follows:

Buttons, Backlight, Timers & Settings

1. ꢀAM12" will flash on the screen. Press the UP/DOWN

arrows to change the hour. (ꢀAM" or ꢀPM" must corre-

spond to time of day.) Press DAY/TIME OR, if adjust-

ing for daylight savings time, pressing the ENTER but-

ton stores the single change and exits to the HOME

screen, bypassing minutes and day of week.

Buttons are located behind the small door on the right−

hand side of the thermostat (see figure 3).

IMPORTANT

Do NOT begin pressing buttons until after you read

the following section describing each button.

2. Minutes will flash. Use the UP/DOWN arrow button to

display the minutes past the hour. Press DAY/TIME.

A pale blue display backlight illuminates for 30 seconds

each time any button is pressed.

3. Day ꢀMO" (Monday) will flash. Use UP or DOWN arrow

buttons to display the current day. Day selections are

abbreviated as ꢀMOꢀ, ꢀTU", ꢀWE", ꢀTH", ꢀFR", ꢀSA",

and ꢀSU". Press DAY/TIME.

When PROG or DAY/TIME button is pressed, a field be-

gins flashing, expecting another input. Start making

changes within 15 seconds or the HOME screen will re-

turn.

4. The HOME screen reappears; confirm day and time

are correct. This completes day and time setting.

Page 2

505068M 07/05

HEAT − Using the Heat Mode

COOL − Using the Cool Mode

Enabling and Disabling Heat Mode

Enabling and Disabling Cool Mode

Use the COOL button to enable or disable cool mode as

desired. If the thermostat is in heat or off mode, cool mode

is enabled when the COOL button is pressed. This is indi-

cated by COOL on the right side of the display (see figure

8).

The thermostat must be in the heat mode to control the

heating equipment. Press the HEAT button to enable or

disable HEAT mode. If the thermostat is in cool or off

mode, pressing the HEAT button enables the heat mode

(indicated by HEAT in the lower right corner − see figure 4).

Turn Heat ON/OFF

Turn Cool ON/OFF

Heat Cool

Prog Auto

5:30

i0:30

Copy Day/Time

Fan Hold

(DOOR

OPEN)

Fan Hold

(DOOR

OPEN)

Settings Enter

(Outside)

Settings Enter

(Outside)

Figure 4

Figure 7

If the thermostat is in heat mode, heat mode is disabled

when the HEAT button is pressed. This is indicated by OFF

near the right side of the display as shown in figure 5.

If the thermostat is in cool mode, pressing the COOL but-

ton disables cool mode (indicated by OFF in the SYSTEM

box − see figure 5).

Heat/Cool Mode Disabled

ROOM

Cooling Demand

OFF

Set the thermostat to cool mode to control the cooling

equipment. Then, if the room temperature is higher than

the temperature setpoint, as shown in figure 7, the thermo-

stat detects a cooling demand and will activate the cooling

equipment to satisfy the demand.

HEAT SETPOINT

75^°

Figure 5

Cooling operation is indicated by flashing ꢀsnowflake"

icons on the right side of the display. When the actual tem-

perature drops below the temperature setpoint, the snow-

flake icons will disappear. This indicates that the cooling

demand has been satisfied and that the cooling equipment

has been turned off.

Heating Demand

Set the thermostat to heat mode to control the heating

equipment. Then, if the room temperature is lower than the

temperature setpoint, as shown in figure 6, the thermostat

detects a heating demand and will activate the heating

equipment to satisfy the demand.

If your system supports 2−stage cooling (as does the

51M39 thermostat), you may notice various cooling levels

being delivered during a demand. Also, if a small cooling

demand is present, "Lo" will be displayed in the SYSTEM

box. However, if a large cooling demand is present, "Hi" will

be displayed in the lower right corner (shown in figure 8).

Heating Demand

ROOM

HEAT SETPOINT

72^°

HEAT

75^°

Figure 6

Cooling Demand

Heating operation is indicated by a flame icon in the lower

right corner. When the actual temperature rises above the

temperature setpoint, the flame icon will disappear. This

indicates that the heating demand has been satisfied and

that the heating equipment has been turned off.

COOL SETPOINT

FAN

AUTO

76^°

COOL

ROOM

80^°

If your system supports 2−stage heating (as does the

51M39 thermostat), you may notice various heating levels

being delivered during a demand.

HEAT SETPOINT

68^°

NOTE − Heating equipment is activated for at least 3 min-

utes if no buttons are pressed during the demand interval.

Figure 8

Page 3

51M39 SERIES 5/2

NOTE − If no buttons are pressed during a demand for

cooling, the equipment must operate for at least 4 minutes.

After a demand has been satisfied, cooling equipment op-

eration is locked out for 5 minutes. If another cooling de-

mand occurs during this 5−minute interval, ꢀCOOL" and the

snowflakes will flash; however, the cooling equipment will

not operate until the 5−minute delay has elapsed.

After the desired setpoint is reached, the HOME screen

will reappear after about 15 seconds.

Permanent Hold Mode

At any time the program is running, from the HOME

screen, set a permanent hold (program override) by press-

ing the HOLD button (see figure 9). The thermostat now

functions much like a non−programmable thermostat. Use

the Up/Down arrow buttons to adjust the hold setpoint. To

return to the program, press the HOLD button again.

AUTO − Using the Autochangeover Mode

When in either heat mode or cool mode, autochangeover

can be enabled or disabled by pressing the AUTO button. If

enabled, AUTO appears at the lower right corner of the dis-

play. The thermostat will then automatically change over

from heating to cooling and vice versa, to keep the room

temperature in between the heating and cooling setpoints.

Autochangeover can be used in either of the thermostat

hold modes, or when the thermostat program is running.

These modes are described later.

Temporary (2-Hour) Hold

At any time the program is running, from the HOME

screen, set a temporary 2-hour hold by pressing the Up/

Down arrow buttons until the desired setpoint is displayed;

ꢀHOLD" flashes (see figure 10). This overrides the pro-

gram for 2 hours from the last button press, then returns to

the program.

While in Temporary Hold, press the HOLD button once to

switch to Permanent Hold (HOLD displays solid; PROG

not displayed); press the HOLD button again to return to

the program (PROG displays; HOLD not displayed).

HOLD − Using Temperature Hold Modes

When HOLD is displayed at the HOME screen, the ther-

mostat is in a temperature hold condition. This means that

the temperature program data is ignored and the thermo-

stat functions much like a non−programmable thermostat.

Temporary Hold Temperature Mode

Heat Cool

Adjusting Temperature Setpoint in Hold Mode

Prog Auto

The temperature setpoint represents the desired tempera-

ture of the space around the thermostat. The default heat

setpoint in hold mode is 70°F; the default cool setpoint in

hold mode is 78°F.

i0:30

Copy Day/Time

Fan Hold

(DOOR

OPEN)

Hold Temperature Mode

Settings Enter

(Outside)

Heat Cool

Prog Auto

i0:30

Figure 10

Copy Day/Time

PROG − Thermostat Programming

Fan Hold

(DOOR

OPEN)

The 51M39 thermostat can be programmed to perform a

set of both heating and cooling events for each day of the

week. Each day can be programmed for 4 unique events

per day, and each day can be different from any other day.

Settings Enter

(Outside)

To Set Program Events and Temperatures

Figure 11 gives an example of how up to 7 different pro-

grams can be set. Programs A and C reflect the desired

warmth while the home IS occupied (72°); B allows less

heating while the home is NOT occupied; D reflects a cool

sleeping temperature.

Figure 9

To adjust the setpoint, press the UP or DOWN (YB) arrow

buttons; the existing setpoint is displayed to the right of the

actual room temperature. Each button press adjusts the

setpoint up or down by 1 degree.

NOTE − This example shows heat setpoints; a similar pro-

gram may be set for cool setpoints.

Page 4

505068M 07/05

Repeat steps A and B for as many days as you

want the same program.

Program Example

MONDAY PROGRAM: 72°

68°

64°

Copy a Programmed Day to Another Day

Heat Cool

TUESDAY PROGRAM: 72°

68°

64°

Prog Auto

Copy Day/Time

Fan Hold

Copy from MO

WEDNESDAY PROGRAM: 72°

68°

64°

(DOOR

OPEN)

THURSDAY PROGRAM: 72°

68°

64°

Settings Enter

(outside)

FRIDAY PROGRAM: 72°

68°

64°

Figure 12

SATURDAY PROGRAM: 72°

68°

64°

7. Repeat steps 1 − 5 to program different times and tem-

peratures for days not yet programmed.

SUNDAY PROGRAM: 72°

FAN − Controlling the Fan Operation

68°

64°

Use the FAN button to select either continuous fan mode or

auto fan mode.

6AM

NOON

6PM

MID

NIGHT

6AM

To change from continuous to auto fan mode (or vice ver-

sa), press the FAN button. Note whether a fan icon in the

FAN box is present (indicating that the fan is running) or not

(fan not running).

Figure 11

NOTE − Pressing the ENTER button during the following

programming steps, saves and exits to the HOME screen.

If continuous fan mode is enabled (ON displayed in FAN

box − see figure 13), the fan will run continuously regard-

less of whether the heating or cooling equipment is run-

ning.

To program events and temperatures, perform the follow-

ing steps.

1. Press and release the PROG button. "MO" (Monday),

"AM 6:00", period "A", "COOL SETPOINT 78", and

"HEAT SETPOINT 70" are displayed. "AM 6" flashes.

Heat Cool

Using Fan ON

2. Use the Up/Down arrow buttons to select the desired

hour; press the PROG button when the desired hour

is reached; use the Up/Down arrow buttons again to

select desired minute. The selected hour/minute is

when the program will start. Press the PROG button.

Prog Auto

i0:30

Copy Day/Time

Fan Hold

3. Use the Up/Down arrow buttons to select the desired

heat setpoint; press the PROG button.

4. Use the Up/Down arrow buttons to select the desired

cool setpoint; press the PROG button.

Figure 13

If auto fan mode is selected (AUTO displayed in FAN box −

see figure 14), the fan will only run when the heating or

cooling equipment is running.

5. Repeat steps 1 − 4 for Monday, periods B, C, and D.

6. COPY A PROGRAM − If the program entered for

Monday is satisfactory for other days of the week, do

the following to repeat the same program for other

days:

Heat Cool

Using Fan AUTO

Press the PROG button, then press the COPY

button. The display clears the screen except for

ꢀCopy from MOꢀ with ꢀMO" flashing. Press the

COPY button again to copy Monday’s program.

The display changes to ꢀCopy to MO". Use the ar-

row buttons to scroll to the desired day where you

want the same program as Monday. Press the

COPY button.

Prog Auto

i0:30

Copy Day/Time

Fan Hold

Figure 14

Page 5

51M39 SERIES 5/2

After either programmed interval has elapsed, the remind-

er will be displayed as shown in figure 17.

Displaying Outside Temperature

Pressing the ENTER (Outside) button at the HOME

screen displays the outside temperature (see figure 15) for

several seconds, then returns to room temperature.

Reminders

PROG

A B C D

HOLD

PM I0:26

Check Outside Temperature

FAN ON

AUTO

Filter &

Maintenance

Indicators

Fan Hold

FILTER SERVICE

MAINTENANCE

ROOM

COOL

OFF

AUTO

EMG.

HEAT

Settings Enter

(Outside)

76^°

Figure 15

FAN ON

AUTO

FILTER SERVICE

MAINTENANCE

ROOM

Service

Indicator

Flashing

COOL

OFF

SETTINGS − Filter/Maintenance Reminders

AUTO

EMG.

HEAT

The 51M39 thermostat is designed to remind you when the

filter needs changing or when routine maintenance is re-

quired, as (and if) defined, by you. These optional remind-

ers are not enabled until you activate them. To do so, press

the SETTINGS button once or twice for the desired re-

minder as shown in figure 16 and as described in table 1.

76^°

Figure 17

After the filter has been changed or maintenance per-

formed, reset the reminder by pressing the SETTINGS

button for 4 seconds at the corresponding reminder set-

tings screen. The screen will blink for a few moments to in-

dicate that the timer has been reset.

Reminder Settings Display

OFF

Service Indicator

For FILTER, press SETTINGS

button 1 time

When abnormal equipment operation is detected, the

SERVICE indicator will flash on the screen (see figure 17).

This indicates that the equipment requires service from a

qualified service technician.

FILTER

For MAINTENANCE, press

SETTINGS button 2 times

MAINTENANCE

Figure 16

Thermostat RESET

The default setting for the reminders is OFF (disabled).

Press Up/Down arrow buttons to select the desired re-

minder intervals.

Under some abnormal conditions, it may be necessary to

ꢀreset" the thermostat to its default condition. Such a RE-

SET would delete all programming and settings and there-

fore should only be used on rare occasions when the the

thermostat fails to function as designed and/or as pro-

grammed. Such an instance can occur as a result of a pow-

er surge or similar electrical disturbance (e.g. after an elec-

trical storm or power outage). The RESET button can be

used to recover from this situation.

Table 1

Filter and Maintenance Reminders

Buttons to

Use

Available Settings and

How to Use

Reminder

FILTER

Settings

(1st press)

then Arrows

to scroll

Total fan run time expressed in

months (Off, 1, 3, 6, 12); for

example, if fan runs 12 hours a day,

1 month reminder displays in 2

calendar months.

selections

CAUTION

When the RESET button is pressed, ALL settings re-

vert back to the defaults, including the default pro-

gram (see tables 2 and 3).

Settings

(2nd press)

then Arrows

to scroll

Elapsed chronological time in

months (Off, 6, 12). Use this, for

example, to remind yourself when to

perform routine checks or when to

call a technician for perodic

MAINTE-

NANCE

selections

preventive maintenance.

The RESET button is an unlabeled, recessed button lo-

cated behind the door on the right−hand side of the thermo-

stat, below the SETTINGS button (see figure 3). Use a pa-

per clip or small pencil to press the RESET button; ALL

thermostat settings will be reset to the defaults listed in the

Default Thermostat Settings section.

Enter

Stores settings.

NOTE − The HOME screen will reappear about 15 seconds

after the final arrow button press. OR, press the ENTER

button at any time to store any changes and exit to the

HOME screen.

Page 6

505068M 07/05

Table 4

Removing and Installing Thermostat

Thermostat Outputs

Condition

W1 W2 Y1 Y2

The thermostat hinges on tabs on the top of the subbase;

no tool is needed to remove the thermostat from the sub-

base. Pivot the bottom of the thermostat outward (releas-

ing the snaps), then lift up to remove.

Gas Heat, Auto Fan

Heat Demand, SMALL

Heat Demand, LARGE

Cool Demand, SMALL

Cool Demand, LARGE

No Demand

To install it, first position the top tilted toward the wall brack-

et and align it until you feel the tabs and slots engage; then,

while the top is in place, pivot the bottom toward the wall

until the thermostat snaps into place.

Gas Heat, Continuous Fan

Heat Demand, SMALL

Heat Demand, LARGE

Cool Demand, SMALL

Cool Demand, LARGE

No Demand

Default Thermostat Settings

Default thermostat settings are in table 2 and the default

program is shown in table 3.

Table 2

Electric Heat, Auto Fan

Heat Demand, SMALL

Heat Demand, LARGE

Cool Demand, SMALL

Cool Demand, LARGE

No Demand

Default Thermostat Settings

Mode

Heat (Perm. Hold)

70°F (or 21°C)

78°F (or 26°C)

Auto

Heat Setpoint (Perm Hold)

Cool Setpoint (Perm Hold)

Fan

Filter Reminder

Maintenance Reminder

OFF

Electric Heat, Continuous Fan

OFF

Heat Demand, SMALL

Heat Demand, LARGE

Cool Demand, SMALL

Cool Demand, LARGE

No Demand

Equipment Protection Timers Reset Back to Zero

Autochangeover OFF

Table 3

Default Program Settings (Time & Temp.)

NOTE − X = output is activated with 24VAC.

NOTE − The temperature ranges expressed in the follow-

ing definitions of ꢀSMALL/LARGE" demands are for guid-

ance only; actual temperatures may vary.

Programs

Time

Temp.− Heat Temp.− Cool

All days − A 6:00am 70_F / 21°C

All days − B 8:00am 62_F / 17°C

All days − C 6:00pm 70_F / 21°C

All days − D 10:00pm 62_F / 17°C

78_F / 26°C

85_F / 29°C

78_F / 26°C

82_F / 28°C

With a SMALL heat demand, temperature is:

below setpoint –0.5F but above setpoint –1.5F

AND 30-minute upstage timer HAS NOT expired.

With a LARGE heat demand, temperature is:

less than setpoint –1.5F

OR 30-minute upstage timer HAS expired.

With a SMALL cool demand, temperature is:

above setpoint +0.5F but below setpoint +1.5F

AND 30-minute upstage timer HAS NOT expired.

Thermostat Output Table

Table4 depicts the 51M39 thermostat output states for var-

ious input conditions. The following notes described terms

used in the table.

With a LARGE cool demand, temperature is:

above setpoint +1.5F

OR 30-minute upstage timer HAS expired.

Page 7

51M39 SERIES 5/2

Display range: 35°F (2°C) to 99°F (37°C)

Display resolution: 1°F (1°C)

Technical Specifications

Display Accuracy: +/−1°F

Thermostat Type

Electronic programmable thermostat for 2−Stage (gas or

electric) Heat/2-Stage Cool, non−heat pump, non−power

robbing applications.

If the Fahrenheit/Celsius display must be changed, use a

plastic, non−conductive tool to push the dip switch to the

right position (see figure 18).

Table 5

51M39 Terminal Designations

Terminal Description

Indoor Temperature Measurement Range

Measurement Scale: Fahrenheit

Measurement Range: 35°F to 99°F

Measurement Resolution: 0.5°F

Measurement Accuracy: +/−1°F

Field Offset: via DIP switches to +/−3°F

Sampling Method: temperature measurements sampled

every 15 seconds. Displayed temperature is the average

of the last four measurements.

24VAC

First-stage cooling

First-stage heating

Second-stage cooling

Second-stage heating

Fan control

Outdoor Temperature Measurement Range

Measurement Scale: Fahrenheit

Measurement Range: −22°F to 122°F

Measurement Resolution: 1°F

Measurement Accuracy: +/−2°F

Service Indicator

24VAC common

Outdoor temperature sensor connection 1

Outdoor temperature sensor connection 2

Temperature Setpoint Range

Setting range: 50°F (10°C) to 90°F (32°C)

Setting resolution: 1°F (1°C)

Power Supply Range

18VAC − 30VAC (24VAC nominal), 60Hz

Smart Setback Recovery (via DIP switch #6)

Smart Setback Recovery (SSR) affects the way the ther-

mostat responds to program events. If SSR is disabled, the

thermostat will react to a program event at the time the

event occurs. However, if SSR is enabled, the thermostat

will react to a program event before the event occurs such

that the desired temperature is reached at the time of the

event, not after.

CAUTION

24VAC is present on the terminals of the thermostat

bracket. If removing the thermostat from the wall,

use caution and avoid touching any of the connec-

tor terminals on the wall bracket.

Also, when working with the thermostat dip

switches, use a non−conductive tool and take cau-

tion to avoid making any contact with the circuit

board, its imprinted circuitry and its connector

prongs.

Autochangeover Deadband Selection (via DIP

switch #7)

Autochangeover deadband can be set to 4 or 6 degrees.

When autochangeover is enabled (via the AUTO button),

the thermostat will automatically change over from heating

to cooling and vice versa, to keep the room temperature in

between the heating and cooling setpoints. The deadband

is the minimum difference between the heating and cooling

setpoints.

Temperature Display

Display Scale: Fahrenheit or Celsius user selectable (via

DIP switch; see figure 18)

Fan Control

AUTO or ON modes, gas or electric heat compatible via

DIP switches (also see Thermostat Output section).

Changing Fahrenheit/Celsius Setting

I/O Relays

All thermostat relays are latching type to minimize power

consumption.

Equipment Protection Timers

Minimum Compressor OFF time: 5 minutes

Minimum Compressor ON time: 4 minutes

Minimum Furnace ON time: 3 minutes

PLUG IN (Use Care To Avoid Bending Prongs.)

Minimum furnace cycle time (elapsed time between any

furnace activation and the next furnace activation): 6 min-

utes.

F/C (Fahrenheit/Celsius) switch

shown in Celsius position

Minimum elapsed time between any compressor activa-

tion and the next compressor activation: 6 minutes.

Figure 18

Page 8

505068M 07/05

NOTE − All protection timers (except the compressor OFF

timer) can be over−ridden if a heating or cooling demand is

initiated or terminated using the UP, DOWN, HEAT, or

COOL buttons.

if the thermostat Y1 terminal has been activated with

24VAC for at least 5 minutes, AND the L terminal is

shorted to the C terminal.

Power Loss/Recovery

Equipment Protection Override

Thermostat memory is retained for a minimum of 24 hours

during a power loss (includes retention of clock setting,

program information, HOLD status, programmed temper-

ature setpoint, heat/cool and fan mode settings, filter re-

minder status, maintenance reminder status, and equip-

ment protection timers). After 24 hours of power loss,

programmed settings will be lost and replaced with default

settings.

Both the minimum compressor OFF timer and the mini-

mum equipment cycle timer can be over−ridden by press-

ing and holding either the HEAT or COOL button down for 4

seconds.

Over−Temperature Protection

Thermal-mechanical switch opens W1 and W2 at

93°F+/−6°F.

Filter Reminder

IMPORTANT

Power must be applied for at least six consecutive

hours prior to a power loss in order for memory to

be retained for the specified time.

Settings of Off, 1, 3, 6 or 12 (months of fan run time) are

available. When programmed time has elapsed, a FILTER

indicator is displayed.

Maintenance Reminder

LCD Backlight

Activated for 30 seconds when any button is pressed.

Settings of Off, 6 or 12 (months of chronological time) are

available. When programmed time has elapsed, a mainte-

nance indicator ꢀMAINTENANCE" is displayed.

NOTE − During an electrical storm or similar disturbance,

the backlight may activate for a few seconds. This is nor-

mal and will no longer occur after the electrical disturbance

has passed.

Service Reminder

The SERVICE indicator is displayed only under the follow-

ing conditions:

Thermostat Operating Conditions

35°F to 105°F, 5% to 90% RH

if the thermostat Y1 terminal has been activated with

24VAC for at least 5 minutes, AND the L terminal is

shorted to the R terminal;

Thermostat Storage Conditions

−40°F to 185°F, 5% to 95% RH

Page 9

51M39 SERIES 5/2

OPERATION

MANUAL

E2005 Lennox Industries Inc.

Dallas, Texas, USA

51M42 Merit^®Series

7−Day Programmable Thermostat

CONTROLS

505,069M

07/05

Litho U.S.A.

Table of Contents

51M42 Series Thermostat . . . . . . . . . . . . . . . . . . . . . .

General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Initial Thermostat Power-up . . . . . . . . . . . . . . . . . . . . .

Buttons, Backlight, Timers & Settings . . . . . . . . . . . .

DAY/TIME − Set Day and Time . . . . . . . . . . . . . . . . . .

HEAT − Heat Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . .

COOL − Cool Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . .

AUTO − Using the Autochangeover Mode . . . . . . . . .

HOLD − Temperature Hold Mode . . . . . . . . . . . . . . . .

PROG − Thermostat Programming . . . . . . . . . . . . . . .

COPY − Copy a Program . . . . . . . . . . . . . . . . . . . . . . .

FAN − Controlling Fan Operation . . . . . . . . . . . . . . . . .

Displaying Outside Temperature . . . . . . . . . . . . . . . . .

SETTINGS − Filter and Maintenance Reminders . . .

SETTINGS − Balance Point . . . . . . . . . . . . . . . . . . . . .

Service Indicator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Thermostat RESET . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Removing/Installing Thermostat . . . . . . . . . . . . . . . . .

Default Thermostat Settings . . . . . . . . . . . . . . . . . . . .

Technical Specifications . . . . . . . . . . . . . . . . . . . . . . . .

Thermostat Output Table . . . . . . . . . . . . . . . . . . . . . . .

RETAIN THESE INSTRUCTIONS

FOR FUTURE REFERENCE

51M42 Merit^®Series Thermostat

The Lennox Merit Series 7−day programmable electronic

thermostat 51M42 provides excellent temperature control

and a large, easy-to-read display. It includes a

programmable filter change reminder, an equipment

maintenance reminder, and a system check indicator to

notify the user when the equipment requires service.

Thermostat 51M42 is suitable for heat pump, three-stage

heat/two-stage cool applications using a gas or electric

auxiliary heat source. In addition to displaying outdoor

temperature, the outdoor temperature sensor provides

auxiliary heat lockout, balance point operation, and dual−

fuel compatibility. Also, the thermostat provides auto-

changeover capability.

General

Initial Power−Up Display

Copy tofrom MO TU WE TH FR SA SU

These instructions are intended as a general guide and do

not supersede local codes in any way. Consult authorities

having jurisdiction before installation.

PROG

A B C D

I8:88

HOLD

Check equipment for shipping damage. If you find any

damage, immediately contact the last carrier.

FAN ON

AUTO

COOL SETPOINT

FILTER

SERVICE

COOL

OFF

MAINTENANCE

Introduction

AUTO

EMG.

HEAT

OUTROOM

i88^°

HEAT SETPOINT

This document describes the operation of Lennox thermostat

51M42. Refer to the installation manual for instructions re-

garding installation and wiring of the thermostat.

Initial Thermostat Power-up

Figure 1

When power is initially applied to the thermostat, the dis-

play will appear as shown in figure 1.

All display segments are momentarily activated. This oc-

curs as a normal part of thermostat initialization.

07/05

505,069M

Page 1

*2P0705*

*P505069M*

Within a few seconds, the HOME screen appears (see fig-

ure 2) with default settings as shown. After about 1 minute

of initialization time, the actual room temperature will be

displayed.

Thermostat Buttons

Heat Cool

Prog Auto

Copy Day/Time

Fan Hold

NOTE − Tables 2 and 3 on page 8 show all system and

programming defaults.

HOME Screen

(DOOR

OPEN)

PROG

A B C D

HOLD

Settings Enter

(Outside)

PM I0:26

FAN ON

AUTO

COOL

Figure 3

OFF

ROOM

AUTO

EMG.

70^°

HEAT

When an Arrow, HOLD, HEAT, or COOL button is pressed,

SETPOINT and the temperature setting appears for 15

seconds. If desired, start making changes within 15 sec-

onds or the HOME screen will return. The backlight will

turn off 15 seconds after the HOME screen reappears.

Figure 2

At this point, the thermostat will be fully functional; its de-

fault temperature setpoint (not shown) is 70°F. At this

point, if the equipment has been fully powered and if a heat

demand were present, the system would begin operating.

DAY/TIME − Setting the Day and Time

NOTE − Temperature scale default is Fahrenheit units but

may be reset to show Celsius, if desired. See page 9.

Press the DAY/TIME button and set the CURRENT hour,

minute, and day of week as follows:

Buttons, Backlight, Timers & Settings

1. ꢀAM12" will flash on the screen. Press the Up/Down ar-

rows buttons to change the hour. (ꢀAM" or ꢀPM" must

correspond to time of day.) Press DAY/TIME OR, if ad-

justing for daylight savings time, pressing the ENTER

button stores the single change and exits to the HOME

screen, bypassing minutes and day of week.

Buttons are located behind the small door on the right−

hand side of the thermostat (see figure 3).

IMPORTANT

Do NOT begin pressing buttons until after you read

the following section describing each button.

2. Minutes will flash. Use the Up/Down arrow buttons to

display the minutes past the hour. Press DAY/TIME.

3. Day ꢀMO" (Monday) will flash. Use Up/Down arrow

buttons to display the current day. Day selections are

abbreviated as ꢀMOꢀ, ꢀTU", ꢀWE", ꢀTH", ꢀFR", ꢀSA",

and ꢀSU". Press DAY/TIME.

A pale blue display backlight illuminates for 30 seconds

each time any button is pressed.

When PROG or DAY/TIME button is pressed, a field be-

gins flashing, expecting another input. Start making

changes within 15 seconds or the HOME screen will re-

turn.

4. The HOME screen reappears; confirm day and time

are correct. This completes day and time setting.

Page 2

505069M 07/05

HEAT − Using the Heat Mode

Heat Mode Disabled

ROOM

OFF

Normal Heat Mode

HEAT SETPOINT

75^°

In normal heat mode, both the heat pump and the backup

heat source are used to provide heat. If the thermostat de-

tects that the heat pump is not able to provide enough heat

(as may be the case in very cold weather), then the backup

heat source is activated.

75^°

Figure 6

Heating Demand

Emergency Heat Mode

The thermostat must be in either normal or emergency

heat mode in order to properly control the heating equip-

ment. In either heat mode, when the actual temperature is

lower than the temperature setpoint (as shown in figure 7),

the thermostat detects a heating demand and activates the

heating equipment to satisfy the demand. Heating opera-

tion is indicated by a flame icon in bottom right−hand corner of

the display.

In emergency heat mode, only the backup heat source

provides heat ꢁthe heat pump is disabled. The backup

heat source is activated only when there is a heat demand.

Enabling Normal Heat Mode

Use the HEAT button to select normal heat mode, emer-

gency heat mode, or to disable heat modes as desired. If

the thermostat is in off or cool mode, pressing the HEAT

button enables heat mode. This is indicated by HEAT in the

lower right corner (see figure 4).

Heating Demand

ROOM

Normal Heat Mode Enabled

HEAT SETPOINT

72^°

HEAT

75^°

Heat Cool

Prog Auto

Figure 7

5:30

When the actual temperature rises above the temperature

setpoint, the flame icon will disappear. This indicates that

the heating demand has been satisfied and that the heat-

ing equipment has been turned off.

Copy Day/Time

Fan Hold

(DOOR

OPEN)

Settings Enter

(Outside)

Heat pump operation is locked out for 5 minutes after a de-

mand has been satisfied. If another heat pump demand

occurs during this 5−minute interval, the flame icon will

flash; however, the heat pump will not run until the 5−min-

ute delay has elapsed.

Figure 4

Enabling Emergency Heat Mode

NOTE − The heat pump is activated for at least 4 minutes if

no buttons are pressed during the demand interval. The

backup heat source is activated for at least 3 minutes if no

buttons are pressed during the demand interval.

If the thermostat is in heat mode, pressing the HEAT button

enables emergency heat mode (see figure 5). If the ther-

mostat is in normal heat mode when the HEAT button is

pressed, then emergency heat mode is enabled.

If your system supports 3−stage heating (as does the

51M42 thermostat), you may notice various heating levels

being delivered during a demand.

Emergency Heat Mode Enabled

ROOM

If a small heat pump heating demand is present, "Lo" is dis-

played in the SYSTEM box; if a large heat pump heating

demand is present, "Hi" is displayed (see figure 8).

HEAT SETPOINT

EMG.

HEAT

62^°

75^°

Large Heat Heating Demand

Figure 5

ROOM

65^°

Disabling Heat Mode

HEAT SETPOINT

HEAT

If the thermostat is in emergency heat mode when the

HEAT button is pressed, then heat modes are disabled.

This is indicated by OFF near the right side of the display

as shown in figure 6.

75^°

Figure 8

Page 3

51M42 SERIES 5/2

NOTE − If no buttons are pressed during a demand for

cooling, the equipment must operate for at least 4 minutes.

After a demand has been satisfied, cooling equipment op-

eration is locked out for 5 minutes. If another cooling de-

mand occurs during this 5−minute interval, ꢀCOOL" and the

snowflakes will flash; however, the cooling equipment will

not operate until the 5−minute delay has elapsed.

COOL − Using the Cool Mode

Enabling and Disabling Cool Mode

Use the COOL button to enable or disable cool mode as

desired. If the thermostat is in heat or off mode, cool mode

is enabled when the COOL button is pressed. This is indi-

cated by COOL on the right side of the display (see figure

9).

AUTO − Using the Autochangeover Mode

When in either heat mode or cool mode, autochangeover

can be enabled or disabled by pressing the AUTO button. If

enabled, AUTO appears at the lower right corner of the dis-

play. The thermostat will then automatically change over

from heating to cooling and vice versa, to keep the room

temperature in between the heating and cooling setpoints.

Autochangeover can be used in either of the thermostat

hold modes, or when the thermostat program is running.

These modes are described later.

Turn Cool ON/OFF

Heat Cool

Prog Auto

i0:30

Copy Day/Time

Fan Hold

(DOOR

OPEN)

HOLD − Using Temperature Hold Modes

Settings Enter

(Outside)

When the HOLD button is displayed at the HOME screen,

the thermostat is in a temperature hold condition. This

means that the temperature program data is ignored and

the thermostat functions much like a non−programmable

thermostat.

Figure 9

If the thermostat is in cool mode, pressing the COOL but-

ton disables cool mode (indicated by OFF on the right side

of the display).

Adjusting Temperature Setpoint in Hold Mode

The temperature setpoint represents the desired tempera-

ture of the space around the thermostat. The default heat

setpoint in hold mode is 70°F; the default cool setpoint in

hold mode is 78°F.

Cooling Demand

Set the thermostat to cool mode to control the cooling

equipment. Then, if the room temperature is higher than

the temperature setpoint, as shown in figure 10, the ther-

mostat detects a cooling demand and will activate the cool-

ing equipment to satisfy the demand.

To adjust the setpoint, press the UP or DOWN (YB) arrow

buttons (see figure 11); the existing setpoint is displayed to

the right of the actual room temperature. Each button

press adjusts the setpoint up or down by 1 degree.

Hold Temperature Mode

Cooling Demand

COOL SETPOINT

Heat Cool

FAN

AUTO

76^°

Prog Auto

COOL

i0:30

Copy Day/Time

ROOM

80^°

HEAT SETPOINT

Fan Hold

(DOOR

OPEN)

68^°

Settings Enter

(Outside)

Figure 10

Cooling operation is indicated by flashing ꢀsnowflake"

icons. When the actual temperature drops below the tem-

perature setpoint, the snowflake icons will disappear. This

indicates that the cooling demand has been satisfied and

that the cooling equipment has been turned off.

Figure 11

After the desired setpoint is reached, the HOME screen

will reappear after about 15 seconds.

Permanent Hold Mode

If your system supports 2−stage cooling (as does the

51M42 thermostat), you may notice various cooling levels

being delivered during a demand. Also, if a small cooling

demand is present, "Lo" will be displayed in the SYSTEM

box. However, if a large cooling demand is present, "Hi" will

be displayed in the lower right side of the display (shown in

figure 10).

At any time the program is running, from the HOME

screen, set a permanent hold (program override) by press-

ing the HOLD button (see figure 11). The thermostat now

functions much like a non−programmable thermostat. Use

the Up/Down arrow buttons to adjust the hold setpoint. To

return to the program, press the HOLD button again.

Page 4

505069M 07/05

Temporary (2-Hour) Hold

Program Example

At any time the program is running, from the HOME

screen, set a temporary 2-hour hold by pressing the Up/

Down arrow buttons until the desired setpoint is displayed;

ꢀHOLD" flashes (see figure 12). This overrides the pro-

gram for 2 hours from the last button press, then returns to

the program.

MONDAY PROGRAM: 72°

68°

64°

TUESDAY PROGRAM: 72°

68°

64°

WEDNESDAY PROGRAM: 72°

68°

64°

Temporary Hold Temperature Mode

Heat Cool

THURSDAY PROGRAM: 72°

68°

64°

Prog Auto

i0:30

FRIDAY PROGRAM: 72°

Copy Day/Time

68°

64°

Fan Hold

(DOOR

OPEN)

SATURDAY PROGRAM: 72°

68°

64°

Settings Enter

(Outside)

SUNDAY PROGRAM: 72°

68°

64°

Figure 12

6AM

NOON

6PM

MID

NIGHT

6AM

While in Temporary Hold, press the HOLD button once to

switch to Permanent Hold (HOLD displays solid; PROG

not displayed); press the HOLD button again to return to

the program (PROG displays; HOLD not displayed).

Figure 13

5. Repeat steps 1 − 4 for Monday, periods B, C, and D.

6. COPY A PROGRAM − If the program entered for

Monday is satisfactory for other days of the week, do

the following to repeat the same program for other

days:

PROG − Thermostat Programming

The 51M42 thermostat can be programmed to perform a

set of both heating and cooling events for each day of the

week. Each day can be programmed for 4 unique events

per day, and each day can be different from any other day.

Press the PROG button, then press the COPY

button. The display clears the screen except for

ꢀCopy from MOꢀ with ꢀMO" flashing. Press the

COPY button again to copy Monday’s program.

The display changes to ꢀCopy to MO". Use the ar-

row buttons to scroll to the desired day where you

want the same program as Monday. Press the

COPY button.

To Set Program Events and Temperatures

Figure 13 gives an example of how up to 7 different pro-

grams can be set. Programs A and C reflect the desired

warmth while the home IS occupied (72°); B allows less

heating while the home is NOT occupied; D reflects a cool

sleeping temperature.

Repeat steps A and B for as many days as you

want the same program.

NOTE − The example in figure 13 shows heat setpoints; a

similar program may be set for cool setpoints.

Copy a Programmed Day to Another Day

1. Press and release the PROG button. "MO" (Monday),

"AM 6:00", period "A", "COOL SETPOINT 78", and

"HEAT SETPOINT 70" are displayed. "AM 6" flashes.

Heat Cool

Copy from MO

Prog Auto

2. Use the Up/Down arrow buttons to select the desired

hour; press the PROG button when the desired hour

is reached; use the Up/Down arrow buttons again to

select desired minute. The selected hour/minute is

when the program will start. Press the PROG button.

Copy Day/Time

Fan Hold

(DOOR

OPEN)

3. Use the Up/Down arrow buttons to select the desired

heat setpoint; press the PROG button.

Settings Enter

4. Use the Up/Down arrow buttons to select the desired

cool setpoint; press the PROG button.

(Outside)

NOTE − Pressing the ENTER button during the following

programming steps, saves and exits to the HOME screen.

Figure 14

To program events and temperatures, perform the follow-

ing steps.

7. Repeat steps 1 − 5 to program different times and tem-

peratures for days not yet programmed.

Page 5

51M42 SERIES 5/2

FAN − Controlling the Fan Operation

SETTINGS − Filter/Maintenance Reminders

Use the FAN button to select either continuous fan mode or

auto fan mode.

The 51M42 thermostat is designed to remind you when the

filter needs changing or when routine maintenance is re-

quired, as (and if) defined, by you. These optional remind-

ers are not enabled until you activate them. To do so, press

the SETTINGS button (shown below the FAN button in fig-

ure 16) once or twice for the desired reminder as shown in

figure 18 and as described in table 1.

To change from continuous to auto fan mode (or vice ver-

sa), press the FAN button. Note whether a fan icon in the

FAN box is present (indicating that the fan is running) or not

(fan not running).

If continuous fan mode is enabled (ON displayed in FAN

box − see figure 15), the fan will run continuously regard-

less of whether the heating or cooling equipment is run-

ning.

Reminder Settings Display

Heat Cool

Using Fan ON

OFF

Prog Auto

i0:30

For FILTER, press SETTINGS

button 1 time

Copy Day/Time

FILTER

For MAINTENANCE, press

SETTINGS button 2 times

Fan Hold

MAINTENANCE

Figure 18

Figure 15

The default setting for the reminders is OFF (disabled).

Press Up/Down arrow buttons to select the desired re-

minder intervals.

If auto fan mode is selected (AUTO displayed in FAN box −

see figure 16), the fan will only run when the heating or

cooling equipment is running.

Table 1

Heat Cool

Using Fan AUTO

Filter and Maintenance Reminders

Prog Auto

i0:30

Buttons to

Use

Available Settings and

How to Use

Reminder

FILTER

Copy Day/Time

Settings

(1st press)

then Arrows

to scroll

Total fan run time expressed in

months (Off, 1, 3, 6, 12); for

example, if fan runs 12 hours a

day, 1 month reminder displays

in 2 calendar months.

Fan Hold

selections

Figure 16

Settings

(2nd press)

then Arrows

to scroll

Elapsed chronological time in

months (Off, 6, 12). Use this,

for example, to remind yourself

when to perform routine checks

or when to call a technician for

perodic preventive

MAINTE-

NANCE

Displaying Outside Temperature

Pressing the ENTER (Outside) button at the HOME

screen displays the outside temperature (see figure 17) for

several seconds, then returns to room temperature.

selections

maintenance.

Check Outside Temperature

Enter

Stores settings.

Fan Hold

NOTE − The HOME screen will reappear about 15 seconds

after the final arrow button press. OR, press the ENTER

button at any time to store any changes and exit to the

HOME screen.

Settings Enter

(Outside)

After either programmed interval has elapsed, the remind-

er will be displayed as shown in figure 19.

Figure 17

Page 6

505069M 07/05

To adjust the balance point up or down, press the Up/Down

arrow buttons to the right of the display screen. The bal-

ance point setting increases by 1°F each time the Up but-

ton is pressed and decreases by 1°F each time the Down

button is pressed. After the desired balance point setting is

reached, the HOME screen will reappear after about 15

seconds.

Reminders

PROG

A B C D

HOLD

PM I0:26

FAN ON

AUTO

Filter &

Maintenance

Indicators

FILTER SERVICE

MAINTENANCE

ROOM

COOL

OFF

NOTE − The balance point feature allows the outdoor tem-

perature to rise or fall 3°F above or below the balance

point. This prevents excessive cycling of the equipment

when the outdoor temperature is near the balance point.

For example, if the balance point is 40°F and the actual

outdoor temperature is 35°F, the outdoor temperature

must rise to 43°F before equipment adjustment occurs.

Conversely, if the balance point is 40°F and the actual out-

door temperature is 45°F, the outdoor temperature must

drop to 37°F before equipment adjustment occurs.

AUTO

EMG.

HEAT

76^°

FILTER SERVICE

MAINTENANCE

ROOM

FAN ON

AUTO

Service

Indicator

Flashing

COOL

OFF

AUTO

EMG.

HEAT

76^°

Service Indicator

When abnormal equipment operation is detected, the

SERVICE indicator will flash on the screen (see figure 19

on page 7). This indicates that the equipment requires

service from a qualified service technician.

Figure 19

After the filter has been changed or maintenance per-

formed, reset the reminder by pressing the SETTINGS

button for 4 seconds at the corresponding reminder set-

tings screen. The screen will blink for a few moments to in-

dicate that the timer has been reset.

Thermostat RESET

Under some abnormal conditions, it may be necessary to

ꢀreset" the thermostat to its default condition. Such a RE-

SET would delete all programming and settings and there-

fore should only be used on rare occasions when the the

thermostat fails to function as designed and/or as pro-

grammed. Such an instance can occur as a result of a pow-

er surge or similar electrical disturbance (e.g. after an elec-

trical storm or power outage). The RESET button can be

used to recover from this situation.

SETTINGS − Balance Point

NOTE − The balance point is only available with the out-

door sensor.

When the outdoor sensor (X2658) is connected to the

51M42 thermostat, balance point adjustment is available.

The balance point feature allows the measured outdoor

temperature to govern operation of the heat pump and

backup heat source.

CAUTION

When the RESET button is pressed, ALL settings re-

vert back to the defaults, including the default pro-

gram (see tables 2 and 3).

Press the SETTINGS button three times to access the Bal-

ance Point Settings screen (see figure 20).

The RESET button is an unlabeled, recessed button lo-

cated behind the door, on the right−hand side of the ther-

mostat, below the SETTINGS button (see figure 3). Use a

paper clip or small pencil to press the RESET button; ALL

thermostat settings will be reset to the defaults listed in the

Default Thermostat Settings section.

Balance Point Setting

SETPOINT

Removing/Installing Thermostat

Figure 20

The thermostat hinges on tabs on the top of the subbase;

no tool is needed to remove the thermostat from the sub-

base. Pivot the bottom of the thermostat outward (releas-

ing the snaps), then lift up to remove.

The balance point can be set at any temperature between

1°F and 55°F (−17°C to 13°C). A balance point setting of

zero disables the balance point function (in this case, the

thermostat behaves as if no outdoor sensor is attached).

The default balance point setting is 55°F.

To replace it, first position the top tilted toward the wall

bracket and align it until you feel the tabs and slots engage;

then, while the top is in place, pivot the bottom toward the

wall until the thermostat snaps into place.

Page 7

51M42 SERIES 5/2

With a VERY LARGE heat demand, temperature is:

below setpoint –2.5F

OR 15−minute 2 upstage timer HAS expired.

Default Thermostat Settings

Default thermostat settings are shown in table 2 and the

default program is shown in table 3.

Emergency heat demand (only if emergency heat is

enabled): temperature is below setpoint –0.5F.

Table 2

Default Thermostat Settings

With a SMALL cool demand, temperature is:

above setpoint +0.5F but below setpoint +1.5F

AND 30-minute upstage timer HAS NOT expired.

Mode

Heat (Perm. Hold)

70°F (or 21°C)

78°F (or 26°C)

Auto

Heat Setpoint (Perm Hold)

Cool Setpoint (Perm Hold)

Fan

With a LARGE cool demand, temperature is:

above setpoint +1.5F

OR 30-minute upstage timer HAS expired.

Filter Reminder

Maintenance Reminder

OFF

Table 4

OFF

THERMOSTAT OUTPUTS

Equipment Protection Timers Reset Back to Zero

Autochangeover OFF

Y1 Y2

Demand Condition

Cooling Demands

Table 3

Default Program Settings − Time & Temp.

SMALL

LARGE

No Demand

Programs

Time

Temp. − Heat Temp. − Cool

All days − A 6:00am 70_F / 21°C

All days − B 8:00am 62_F / 17°C

All days − C 6:00pm 70_F / 21°C

All days − D 10:00pm 62_F / 17°C

78_F / 26°C

85_F / 29°C

78_F / 26°C

82_F / 28°C

Heating Demands

With Electric Backup (no outdoor sensor)

SMALL

LARGE

VERY LARGE

Emergency

No Demand

Thermostat Output Table

Table4 depicts the 51M42 thermostat output states for var-

ious input conditions. The following notes described terms

used in the table.

With Gas Backup (dual fuel; no outdoor sensor)

SMALL

NOTES:

LARGE

S X = output is activated with 24VAC.

S BBP: outdoor temperature is below balance point

S ABP: outdoor temperature is above balance point

VERY LARGE

Emergency

No Demand

With Electric Backup (outdoor sensor attached)

S In all cases, the state of the B terminal is opposite that

SMALL (Above Balance Point)

LARGE (ABP)

of the O terminal

S Data are tabulated for AUTO fan setting. If the fan set-

VERY LARGE (ABP)

SMALL (Below Balance Point)

LARGE (BBP)

ting is ON, the G output is activated in all cases

S Upstage timers:

30 minutes (when upstaging from small to large

demand)

VERY LARGE (BBP)

Emergency

15 minutes (HEAT MODE ONLY − when upstaging

from large to very large demand)

No Demand

With Gas Backup (dual fuel; outdoor sensor attached)

S The temperature ranges expressed in the following

definitions of ꢀSMALL/LARGE/VERY LARGE" de-

mands are for guidance only; actual temperatures

may vary:

SMALL (ABP)

LARGE (ABP)

VERY LARGE (ABP)

SMALL (BBP)

LARGE (BBP)

VERY LARGE (BBP)

Emergency

With a SMALL heat demand, temperature is:

below setpoint –0.5F but above setpoint –1.5F

AND 30-minute upstage timer HAS NOT expired.

With a LARGE heat demand, temperature is:

less than setpoint –1.5F

OR 30-minute upstage timer HAS expired.

No Demand

Page 8

505069M 07/05

Indoor Temperature Measurement Range

Technical Specifications

Measurement Scale: Fahrenheit

Measurement Range: 35°F to 99°F

Thermostat Type

Electronic programmable thermostat for heat pump,

3-stage heat/2-stage cool.

Measurement Resolution: 0.5°F

Measurement Accuracy: +/−1°F

NOTE − 2 stages of heat pump heat and one stage of auxil-

iary (gas or electric) backup heat are supported.

Field Offset: via DIP switches to +/−3°F

Sampling Method: temperature measurements sampled

every 15 seconds. Displayed temperature is the average

of the last four measurements.

Power Supply Range

18VAC − 30VAC (24VAC nominal), 60Hz

Outdoor Temperature Measurement Range

CAUTION

24VAC is present on the terminals of the thermostat

bracket. If removing the thermostat from the wall,

use caution and avoid touching any of the connec-

tor terminals on the wall bracket.

Measurement Scale: Fahrenheit

Measurement Range: −22°F to 122°F

Measurement Resolution: 1°F

Measurement Accuracy: +/−2°F

Also, when working with the thermostat dip

switches, use a non−conductive tool and take cau-

tion to avoid making any contact with the circuit

board, its imprinted circuitry and its connector

prongs.

Temperature Setpoint Range

Setting range: 50°F (10°C) to 90°F (32°C)

Setting resolution: 1°F (1°C)

Smart Setback Recovery (via DIP switch #6)

Smart Setback Recovery (SSR) affects the way the ther-

mostat responds to program events. If SSR is disabled, the

thermostat will react to a program event at the time the

event occurs. However, if SSR is enabled, the thermostat

will react to a program event before the event occurs such

that the desired temperature is reached at the time of the

event, not after.

Temperature Display

Display Scale: Fahrenheit or Celsius user selectable (via

DIP switch; see figure 21)

Display range: 35°F (2°C) to 99°F (37°C)

Display resolution: 1°F (1°C)

Display Accuracy: +/−1°F

If the Fahrenheit/Celsius display must be changed, use a

plastic, non−conductive tool to push the dip switch to the

right position (see figure 21).

Autochangeover Deadband Selection (via DIP

switch #7)

Changing Fahrenheit/Celsius Setting

Autochangeover deadband can be set to 4 or 6 degrees.

When autochangeover is enabled (via the AUTO button),

the thermostat will automatically change over from heating

to cooling and vice versa, to keep the room temperature in

between the heating and cooling setpoints. The deadband

is the minimum difference between the heating and cooling

setpoints.

Fan Control

PLUG IN (Use Care To Avoid Bending Prongs.)

AUTO or ON modes.

I/O Relays

F/C (Fahrenheit/Celsius) switch

shown in Celsius position

All thermostat relays are latching type to minimize power

consumption.

Figure 21

Page 9

51M42 SERIES 5/2

Table 5

Over−Temperature Protection

Thermal-mechanical switch opens W1, Y1, Y2, and E at

93°F+/−6°F.

51M42 Terminal Designations

Term. Description

Filter Reminder

Reversing valve, heat active

Reversing valve, cool active

24VAC

Settings of Off, 1, 3, 6 or 12 (months of fan run time) are

available. When programmed time has elapsed, a FILTER

indicator is displayed.

First-stage cooling/heating, compressor

energized

Maintenance Reminder

Settings of Off, 6 or 12 (months of chronological time) are

available. When programmed time has elapsed, a mainte-

nance indicator ꢀMAINTENANCE" is displayed.

W1*

Auxiliary heating, furnace energized

Second-stage cooling/heating, compressor

energized

Service Reminder

The SERVICE indicator is displayed only under the follow-

ing conditions:

Emergency heat

Fan control

Service Indicator

if the thermostat Y1 terminal has been activated with

24VAC for at least 5 minutes, AND the L terminal is

shorted to the R terminal;

24VAC common

Outdoor temperature sensor connection 1

Outdoor temperature sensor connection 2

if the thermostat Y1 terminal has been activated with

24VAC for at least 5 minutes, AND the L terminal is

shorted to the C terminal.

* For most applications, E will be jumpered to W1. If sep-

arate wires are not provided for both E and W1, jumper

the E terminal to the W1 terminal on the thermostat sub-

base. For applications involving the use of a balance

point (whereby the outdoor temperature is to be used to

restrict either heat pump operation or backup heat

source operation), the outdoor sensor (part number

X2658) MUST be installed.

Power Loss/Recovery

Thermostat memory is retained for a minimum of 24 hours

during a power loss (includes retention of clock setting,

program information, HOLD status, programmed temper-

ature setpoint, heat/cool and fan mode settings, filter re-

minder status, maintenance reminder status, and equip-

ment protection timers). After 24 hours of power loss,

programmed settings will be lost and replaced with default

settings.

Equipment Protection Timers

Minimum Compressor OFF time: 5 minutes

Minimum Compressor ON time: 4 minutes

Minimum Furnace ON time: 3 minutes

IMPORTANT

Power must be applied for at least six consecutive

hours prior to a power loss in order for memory to

be retained for the specified time.

Minimum furnace cycle time (elapsed time between any

furnace activation and the next furnace activation): 6 min-

utes.

Minimum elapsed time between any compressor activa-

tion and the next compressor activation: 6 minutes.

LCD Backlight

Activated for 30 seconds when any button is pressed.

NOTE − All protection timers (except the compressor OFF

timer) can be over−ridden if a heating or cooling demand is

initiated or terminated using the UP, DOWN, HEAT, or

COOL buttons.

NOTE − During an electrical storm or similar disturbance,

the backlight may activate for a few seconds. This is nor-

mal and will no longer occur after the electrical disturbance

has passed.

Equipment Protection Override

Thermostat Operating Conditions

35°F to 105°F, 5% to 90% RH

Both the minimum compressor OFF timer and the mini-

mum equipment cycle timer can be over−ridden by press-

ing and holding either the HEAT or COOL button down for 4

seconds.

Thermostat Storage Conditions

−40°F to 185°F, 5% to 95% RH

Page 10

505069M 07/05

OPERATION

MANUAL

E2005 Lennox Industries Inc.

Dallas, Texas, USA

51M38 Merit^®Series

7 Day Programmable Thermostat

CONTROLS

505,067M

07/05

Litho U.S.A.

Table of Contents

51M38 Series Thermostat . . . . . . . . . . . . . . . . . . . . . .

General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Initial Thermostat Power-up . . . . . . . . . . . . . . . . . . . . .

Buttons, Backlight, Timers & Settings . . . . . . . . . . . .

DAY/TIME − Set Day and Time . . . . . . . . . . . . . . . . . .

HEAT − Heat Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . .

COOL − Cool Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . .

AUTO − Using the Autochangeover Mode . . . . . . . . .

HOLD − Temperature Hold Mode . . . . . . . . . . . . . . . .

PROG − Thermostat Programming . . . . . . . . . . . . . . .

COPY − Copy a Program . . . . . . . . . . . . . . . . . . . . . . .

FAN − Controlling Fan Operation . . . . . . . . . . . . . . . . .

Displaying Outside Temperature . . . . . . . . . . . . . . . . .

SETTINGS − Filter /Maintenance Reminder . . . . . . .

Service Indicator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Thermostat RESET . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Removing/Installing Thermostat . . . . . . . . . . . . . . . . .

Default Thermostat Settings . . . . . . . . . . . . . . . . . . . .

Thermostat Output Table . . . . . . . . . . . . . . . . . . . . . . .

Technical Specifications . . . . . . . . . . . . . . . . . . . . . . . .

RETAIN THESE INSTRUCTIONS

FOR FUTURE REFERENCE

51M38 Merit^®Series Thermostat

The Lennox Merit Series 7 day programmable electronic

thermostat 51M38 provides excellent temperature control

and a large, easy-to-read display. This product includes a

programmable filter change reminder, an equipment

maintenance reminder, and a system check indicator to

notify the user when the equipment requires service.

Thermostat 51M38 is suitable for non−heat pump, single-

stage heat/single-stage cool applications with either gas or

electric furnace. Also, the thermostat provides auto-

changeover capability.

General

Initial Power−Up Display

Copy tofrom MO TU WE TH FR SA SU

These instructions are intended as a general guide and do

not supersede local codes in any way. Consult authorities

having jurisdiction before installation.

PROG

A B C D

I8:88

HOLD

Check equipment for shipping damage. If you find any

damage, immediately contact the last carrier.

FAN ON

AUTO

COOL SETPOINT

FILTER

SERVICE

COOL

OFF

MAINTENANCE

Introduction

AUTO

EMG.

HEAT

OUTROOM

i88^°

HEAT SETPOINT

This document describes the operation of Lennox thermo-

stat 51M38. Refer to the installation manual for instructions

regarding installation and wiring of the thermostat.

Initial Thermostat Power-up

Figure 1

When power is initially applied to the thermostat, the dis-

play will appear as shown in figure 1.

All display segments are momentarily activated. This oc-

curs as a normal part of thermostat initialization.

07/05

505,067M

Page 1

*2P0705*

*P505067M*

Within a few seconds, the HOME screen appears (see fig-

ure 2) with default settings as shown. After about 1 minute

of initialization time, the actual room temperature will be

displayed.

Thermostat Buttons

Heat Cool

Prog Auto

Copy Day/Time

Fan Hold

HOME Screen

PROG

A B C D

HOLD

(DOOR

OPEN)

PM I0:26

Settings Enter

(Outside)

FAN ON

AUTO

COOL

OFF

ROOM

AUTO

Figure 3

EMG.

70^°

HEAT

When an Arrow, HOLD, HEAT, or COOL button is pressed,

SETPOINT and the temperature setting appears for 15

seconds. If desired, start making changes within 15 sec-

onds or the HOME screen will return. The backlight will

turn off 15 seconds after the HOME screen reappears.

Figure 2

At this point, the thermostat will be fully functional; its de-

fault temperature setpoint (not shown) is 70°F. At this

point, if the equipment has been fully powered and if a heat

demand were present, the system would begin operating.

DAY/TIME − Setting the Day and Time

NOTE − Temperature scale default is Fahrenheit units but

may be reset to show Celsius, if desired. See page 7.

Press the DAY/TIME button and set the CURRENT hour,

minute, and day of week as follows:

1. ꢀAM12" will flash on the screen. Press the UP/DOWN

arrows to change the hour. (ꢀAM" or ꢀPM" must corre-

spond to time of day.) Press the DAY/TIME button OR,

if adjusting for daylight savings time, pressing the EN-

TER button stores the single change and exits to the

HOME screen, bypassing minutes and day of week.

Buttons, Backlight, Timers & Settings

Buttons are located behind the small door on the right−

hand side of the thermostat (see figure 3).

IMPORTANT

Do NOT begin pressing buttons until after you read

the following section describing each button.

2. Minutes will flash. Use the UP/DOWN arrow button to

display the minutes past the hour. Press the DAY/

TIME button.

3. Day ꢀMO" (Monday) will flash. Use UP or DOWN arrow

buttons to display the current day. Day selections are

abbreviated as ꢀMOꢀ, ꢀTU", ꢀWE", ꢀTH", ꢀFR", ꢀSA",

and ꢀSU". Press the DAY/TIME button.

A pale blue display backlight illuminates for 30 seconds

each time any button is pressed.

When the PROG or DAY/TIME button is pressed, a field

begins flashing, expecting another input. Start making

changes within 15 seconds or the HOME screen will re-

turn.

4. The HOME screen reappears; confirm day and time

are correct. This completes day and time setting.

Page 2

505067M 7/13/05

HEAT − Using the Heat Mode

COOL − Using the Cool Mode

Enabling and Disabling Cool Mode

Use the COOL button to enable or disable cool mode as

desired. If the thermostat is in heat or off mode, cool mode

is enabled when the COOL button is pressed. This is indi-

cated by COOL on the right side of the display (see figure

7).

Enabling and Disabling Heat Mode

The thermostat must be in the heat mode to control the

heating equipment. Press the HEAT button to enable or

disable heat mode. If the thermostat is in cool or off mode,

pressing the HEAT button enables the heat mode (indi-

cated by HEAT in the lower right corner − see figure 4).

Turn Cool ON/OFF

Heat Cool

Turn Heat ON/OFF

Heat Cool

Prog Auto

i0:30

Prog Auto

Copy Day/Time

5:30

Copy Day/Time

Fan Hold

(DOOR

OPEN)

Fan Hold

(DOOR

OPEN)

Settings Enter

(Outside)

Figure 7

If the thermostat is in cool mode, pressing the COOL but-

ton disables cool mode (indicated by OFF, similar to figure

5).

Figure 4

If the thermostat is in heat mode, heat mode is disabled

when the HEAT button is pressed. This is indicated by OFF

near the right side of the display as shown in figure 5.

Cooling Demand

Set the thermostat to cool mode to control the cooling

equipment. Then, if the room temperature is higher than

the temperature setpoint, as shown in figure 7, the thermo-

stat detects a cooling demand and will activate the cooling

equipment to satisfy the demand.

Heat/Cool Mode Disabled

OFF

ROOM

75^°

HEAT SETPOINT

Cooling operation is indicated by flashing ꢀsnowflake"

icons in the display. When the actual temperature drops

below the temperature setpoint, the snowflake icons will

disappear. This indicates that the cooling demand has

been satisfied and that the cooling equipment has been

turned off.

75^°

Figure 5

Heating Demand

NOTE − If no buttons are pressed during a demand for

cooling, the equipment must operate for at least 4 minutes.

After a demand has been satisfied, cooling equipment op-

eration is locked out for 5 minutes. If another cooling de-

mand occurs during this 5−minute interval, ꢀCOOL" and the

snowflakes will flash; however, the cooling equipment will

not operate until the 5−minute delay has elapsed.

Set the thermostat to heat mode to control the heating

equipment. Then, if the room temperature is lower than the

temperature setpoint, as shown in figure 6, the thermostat

detects a heating demand and will activate the heating

equipment to satisfy the demand.

Heating Demand

NOTE − The cooling setpoint cannot be lower than the

heating setpoint. Also, the difference between the cooling

and heating setpoint must be at least 4 degrees.

ROOM

72^°

HEAT SETPOINT

HEAT

75^°

AUTO − Using the Autochangeover Mode

When in either heat mode or cool mode, autochangeover

can be enabled or disabled by pressing the AUTO button. If

enabled, AUTO appears at the lower right corner of the dis-

play. The thermostat will then automatically change over

from heating to cooling and vice versa, to keep the room

temperature in between the heating and cooling setpoints.

Autochangeover can be used in either of the thermostat

hold modes, or when the thermostat program is running.

These modes are described later.

Figure 6

Heating operation is indicated by a flame icon in the lower

right corner. When the actual temperature rises above the

temperature setpoint, the flame icon will disappear. This

indicates that the heating demand has been satisfied and

that the heating equipment has been turned off.

NOTE − Heating equipment is activated for at least 3 min-

utes if no buttons are pressed during the demand interval.

Page 3

51M38 SERIES 5/2

Temporary Hold Temperature Mode

HOLD − Using Temperature Hold Modes

Heat Cool

When HOLD is displayed at the HOME screen, the ther-

mostat is in a temperature hold condition. This means that

the temperature program data is ignored and the thermo-

stat functions much like a non−programmable thermostat.

Prog Auto

i0:30

Copy Day/Time

Adjusting Temperature Setpoint in Hold Mode

Fan Hold

(DOOR

OPEN)

The temperature setpoint represents the desired tempera-

ture of the space around the thermostat. The default heat

setpoint in hold mode is 70°F; the default cool setpoint in

hold mode is 78°F.

Settings Enter

(Outside)

To adjust the setpoint, press the UP or DOWN (YB) arrow

buttons (see figure 8); the existing setpoint is displayed to

the right of the actual room temperature. Each button

press adjusts the setpoint up or down by 1 degree.

Figure 9

PROG − Thermostat Programming

The 51M38 thermostat can be programmed to perform a

set of both heating and cooling events for each day of the

week. Each day can be programmed for 4 unique events

per day, and each day can be different from any other day.

Hold Temperature Mode

To Set Program Events and Temperatures

Heat Cool

Figure 10 gives an example of how up to 7 different pro-

grams can be set. Programs A and C reflect the desired

warmth while the home IS occupied (72°); B allows less

heating while the home is NOT occupied; D reflects a cool

sleeping temperature.

Prog Auto

i0:30

Copy Day/Time

NOTE − The example in figure 10 shows heat setpoints; a

similar program may be set for cool setpoints.

Fan Hold

(DOOR

OPEN)

Settings Enter

(Outside)

Program Example

MONDAY PROGRAM: 72°

68°

64°

Figure 8

TUESDAY PROGRAM: 72°

68°

64°

After the desired setpoint is reached, the HOME screen

will reappear after about 15 seconds.

WEDNESDAY PROGRAM: 72°

68°

64°

Permanent Hold Mode

THURSDAY PROGRAM: 72°

At any time the program is running, from the HOME

screen, set a permanent hold (program override) by press-

ing the HOLD button (see figure 8). The thermostat now

functions much like a non−programmable thermostat. Use

the Up/Down arrow buttons to adjust the hold setpoint. To

return to the program, press HOLD again.

68°

64°

FRIDAY PROGRAM: 72°

68°

64°

SATURDAY PROGRAM: 72°

68°

64°

Temporary (2-Hour) Hold

At any time the program is running, from the HOME

screen, set a temporary 2-hour hold by pressing the Up/

Down arrow buttons until the desired setpoint is displayed;

ꢀHOLD" flashes (see figure 9). This overrides the program

for 2 hours from the last button press, then returns to the

program.

SUNDAY PROGRAM: 72°

68°

64°

6AM

NOON

6PM

MID

NIGHT

6AM

Figure 10

While in Temporary Hold, press HOLD once to switch to

Permanent Hold (HOLD displays solid; PROG not dis-

played); press HOLD again to return to the program

(PROG displays; HOLD not displayed).

NOTE − Pressing the ENTER button during the following

programming steps, saves and exits to the HOME screen.

To program events and temperatures, perform the follow-

ing steps.

Page 4

505067M 7/13/05

1. Press and release the PROG button. "MO" (Monday),

"AM 6:00", period "A", "COOL SETPOINT 78", and

"HEAT SETPOINT 70" are displayed. "AM 6" flashes.

Heat Cool

Prog Auto

Using Fan ON

2. Use the Up/Down arrow buttons to select the desired

hour; press the PROG button when the desired hour

is reached; use the Up/Down arrow buttons again to

select desired minute. The selected hour/minute is

when the program will start. Press the PROG button.

i0:30

Copy Day/Time

Fan Hold

3. Use the Up/Down arrow buttons to select the desired

heat setpoint; press the PROG button.

Figure 12

4. Use the Up/Down arrow buttons to select the desired

cool setpoint; press the PROG button.

If auto fan mode is selected (AUTO displayed in FAN box −

see figure 13), the fan will only run when the heating or

cooling equipment is running.

5. Repeat steps 1 − 4 for Monday, periods B, C, and D.

6. COPY A PROGRAM − If the program entered for

Monday is satisfactory for other days of the week, do

the following to repeat the same program for other

days:

Heat Cool

Using Fan Auto

Prog Auto

i0:30

Press the PROG button, then press the COPY

button. The display clears the screen except for

ꢀCopy from MOꢀ with ꢀMO" flashing. Press the

COPY button again to copy Monday’s program.

The display changes to ꢀCopy to MO". Use the ar-

row buttons to scroll to the desired day where you

want the same program as Monday. Press the

COPY button.

Copy Day/Time

Fan Hold

Figure 13

Displaying Outside Temperature

Repeat steps A and B for as many days as you

want the same program.

Pressing the ENTER (Outside) button at the HOME

screen displays the outside temperature (see figure 14) for

several seconds, then returns to room temperature.

Copy a Programmed Day to Another Day

Check Outside Temperature

Heat Cool

Prog Auto

Copy Day/Time

Fan Hold

Copy from MO

Fan Hold

Settings Enter

(Outside)

(DOOR

OPEN)

Figure 14

Settings Enter

(Outside)

SETTINGS − Filter/Maintenance Reminders

The 51M38 thermostat is designed to remind you when the

filter needs changing or when routine maintenance is re-

quired, as (and if) defined, by you. These optional remind-

ers are not enabled until you activate them. To do so, press

the SETTINGS button once or twice for the desired re-

minder as shown in figure 15 and as described in table 1.

Figure 11

7. Repeat steps 1 − 5 to program different times and tem-

peratures for days not yet programmed.

FAN − Controlling the Fan Operation

Reminder Settings Display

Use the FAN button to select either continuous fan mode or

auto fan mode.

To change from continuous to auto fan mode (or vice ver-

sa), press the FAN button. Note whether a fan icon in the

FAN box is present (indicating that the fan is running) or not

(fan not running).

OFF

For FILTER, press SETTINGS

button 1 time

If continuous fan mode is enabled (ON displayed in FAN

box − see figure 12), the fan will run continuously regard-

less of whether the heating or cooling equipment is run-

ning.

FILTER

For MAINTENANCE, press

SETTINGS button 2 times

MAINTENANCE

Figure 15

Page 5

51M38 SERIES 5/2

The default setting for the reminders is OFF (disabled).

Press Up/Down arrow buttons to select the desired re-

minder intervals.

Service Indicator

When abnormal equipment operation is detected, the

SERVICE indicator will flash on the screen (see figure 16).

This indicates that the equipment requires service from a

qualified service technician.

Table 1

Filter and Maintenance Reminders

Buttons to

Use

Available Settings and

How to Use

Thermostat RESET

Reminder

FILTER

Under some abnormal conditions, it may be necessary to

ꢀreset" the thermostat to its default condition. Such a RE-

SET would delete all programming and settings and there-

fore should only be used on rare occasions when the the

thermostat fails to function as designed and/or as pro-

grammed. Such an instance can occur as a result of a pow-

er surge or similar electrical disturbance (e.g. after an elec-

trical storm or power outage). The RESET button can be

used to recover from this situation.

SETTINGS

(1st press)

then Arrows

to scroll

Total fan run time expressed in

months (Off, 1, 3, 6, 12); for

example, if fan runs 12 hours a

day, 1 month reminder displays

in 2 calendar months.

selections

SETTINGS

(2nd press)

then Arrows

to scroll

Elapsed chronological time in

months (Off, 6, 12). Use this,

for example, to remind yourself

when to perform routine checks

or when to call a technician for

perodic preventive

MAINTE-

NANCE

selections

CAUTION

When the RESET button is pressed, ALL settings re-

vert back to the defaults, including the default pro-

gram (see tables 2 and 3).

maintenance.

Enter

Stores settings.

NOTE − The HOME screen will reappear about 15 seconds

after the final arrow button press. OR, press the ENTER

button at any time to store any changes and exit to the

HOME screen.

The RESET button is an unlabeled, recessed button lo-

cated behind the door on the right−hand side of the thermo-

stat, below the SETTINGS button (see figure 3). Use a pa-

per clip or small pencil to press the RESET button; ALL

thermostat settings will be reset to the defaults listed in the

Default Thermostat Settings section.

After either programmed interval has elapsed, the remind-

er will be displayed as shown in figure 16.

Reminders

Removing/Installing Thermostat

PROG

A B C D

HOLD

PM I0:26

The thermostat hinges on tabs on the top of the subbase;

no tool is needed to remove the thermostat from the sub-

base. Pivot the bottom of the thermostat outward (releas-

ing the snaps), then lift up to remove.

FAN ON

AUTO

FILTER SERVICE

MAINTENANCE

ROOM

Filter &

Maintenance

Indicators

COOL

OFF

To replace it, first position the top tilted toward the wall

bracket and align it until you feel the tabs and slots engage;

then, while the top is in place, pivot the bottom toward the

wall until the thermostat snaps into place.

AUTO

EMG.

HEAT

76^°

Default Thermostat Settings

FAN ON

AUTO

Default thermostat settings are in table 2 and the default

program is shown in table 3.

FILTER SERVICE

MAINTENANCE

ROOM

Service

Indicator

Flashing

COOL

OFF

Table 2

AUTO

EMG.

HEAT

Default Thermostat Settings

76^°

Mode

Heat (Perm. Hold)

70°F (or 21°C)

78°F (or 26°C)

Auto

Heat Setpoint (Perm Hold)

Cool Setpoint (Perm Hold)

Fan

Figure 16

Filter Reminder

Maintenance Reminder

OFF

After the filter has been changed or maintenance per-

formed, reset the reminder by pressing the SETTINGS

button for 4 seconds at the corresponding reminder set-

tings screen. The screen will blink for a few moments to in-

dicate that the timer has been reset.

OFF

Equipment Protection Timers Reset Back to Zero

Autochangeover OFF

Page 6

505067M 7/13/05

Table 3

CAUTION

24VAC is present on the terminals of the thermostat

bracket. If removing the thermostat from the wall,

use caution and avoid touching any of the connec-

tor terminals on the wall bracket.

Default Program Settings (Time & Temp.)

Programs

Time

Temp. − Heat Temp. Cool

All days − A 6:00am 70_F / 21°C

All days − B 8:00am 62_F / 17°C

All days − C 6:00pm 70_F / 21°C

All days − D 10:00pm 62_F / 17°C

78_F / 26°C

85_F / 29°C

78_F / 26°C

82_F / 28°C

Also, when working with the thermostat dip

switches, use a non−conductive tool and take cau-

tion to avoid making any contact with the circuit

board, its imprinted circuitry and its connector

prongs.

Temperature Display

Display Scale: Fahrenheit or Celsius user selectable (via

DIP switch; see figure 17)

Thermostat Output Table

Display range: 35°F (2°C) to 99°F (37°C)

Display resolution: 1°F (1°C)

Display Accuracy: +/−1°F

Table4 depicts the 51M38 thermostat output states for var-

ious input conditions.

If the Fahrenheit/Celsius display must be changed, use a

plastic, non−conductive tool to push the dip switch to the

right position (see figure 17).

Table 4

Thermostat Outputs

Condition

Changing Fahrenheit/Celsius Setting

Gas Heat, Auto Fan

Heat Demand

Cool Demand

No Demand

Gas Heat, Continuous Fan

Heat Demand

Cool Demand

No Demand

PLUG IN (Use Care To Avoid Bending Prongs.)

Electric Heat, Auto Fan

Heat Demand

F/C (Fahrenheit/Celsius) switch

shown in Celsius position

Cool Demand

No Demand

Figure 17

Electric Heat, Continuous Fan

Indoor Temperature Measurement Range

Measurement Scale: Fahrenheit

Measurement Range: 35°F to 99°F

Measurement Resolution: 0.5°F

Measurement Accuracy: +/−1°F

Heat Demand

Cool Demand

No Demand

NOTE − X = output is activated with 24VAC.

Field Offset: via DIP switches to +/−3°F

Sampling Method: temperature measurements sampled

every 15 seconds. Displayed temperature is the average

of the last four measurements.

Technical Specifications

Thermostat Type

Outdoor Temperature Measurement Range

Measurement Scale: Fahrenheit

Measurement Range: −22°F to 122°F

Measurement Resolution: 1°F

Measurement Accuracy: +/−2°F

Electronic programmable thermostat for 1-Stage

Heat/1-Stage Cool, gas or electric heat, non−heat pump,

non−power robbing applications.

Temperature Setpoint Range

Setting range: 50°F (10°C) to 90°F (32°C)

Setting resolution: 1°F (1°C)

Power Supply Range

18VAC − 30VAC (24VAC nominal), 60Hz

Page 7

51M38 SERIES 5/2

Smart Setback Recovery (via DIP switch #6)

Equipment Protection Override

Both the minimum compressor OFF timer and the mini-

mum equipment cycle timer can be over−ridden by press-

ing and holding either the HEAT or COOL button down for 4

seconds.

Smart Setback Recovery (SSR) affects the way the ther-

mostat responds to program events. If SSR is disabled, the

thermostat will react to a program event at the time the

event occurs. However, if SSR is enabled, the thermostat

will react to a program event before the event occurs such

that the desired temperature is reached at the time of the

event, not after.

Over−Temperature Protection

Thermal-mechanical switch opens W1 at 93°F+/−6°F.

Filter Reminder

Autochangeover Deadband Selection (via DIP

switch #7)

Settings of Off, 1, 3, 6 or 12 (months of fan run time) are

available. When programmed time has elapsed, a FILTER

indicator is displayed.

Autochangeover deadband can be set to 4 or 6 degrees.

When autochangeover is enabled (via the AUTO button),

the thermostat will automatically change over from heating

to cooling and vice versa, to keep the room temperature in

between the heating and cooling setpoints. The deadband

is the minimum difference between the heating and cooling

setpoints.

Maintenance Reminder

Settings of Off, 6 or 12 (months of chronological time) are

available. When programmed time has elapsed, a MAIN-

TENANCE indicator is displayed.

Service Reminder

The SERVICE indicator is displayed only under the follow-

ing conditions:

Fan Control

AUTO or ON modes, gas or electric heat compatible via

DIP switches (also see Thermostat Output section).

if the thermostat Y1 terminal has been activated with

24VAC for at least 5 minutes, AND the L terminal is

shorted to the R terminal;

I/O Relays

All thermostat relays are latching type to minimize power

consumption.

if the thermostat Y1 terminal has been activated with

24VAC for at least 5 minutes, AND the L terminal is

shorted to the C terminal.

Table 5

51M38 Terminal Designations

Terminal Description

Power Loss/Recovery

Thermostat memory is retained for a minimum of 24 hours

during a power loss (includes retention of clock setting,

program information, HOLD status, programmed temper-

ature setpoint, heat/cool and fan mode settings, filter re-

minder status, maintenance reminder status, and equip-

ment protection timers). After 24 hours of power loss,

programmed settings will be lost and replaced with default

settings.

24VAC

First stage cooling

First stage heating

Fan control

Service Indicator

24VAC common

Outdoor temperature sensor connection 1

IMPORTANT

Outdoor temperature sensor connection 2

Power must be applied for at least six consecutive

hours prior to a power loss in order for memory to

be retained for the specified time.

Equipment Protection Timers

Minimum Compressor OFF time: 5 minutes

Minimum Compressor ON time: 4 minutes

Minimum Furnace ON time: 3 minutes

LCD Backlight

Activated for 30 seconds when any button is pressed.

Minimum furnace cycle time (elapsed time between any

furnace activation and the next furnace activation): 6 min-

utes.

NOTE − During an electrical storm or similar disturbance,

the backlight may activate for a few seconds. This is nor-

mal and will no longer occur after the electrical disturbance

has passed.

Minimum elapsed time between any compressor activa-

tion and the next compressor activation: 6 minutes.

Thermostat Operating Conditions

35°F to 105°F, 5% to 90% RH

NOTE − All protection timers (except the compressor OFF

timer) can be over−ridden if a heating or cooling demand is

initiated or terminated using the UP, DOWN, HEAT, or

COOL buttons.

Thermostat Storage Conditions

−40°F to 185°F, 5% to 95% RH

Page 8

505067M 7/13/05

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