Honeywell T874 Thermostat User Manual

T874 Multistage Thermostats

and Q674 Subbases

PRODUCT DATA

FEATURES

•

T874 Thermostat has silent, dust-free mercury

switches operated by coiled bimetal elements.

Heat anticipator(s) are adjustable or fixed cooling

anticipator(s) are fixed.

Individual heat and cool levers and scales (most

models) for temperature setting located on top of

thermostat case.

•

Cover thermometer on most T874 Multistage

Thermostat models.

Locking cover and locking lever screws available for

T874 Multistage Thermostats.

Versaguard™ Thermostat Guard or custom key lock

thermostat guards available for T874 Multistage

Thermostats.

•

T874 Thermostat requires a Q674 Subbase.

Q674 Subbase provides system and fan switching,

wiring terminals and mounting base for T874

Multistage Thermostat.

APPLICATION

These thermostats and subbases provide low voltage control

of multistage heating and cooling systems, including heat

pump systems.

•

Adapter plate available for mounting Q674 on wall or

horizontal outlet box.

Light-emitting diodes (LEDs) located on subbase for

easy reference.

Up to three stages each of heating and cooling control

possible.

Models with setpoint restrictions and locking cover

with no thermometer available for Department of

Defense (DoD) and other special applications.

•

Outdoor reset used on some models to improve

thermal performance.

Contents

Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Ordering Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Checkout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Understanding Circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

Cross Reference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

Wiring Diagrams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96

U.S. Registered Trademark

60- 2485- 8

T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

Tradeline Models

T874 THERMOSTAT

SPECIFICATIONS

IMPORTANT

•

T874 TRADELINE models provide staged heat and/or cool

operation. See Table 1.

The specifications given in this publication do not

include normal manufacturing tolerances. Therefore,

this unit may not exactly match the listed specifica-

tions. This product is tested and calibrated under

closely controlled conditions, and some minor differ-

ences in performance can be expected if those con-

ditions are changed.

Table 1. Heating and Cooling Stages.

Models

Heating Stages

Cooling Stages

—

•

T874A-F are standard models.

Super Tradeline®/Tradeline® Models

SUPER TRADELINE controls offer features not available on

TRADELINE or standard models, and are designed to replace

a wide range of Honeywell and competitive controls.

TRADELINE FEATURES:

•

TRADELINE package with cross reference label and

special instruction sheet.

T874A,C model available with factory stops for DoD

applications.

TRADELINE models are selected and packaged to provide

ease of stocking, ease of handling, and maximum

replacement value. Specifications of SUPER TRADELINE

and TRADELINE controls are the same as those of standard

models except as noted below.

T874A,C model available with adjustable temperature

locking stops.

Q674 SUBBASE

Q674 switching subbases provide system and fan switching.

See Table 2.

Super Tradeline Models

T874 THERMOSTAT

Table 2. System and Fan Switching.

T874D Thermostat. Provides two stages of heating and two

stages of cooling. Use with Q674A-F Subbases.

Y594D (T874D/Q674E/TG504A) Thermostat/Subbase/key

lock cover package. Provides two stages of heating and

two stages of cooling. Includes a key lock cover for setpoint

protection.

Q674

System

Heat-Auto-Cool

Heat-Off-Cool

Fan

Auto-On

None

Auto-On

None

Off-Auto

None

Off-Heat-Auto-Cool

Em. Ht.-Off-Heat-Auto-Cool

Off-Auto

Y594G (T874G/Q674F) Thermostat/Subbase package for

heat pump. Provides two stages of heating and one stage

of cooling. Automatic changeover in heat or cool mode.

Available in beige or Premier White® color.

Y594R (T874R/Q674L) Thermostat/Subbase package for

heat pump. Provides two stages of heating and one stage

of cooling. Manual changeover in heat or cool mode. Avail-

able in beige or Premier White® color.

Em. Ht.-Auto-Off

Em. Ht.-Heat-Off-Cool

Auto-On

SUPER TRADELINE FEATURES:

•

SUPER TRADELINE package with cross reference label

and special instruction sheet.

TRADELINE FEATURE:

•

SUPER TRADELINE model supplied with locking lever and

locking cover accessories.

•

TRADELINE package with cross reference label and

special instruction sheet.

•

Includes adjustable temperature locking stops.

T874D replaces T874A-F TRADELINE or standard models.

•

Q674A-E,G are standard (non-heat pump) models.

Q674F,J,L are heat pump models.

ORDERING INFORMATION

When purchasing replacement and modernization products from your TRADELINE® wholesaler or distributor, refer to the

TRADELINE® Catalog or price sheets for complete ordering number.

If you have additional questions, need further information, or would like to comment on our products or services, please write or

phone:

Your local Home and Building Control Sales Office (check white pages of your phone directory).

Home and Building Control Customer Relations

Honeywell, 1885 Douglas Drive North

Minneapolis, Minnesota 55422-4386 (800) 328-5111

In Canada—Honeywell Limited/Honeywell Limitée, 35 Dynamic Drive, Scarborough, Ontario M1V 4Z9.

International Sales and Service Offices in all principal cities of the world. Manufacturing in Australia, Canada, Finland, France,

Germany, Japan, Mexico, Netherlands, Spain, Taiwan, United Kingdom, U.S.A.

60-2485—8

T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

Thermometer Range: 42° to 88°F (6 to 31°C)

Standard Models

Changeover Differential: 4°F (2°C) minimum between heat-

ing and cooling (5°F [3°C] on T874W). Levers can be set

apart for greater separation.

T874 THERMOSTATS

Models: See Table 3.

Electrical Rating: 24 to 30 Vac.

Interstage Differential:

Standard Models: Mechanical differential is 1°F (0.6°C)

between heating or cooling stages; operating differential is

approximately 1.9°F (1°C) between stages in heating or

cooling.

Switching: Coiled bimetal elements operate mercury

switches.

Special Models: See Table 3.

Temperature Adjustment: Heating and cooling setting

levers, with separate scales located on top of thermostat

base. Common lever for heating and cooling on T874R;

one cooling lever on T874E,V; and one heating lever on

T874F,Q.

Finish: Beige or Premier White^®finish.

Mounting Means: T874 Multistage Thermostat mounts on

Q674 Subbase. Subbase mounts horizontally on wall or

outlet box. Mounts on vertical outlet box with optional

Dimensions: See Fig. 1.

193121A Adapter Plate Assembly.

Temperature: Scale Range: 42° to 88°F (6° to 31°C) stan-

dard; optional ranges available.

THERMOSTAT MOUNTED

ON SUBBASE

SUBBASE

FRONT

SIDE

3/8 (10)

5-5/8 (143)

3-9/32 (83)

HEAT

50 60 70 80

COOL

50 60 70 80

1-1/2

(38)

50 60 70 80

-7/16

(

-1/2

89)

(87)

FAN

AUTO

OFF EM. HT. HEAT AUTO COOL

AUTO

OFF EM. HT. HEAT AUTO COOL

1-7/8 (48)

5-1/8 (130)

2-3/16 (56)

M5849

Fig. 1. T874 Thermostat and Q674 Subbase dimensions in in. (mm).

Optional Specifications (T874 Only):

Thermostat Accessories:

Locking Cover and Locking Lever Assembly: Part no.

194559R with thermometer; 194559S without thermome-

ter. See Fig. 2. Includes cover, screws, and Allen wrench

for locking cover. The screws must be used to assure

proper operation.

Temperature scale ranges are 40° to 75°F (4° to 24°C) heat-

ing and 75° to 90°F (24° to 32°C) cooling with stop; 44° to

8°F (7° to 20°C) heating, 80° to 86°F (27° to 30°C) cool-

ing; 6° to 29°C (43° to 85°F) Celsius scale; 3° to 22°C (38°

to 72°F) and 26° to 32°C (78° to 90°F) cooling with stop.

Nonadjustable factory-added stop limits heating setpoint to

Adjustable Lever Stop: Part no. 4074ECK; includes lever stop

and screws.

2°F (22°C) maximum and cooling setpoint to 78°F (26°C)

minimum.

Universal Versaguard™ Thermostat Guard: Includes wall-

plate, ring base, guard cover, tumbler lock, two keys and

optional Honeywell logo insert. Double-wall construction

provides extra measure of tamper-resistance. Tamper-

resistant lock; key cannot be removed without being in

locked position. Vents in guard base allow airflow for opti-

mum thermostat performance. See form 68-0104 for more

information.

OEM customer personalization.

Locking cover and locking lever (see Thermostat Accesso-

ries).

Thermostat cover without thermometer.

Adjustable locking temperature stops.

Voltage heat anticipation for first or second stage heat or both.

See Table 3.

Fast cycling on heating stage(s) for electric heat applications.

C815A Outdoor Thermistor for improved performance on

specified models.

—

TG511A1000: Clear cover.

TG511B1008: Opaque cover.

TG511D1004: Painted steel (off-white) cover. See

Fig. 2.

60-2485—8

T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

Key Lock Cover: Part no. TG504A replaces existing T874

cover. Mounts on T874 base and covers thermostat settling

levers and subbase switches. Includes LED window and

two keys. Should not be used with 193121A Adapter Plate.

—

TG504A1025: Blank face, internal thermometer.

TG504A1033: External thermometer. See Fig. 2.

194559R Locking Cover

with Thermometer

Allen Wrench

TG504A1033 Key Lock

Cover with External

Thermometer

TG511D1004 Includes

Painted Steel Cover,

Opaque Base and Wallplate

Fig. 2. T874 Thermostat accessories.

Table 3. T874 Thermostat Specifications.

Anticipation

Heating (Adj) Cooling (Fixed)

System Stages

Models and Options

Replaces Applications Heat Cool Other Stage 1 Stage 2 Stage 1 Stage 2

T874A—Standard and TRADELINE®.

T872A

Standard

—

0.1-1.2A

—

0-1.5A

—

72°F/78°F (22°C/26°C)setpoint

stops with locking cover.

—

Adjustable anticipator set 0.4A.

Adjustable locking temperature

stops (TRADELINE®).

—

72°F/78°F (22°C/26°C) setpoint

stops with locking cover, no

thermometer (for DoD ).

T874B—Standard and TRADELINE.

T874C—Standard and TRADELINE.

T872B

T872C

Standard

—

0.1-1.2A

0.1-1.2A 0.1-1.2A 0-1.5A

—

0-1.2A 0-1.0A

—

72°F/78°F (22°C/26°C)set stops

with locking cover, no thermometer

(

for DoD ).

—

12°F (7°C) differential between

H1 and H2 stages (T874C1125).

Fast cycling.

0.12-

.6A

0.12-

0.6A

T874D—Standard and SUPER

TRADELINE®.

T872D

Standard

—

0.1-1.2A 0.1-1.2A 0-1.2A 0-1.0A

—

Adjustable locking temperature

stops (SUPER TRADELINE).

T874E—Standard and TRADELINE.

T874F—Standard and TRADELINE.

T872E

T872F

2-Stage Cool

2-Stage Heat

—

0-1.2A 0-1.0A

0.1-1.2A 0.1-1.2A

—

Locking cover.

Department of Defense.

Changeover stage operates with heating.

Fixed voltage type anticipation.

Changeover stage operates with heating; a secondary changeover is provided in cooling switch.

Provides night setback used with standard T874 and timer-operated remote switching.

Manual changeover stage—use Q674B,L subbase.

Changeover stage operates with cooling.Q674 Subbases

60-2485—8

T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

—

Fast cycling.

Electric Heat

0.12-

.6A

0.12-

0.6A

T874G—Heat pump, cool

T872G Heat Pump or

Standard

₁b

₀_-₁_.₀_Ac

0.1-1.2A 0-1.0A

—

changeover, with fast cycling.

—

Fixed anticipator for H2.

Fast cycling.

0.1-1.5A

0.12-

0.6A

T874H—Use with Q674C.

T872H Heat Pump or

Standard

₁b

₀_-₁_.₀_Ac

—

0-1.0A

T874J—Heat pump.

T874K—Heat pump.

T874L—Heat pump.

None

Heat Pump

₂d

₁b

—

₀_-₁_.₀_Ae

0.1-1.2A

₀_-₁_.₅_Ac ₀_-₁_.₀_Ac

—

None

Heat Pump

₀_-₁_.₅_Ac

0-1.5A

None

T872N

Heat Pump

0.1-1.2A 0.1-1.2A 0-1.5A

0.1-1.2A 0.1-1.2A 0-1.0A

—

T874N—Heat pump, heat

₁b

changeover.

T874P—Heat pump.

T874Q—Night setback heating.

None

T872Q

Heat Pump

Standard

_H_e_a_t_P_u_m_pf

₁e

—

0.1-1.2A 0.1-1.2A 0-1.5A

—

0.1-1.2A

₀_-₁_.₅_Ac

₀_-₁_.₂_Ac

—

T874R—Heat pump.

T872R

None

—

₁g

0.1-1.2A 0-1.5A

₀_-₁_.₂_Ac

—

T874S—Two-speed compressor heat

Heat Pump

0-1.2A 0-1.0A

pump.

T874V—Standard.

T874W—Heat pump and standard.

None

Standard

—

0-1.5A

0-1.2A 0-1.0A

—

T872W Heat Pump or

_-₁_.₂_Ab ₀_-₁_.₂_Ac,d

—

Night setback heating.

Standard

Department of Defense.

Changeover stage operates with heating.

Fixed voltage type anticipation.

Changeover stage operates with heating; a secondary changeover is provided in cooling switch.

Provides night setback used with standard T874 and timer-operated remote switching.

Manual changeover stage—use Q674B,L subbase.

Changeover stage operates with cooling.Q674 Subbases

Models:

Mounting: Designed to mount horizontally on an outlet box

or the wall. Adapter plate assembly available for mounting

on a vertical outlet box (see Subbase Accessory).

See Table 4.

Electrical Ratings:

Switch Contacts: 2.5A at 30 Vac (7.5A inrush).

LED Lights (Optional): 30 Vac.

Finish: Dark brown or gray.

Dimensions in in. (mm): 3-1/2 (89)height; 5-5/8 (143) width;

Switches: Two slide switches (one switch on Q674G and K;

no switches on Q674D) operated by levers. Switch position

is shown on scaleplate.

5/16 (8) depth. See Fig. 1.

Table 4. Q674 Subbase Specifications.

Switch Positions

System

Models and Options

Replaces Application

Fan

Use With

Q674A—Standard and TRADELINE®.

Q672A

Standard

HEAT-AUTO-COOL

AUTO-ON

T874A-D

—

Indicator LEDs.

Q674B—Standard and TRADELINE.

Q672B

Standard

Heat Pump

HEAT-OFF-COOL

AUTO-ON

T874A-D

T874G,H,L,R

T874P

—

Provision for fan relay operation from

external fan switch (isolate G terminal).

Indicator LEDs.

OVERRIDE-HEAT-

OFF-COOL

OFF-AUTO

OVERRIDE-AUTO-

OFF

—

Q674C—Standard and TRADELINE.

Indicator LEDs.

Q672C

Q672D

Standard

Heat Pump

AUTO-ON

T874A-H,W

T874K,L,N

T874P

—

Q674D—Standard and TRADELINE.

Standard

None

T874A-F

—

For use when subbase switching is not

required.

60-2485—8

T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

Q674E—Standard and TRADELINE.

Q674F—Two LED models.

Q672E

Q672F

Standard

OFF-HEAT-AUTO-

COOL

AUTO-ON

T874A-D

Heat Pump OFF-EM.HT.-HEAT-

AUTO-ON T874C,D,G,N,S

AUTO-ON

—

EM.HT. light.

AUTO-COOL

OFF-COOL-AUTO-

HEAT-EM.HT.

Provision for AUTO fan operation in

EM.HT.

Q674G—O and B terminals.

Q674J—Provision for AUTO fan operation

in EM.HT.

Q672G

Q672J

Standard

Heat Pump

OFF-AUTO

None

T874A-F

T874A,D,G,J

EM.HT.-AUTO-OFF

EM.HT.-ON-OFF

SUPL.HT.-ON-OFF

AUTO-ON

None

—

Provision for outdoor thermistor.

Q674K—Standard.

Q672K

Q672L

Standard

OFF-HEAT-AUTO-

T874F

COOL

OFF-WOOD-WOOD/

OIL-OIL

None

Q674L—Provision for AUTO fan operation in

EM.HT.

Heat Pump EM.HT.-HEAT-OFF-

AUTO-ON

T874R,W

COOL

SUPL.HT.-HEAT-

OFF-COOL

—

Indicator LEDs.

Q674N—Standard.

Q674P—Standard.

Q674Q—Standard.

Q674R—Standard.

Q672N

Q672P

None

Evaporative

Cooler

Heat Pump

EVAP-COOL-OFF-

T874C

T874G

T874A

T874V

T874C

HEAT

SUPL.HT.-HEAT-

COOL

HEAT-OFF-COOL LO-MED-HI-

Fan Coil

LO-HI-

CONT.

None

Fan Coil

OFF-COOL

—

International symbols.

Q674S—Indicator LED.

None

Standard

HEAT-COOL

None

Optional Specifications (Q674 Only):

L terminal is used for system monitoring devices.

Common R terminal for heating/cooling.

Changeover in cool or heat mode for heat pumps.

Auto fan in EM.HT. for heat pumps.

Models available with up to four LEDs; for example, LEDs can

show EM. HT, AUX. HT, SERVICE, CHECK, FILTER, and

LOCKOUT. See Fig. 3.

System switching marked HEAT-OFF/RESET-COOL for sys-

tems requiring impedance relay reset. Available on Q674B

only.

Subbase Accessory: 193121A Adapter Plate Assembly for

mounting on vertical outlet box. Assembly includes adapter

ring and cover plate. Use to cover wall marks from

replaced thermostat.

G terminal isolated on heating to provide fan relay operation

from external low voltage fan switch (Q674B only).

Auto fan operation on both heat and cool.

T874 Thermostat with

Q674 Subbase

with Four LEDs

One Setpoint Lever

Separate

Heating and Cooling Levers

Fig. 3. Heating, cooling levers and system LED indicators.

60-2485—8

T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

—

concealed pipes and chimneys.

unheated (uncooled) areas such as an outside wall behind

the thermostat.

MERCURY NOTICE

This control contains mercury in a sealed tube. Do not

place control in the trash at the end of its useful life.

Mount Subbase

If this control is replacing a control that contains

mercury in a sealed tube, do not place your old control

in the trash.

The subbase can be mounted on a vertical outlet box,

horizontal outlet box or directly on the wall.

1. If the subbase is mounted on a vertical outlet box, order

Honeywell part no. 193121A Adapter Assembly. See

Fig. 4. The assembly includes an adapter ring, two

screws and a cover plate to cover marks on the wall.

Install the ring and cover plate on the vertical outlet box.

Contact your local waste management authority for

instructions regarding recycling and the proper

disposal of this control, or of an old control containing

mercury in a sealed tube. If you have questions, call

the Honeywell Customer Response Center at

NOTE: For a wall installation, hold subbase in position and

mark holes for anchors. See Fig. 5. Wall anchors

must be obtained from local hardware store. Be care-

ful that the wires do not fall back into the wall open-

ing. Set aside subbase. Drill four

1-800-468-1502.

INSTALLATION

/16 in. (4.8 mm) holes and gently tap anchors into

the holes until flush with the wall.

When Installing this Product…

Pull wires through the cover plate (if used) and subbase

cable opening. See Fig. 6.

Read these instructions carefully. Failure to follow them

could damage the product or cause a hazardous condi-

tion.

Secure the cover plate (if used) and subbase with the

screws provided. Do not fully tighten the subbase

screws.

Check the ratings given on the product to make sure the

product is suitable for your application.

Installer must be a trained, experienced service techni-

cian.

Level the subbase using a spirit level, see Fig. 7, and firmly

tighten subbase mounting screws. The subbase mounting

holes provide for minor out-of-level adjustments.

After installation is complete, check out product opera-

tion as provided in these instructions.

VERTICAL

OUTLET

MOUNTING

SCREWS (2)

BOX

COVER

PLATE

CAUTION

ADAPTER

RING

Hazardous Voltage.

Can damage heating/cooling system.

Disconnect power supply before beginning instal-

lation to prevent electrical shock or equipment

damage.

Do not short across coil terminals on relay. This

can burn out thermostat heat anticipator.

To prevent interference with the thermostat link-

age, keep wire length to a minimum and run wires

as close as possible to the subbase.

Do not overtighten thermostat captive mounting

screws because damage to subbase threads can

result.

HORIZONTAL

OUTLET

BOX

THERMOSTAT

IMPORTANT

An incorrectly leveled thermostat will cause the tem-

THERMOSTAT

COVER

COOL

perature control to deviate from setpoint. It is not a

calibration problem.

60 70 80

SUBBASE

60 70 80

HEAT

SUBBASE

Location

MOUNTING SCREWS (2)

Install the thermostat about 5 ft (1.5m) above the floor in an

area with good air circulation at average temperature.

60 70 80

CAPTIVE

MOUNTING SCREWS (2)

Do not mount the thermostat where it can be affected by:

NOT INCLUDED WITH UNIT.

—

drafts or dead spots behind doors, in corners or under

cabinets.

ACCESSORY PART AVAILABLE (193121A).

M6009

—

hot or cold air from ducts.

Fig. 4. Installing Q674 Subbase on outlet box.

radiant heat from the sun, fireplace, or appliances.

60-2485—8

T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

All wiring must comply with local electrical codes and

ordinances.

WALL

IMPORTANT

Use 18 gauge, solid-conductor wire whenever possi-

ble. If using 18 gauge stranded wire, no more than

WIRES THROUGH

WALL OPENING

0 wires can be used. Do not use larger than

8 gauge wire.

Follow equipment manufacturer wiring instructions when

available. To wire subbase, proceed as follows:

Connect the system wires to the subbase as shown in

the applicable diagram. A letter code is located near

each terminal for identification. Typical terminal desig-

nation and wiring connections are listed in Table 5. The

terminal barrier permits straight or wraparound wiring

connection. See Fig. 7. The subbase can require one or

more jumpers that may or may not be factory-supplied.

See Fig. 8 and the wiring diagrams for specific terminals

to be jumpered.

WALL

ANCHORS

MOUNTING

HOLES

(

MOUNTING

SCREWS (2)

M926

SUBBASE

Fig. 5. Installing Q674 Subbase on wall.

FOR STRAIGHT

INSERTION–

STRIP 5/16 IN. (8 MM)

FOR WRAPAROUND–

STRIP 7/16 IN. (11 MM)

WIRING

TERMINAL

THERMOSTAT

CABLE OPENING

SPIRIT LEVEL

BARRIER

SUBBASE TERMINAL SCREW

M928

Fig. 7. Barrier configuration.

CAUTION

Equipment Damage Hazard.

POST FOR

MOUNTING

THERMOSTAT (2)

MOUNTING

HOLES (4)

TO SPRING FINGER

CONTACTS ON

THE THERMOSTAT

Never install more than one wire per terminal unless

using factory-supplied jumper with spade terminal.

(UP TO 12)

M927

Firmly tighten each terminal screw.

Fig. 6. Subbase components and leveling procedure.

3. Fit wires as close as possible to the subbase. Push

excess wire back into the hole.

Plug hole with nonflammable insulation to prevent drafts

from affecting the thermostat.

Wire Subbase

Disconnect power supply before beginning installation to

prevent electrical shock or equipment damage.

60-2485—8

T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

JUMPER WIRE

(SUPPLIED WITH SOME MODELS)

TWO ADJACENT TERMINALS SHOWN JUMPERED ARE FOR EXAMPLE ONLY. COMPARE WIRING

DIAGRAM AND SUBBASE TO IDENTIFY TERMINALS TO BE JUMPERED.

M5899

Fig. 8. Jumper adjacent terminals for special system hookup using stripped wire 3/4 in. (19 mm).

For nonadjacent terminals and using jumper wire supplied with subbase.

Table 5. Terminal Designations^a

Standard Terminal

Alternate Designations or

Customer Specials

Designation

Typical Connection

Heating damper motor; changeover valve

Emergency heat relay

Fan relay coil

System monitor

Cooling damper motor; changeover valve

Power connection to transformer (internally connected for heating

and cooling)

Power connection to cooling transformer

Power connection to heating transformer

Stage 1 heating control

Stage 2 heating control

_S_t_a_g_e₃_h_e_a_t_i_n_g_c_o_n_t_r_o_lb

H1, R3

H2, Y, R4

C1, M

Stage 1 cooling control

Stage 2 cooling control

Stage 3 cooling control

Clogged filter switch or common connection

Outdoor thermistor

X1,X2,C

L, C, H

HSII control panel

Defrost

Momentary circuit, changeover

LEDs

A, A1, A2, Z, C, L

Other terminal designations can be used that are not listed on this table. Refer to the hookup drawing and internal schematic

for exact connections.

W3 controls the auxiliary heat like W2, and allows adding additional stages of auxiliary heat with outdoor thermostats while

maintaining the proper second stage anticipation.

60-2485—8

T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

Table 5. Terminal Designations^a

Standard Terminal

Designation

Alternate Designations or

Customer Specials

Typical Connection

External temperature readout, T relay

LO and HI speed fan relays

Cooling contactor

R1, R2

Compressor contactor

Other terminal designations can be used that are not listed on this table. Refer to the hookup drawing and internal schematic

for exact connections.

W3 controls the auxiliary heat like W2, and allows adding additional stages of auxiliary heat with outdoor thermostats while

maintaining the proper second stage anticipation.

mounting screw and two locking screws with insulated heads.

Outdoor Disconnect

When installed, the stop brackets limit the movement of the

T874 HEAT and COOL levers.

The National Electrical Code requires the installation of a

disconnect switch within sight of the outdoor unit of an air

conditioner or heat pump. The switch is for the safety of any

technician working on the unit. The technician can assure that

the unit remains unpowered.

TO INSTALL:

Remove the thermostat cover by pulling the bottom

edge of the cover upward until it snaps free of the

mounting slots.

Turn to the back of the T874 Thermostat. Locate the

hole for the brass insert in the plastic base below the

LED window.

Install and Adjust Stop Brackets

The stop brackets should be installed only if there is a need to

restrict the adjustable range of the heating and cooling

temperature setpoint levers. If adjustable lever stops are

desired, order 4074ECK Envelope Assembly, which contains

two adjustable lever stop brackets, one brass insert, one

Push the brass insert into the hole with finger.

Turn to the front of the T874 Thermostat.

Place the two stop brackets in position with the tabs in

the slot between the HEAT and COOL levers. See

Fig. 9.

HOLES FOR INSULATED

LOCKING LEVER SCREWS

COOL

LEVER

HOLE WITH

BRASS INSERT

HEAT

LEVER

BRACKET

TABS

ADJUSTABLE LOCKING LEVERS

NONADJUSTABLE STOPS

WITH LOCKING LEVER SCREWS

75°F (24°C) MAX. HEAT

ADJUSTABLE

LEVER STOP

BRACKETS

MOUNTING

SCREW

BRACKET

SLOTS

75°F (24°C) MIN. COOL

NONADJUSTABLE D.O.D. STOPS

ADJUSTABLE STOPS

72°F (22°C) MAX. HEAT

8°F (26°C) MIN. COOL

M7626

Fig. 9. Range limiting and lever locking methods.

Insert the mounting screw into the two slots in the stop

brackets and attach to the brass insert. Tighten the

screw to pull the brass insert into the back of the ther-

mostat.

7. Loosen the mounting screw enough to free the stop

brackets for adjustment.

60-2485—8

T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

Move the HEAT and COOL levers to the maximum tem-

perature desired.

Slide the stop brackets until one rests against the HEAT

lever and the other against the COOL lever.

0. Firmly tighten the mounting screw.

1. If the HEAT and COOL levers are to be locked in place

at a specific temperature, use the two insulated head

screws supplied instead of the two adjustable lever stop

brackets.

ALLEN

RETAINING

SCREWS (2)

CAUTION

Equipment Damage Hazard.

Do not use standard screws that provide metal-to-

metal contact with the stop brackets. Short circuit and

potential equipment damage can result.

M956

Fig. 10. Installation of locking cover assembly.

Mount Thermostat

Remove the thermostat cover by pulling the bottom

edge of the cover away from the base until it snaps free

of the cover clip.

SETTINGS

NOTE: The cover is hinged at the top and must be removed

by pulling up at the bottom.

CAUTION

Equipment Damage Hazard.

Carefully remove and discard the polystyrene packing

insert that protects the mercury switches during ship-

ment.

On systems using a gas valve, never apply a jumper

across the valve coil terminals, even temporarily. This

can burn out thermostat heat anticipator(s).

If LED indication (EM.HT., CHECK, etc.) is to be used

with the Q674 Subbase, install the preprinted insert

under the thermostat setpoint scale. To install, push

both thermostat setpoint levers to the far ends of the

thermostat. Use index finger to gently pull out the plastic

setpoint scale about 1/4 in. (6 mm). Position the desired

preprinted insert in the space above the LED lights.

Reposition setpoint levers.

Set the Heat Anticipator

Move the indicator to match the primary control current draw.

When using a T874 Thermostat with two stages of heating,

set each heat anticipator to match its respective primary

control current draw. If you cannot find the current rating on

the primary control, or if further adjustment is necessary, see

NOTE and use the following procedure to determine the

current draw of each stage.

Turn over the thermostat base and note the spring fin-

gers that engage the subbase contacts. Make sure the

spring fingers are not bent flat, preventing proper elec-

trical contact with the subbase.

The current draw of each heating stage must be measured

with the thermostat removed and power on to the heating

system.

Set the heat anticipator indicator(s) to the respective

current setting of each stage. See Set The Heat Antici-

pator section.

If the thermostat provides optional locking cover assem-

bly, start the Allen locking screws in the cover with the

wrench provided. See Fig. 10.

Connect an ac ammeter of appropriate range between

the heating terminals of the subbase:

a. Stage 1—between W1 and RH or R;

b. Stage 2—between W2 and RH or R

c. Stage 3—between W3 and RH or R.

Move the system switch to HEAT or AUTO.

After one minute, read the ammeter and record the

reading:

Note the two tabs along the top inside edge of the ther-

mostat base. The tabs fit into corresponding slots on top

of the subbase. Mount the thermostat on the subbase.

Align the two captive mounting screws in the thermostat

base with the posts on the subbase. Tighten both

screws. Do not overtighten screws or damage to sub-

base posts can result.

a. Stage 1—__________A;

b. Stage 2—__________A;

c. Stage 3—__________A.

NOTE: If equipment cycles too fast, set the indicator to a

higher current rating, but not more than one-half divi-

sion at a time, and recheck the cycle rate. Most con-

ventional two-stage heating equipment is designed

to operate at three cycles per hour per stage, and

one-stage heating equipment at six cycles per hour,

at 50 percent load conditions. When using the T874

Thermostat in heat pump systems, set the heat antic-

ipator at 140 percent of the actual primary control

current draw to reduce the cycling rate. See Fig. 11.

60-2485—8

T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

Most heat pump systems should cycle 2-1/2 to 3 times per hour.

HEAT—heating system is automatically controlled by the

thermostat. Cooling system is off.

Hang the upper edge of the thermostat cover on top of

the thermostat base and swing the cover downward

until it engages with the cover clip.

AUTO—thermostat automatically changes between heat-

ing and cooling system operation, depending on the

indoor temperature.

COOL—cooling system is automatically controlled by the

thermostat. Heating system is off.

STAGE ONE

ANTICIPATOR

HEATING

EM.HT.—emergency heat relay is automatically controlled

by the thermostat. Cooling system is off. Compressor is

de-energized.

CONTROL

SUPL.HT.—supplemental heat relay is energized. Cooling

system is off. Compressor is de-energized.

WOOD—heating system is operating with only the wood-

burning stage.

OIL—heating system is operating with only the oil-burning

stage.

WOOD/OIL—wood and oil stages operate sequentially;

first the WOOD stage operates, then the OIL stage

operates if the WOOD stage cannot handle the load.

EVAP—controls cooling system by water evaporation; see

equipment instructions for further information.

OVERRIDE—night setback is disabled.

.2.8

STAGE TWO

ANTICIPATOR

HEATING CONTROL

MOVE INDICATOR TO

MATCH CURRENT RATING

OF PRIMARY CONTROL

M5069

ON—heating system is controlled by the thermostat. EM.

HT. or SUPL. HT. relay is not energized.

Fig. 11. Adjustable heat anticipator scales.

FAN SWITCH positions control fan operation as follows:

ON or CONT.—fan operates continuously.

AUTO—fan operates as controlled by the thermostat in

heat pump systems or conventional cooling mode; fan

operates as controlled by the plenum switch in conven-

tional heating mode.

Temperature Setting

Move the heating and cooling levers to the desired comfort

positions. See Fig. 12. On some models with two stages of

heating or cooling, the same lever controls both stages. The

minimum differential between heating and cooling setpoints is

LO—fan operates constantly at low speed.

MED—fan operates constantly at medium speed.

HI—fan operates constantly at high speed.

4°F (2°C) (5°F [3°C]) on T874W.

If model has optional screws to lock temperature control

levers, loosen these screws before making temperature

adjustment; tighten the screws when levers are set at desired

position.

To move the subbase switches to the desired control

positions, use thumb and index finger to slide the lever. The

lever must stop over desired function indicator position for

proper circuit operation.

STAGE 1

HEATING

LEVER

COOLING

STAGE 3

HEATING

Spring return momentary position switching feature is

available on selected subbase models. On these models, the

fan switch is positioned to the right of the system switch. By

moving the fan switch to the far right and releasing it, the ON

position circuit makes. The lever springs back on release. This

position is not marked on the subbase.

LEVER

STAGE 1

COOLING

Setting the Adjustable Differential

The adjustable interstage differential feature, on a selected

T874D model only, can be identified by the scale and tension

screw near the heating and cooling mercury switches. See

Fig. 13. On this model, the number of degrees between the

making of the first and second stage mercury bulbs is

adjustable. This feature is especially useful if the first stage

controls the comfort temperature, and the second stage

controls the energy savings temperature. Timers, such as the

S6005, for insertion between the first and second stage

control points must be ordered separately.

STAGE 2

HEATING

CAPTIVE

MOUNTING

SCREWS (2)

STAGE 2

COOLING

M7625

Fig. 12. Internal view of T874W (three stages of

heating, two stages of cooling).

Each mark on the scale represents 1°F (0.6°C) The

differential is factory set at 2°F (1°C) the differential can be set

as high as 12°F (7°C) To set the adjustable interstage

differential, loosen the tension screw. See Fig. 13. Slide the

adjustable scale to align with the number of degrees desired

between stages. Use the lower edge of the tension screw

bracket as a guide for alignment. In heating, slide the lever

wider apart for a larger differential, or closer together for a

Subbase Setting

The subbase switching positions control the system operation

as described below.

SYSTEM SWITCH (see subbase for positions):

OFF—both the heating and cooling systems are off. If the

fan switch is at the AUTO position, the cooling fan is

also off.

60-2485—8

T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

smaller differential. In cooling, slide the lever closer together

for a larger differential, or wider apart for a smaller differential.

While supporting the scale with hand, tighten the tension screw.

CAUTION

Equipment Damage Hazard.

When the thermostat is used to control a two-stage

heating or cooling system, the second stage mercury

bulb must never make before the first stage bulb, or

severe equipment damage could result. To prevent

this problem, provide at least 2°F (1°C) differential

between stage-one and stage-two make points.

Example: in heating, if stage-one makes at 70°F (21°C)

stage-two should make at 68°F (20°C) or lower.

IMPORTANT

Support the scale with hand while tightening tension

screw. See Fig. 13. Failure to do so can result in

twisting and damaging bimetal coil.

HEATING SET-

POINT LEVER

TENSION

SCREW

HEAT

50 60 70 80

FIRST

STAGE

SWITCH

Verify the Adjustment

Heating

Start with the heating setpoint lever all the way to the left.

Slowly move the lever to the right, just until the first stage bulb

makes (mercury rolls to the right side of the bulb). Note the

setting on the temperature scale. Slowly move the lever to the

right until the second stage bulb makes. Note the setting on

the temperature scale. The difference between the two

temperatures is the interstage differential, which should match

the number set on the scale with the tension screw.

SCALE

SECOND STAGE

ADJUSTABLE)

SWITCH

(

Cooling

HEATING

Start with the cooling setpoint lever all the way to the right.

Slowly move the lever to the left, just until the first stage bulb

makes (mercury rolls to the left side of the bulb). Note the

setting on the temperature scale. Slowly move the lever to the

left until the second stage bulb makes. Note the setting on the

temperature scale. The difference between the two

TENSION

SCREW

ALIGN LOWER

EDGE WITH

SCALE

SLIDE LEVER

temperatures is the interstage differential, which should match

the number set on the scale with the tension screw.

NOTCH

0°F

WIDER APART

FOR LARGER

DIFFERENTIAL

SCALE

CHECKOUT

COOLING

Heating

Move the system switch on the Q674 Subbase to HEAT or

AUTO. Move the heat lever on the T874 about 10°F (6°C)

above room temperature. See Fig. 12. Heating system should

start and the fan should run after a short delay. Move the heat

lever about 10°F (6°C) below room temperature. The heating

equipment should shut off, and the fan should run for a short

time, then shut off.

TENSION

SCREW

ALIGN LOWER

EDGE WITH

SCALE

SLIDE LEVER

CLOSER TOGETHER

FOR LARGER

NOTCH

4°F

DIFFERENTIAL

SCALE

In heat pump applications, sometimes time delays are

involved before the compressor and auxiliary heat are

activated. This is due to a minimum-off timer, which prevents

the compressor from restarting for five minutes from when the

thermostat last turned off the compressor, or from when the

system first received power.

EACH MARK ON THE SCALE REPRESENTS 1°F (0.6°C).

M937

Fig. 13. Set adjustable interstage differential.

Cooling

CAUTION

Equipment Damage Hazard.

Do not operate cooling if outdoor temperature is below

50°F (10°C). Refer to manufacturer recommendations.

60-2485—8

T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

Move the system switch on the Q674 Subbase to COOL or

AUTO. Move the cool setting lever on the T874 Multistage

Thermostat about 10°F (6°C) below room temperature. See

Fig. 12. The cooling equipment and fan should start. If the

system has two stages of cooling, both stages should start.

Move the cool lever about 10°F (6°C) above room

Thermometer

The thermometer in your thermostat has been accurately

calibrated at the factory. The thermometer should only need

adjustment if it has been dropped or shifted due to mishandling.

If the setpoint lever and the thermometer reading do not

agree, use the following procedure:

temperature. The cooling equipment and fan should stop.

Remove the thermostat cover by pulling up from the

bottom edge of the cover away from the base until it

snaps free of the cover clip.

Fan

Move the system switch to COOL, OFF, or AUTO. If

necessary, position both temperature setting levers so that the

heating and cooling equipment are off. Move the fan switch to

ON or CONT. The fan should run continuously. When the fan

switch is in AUTO, LO, MED, or HI position, fan operation is

controlled by the heating or cooling system.

Set the thermostat cover on a table near an accurate

thermometer.

Allow ten minutes for cover thermometer to sense area

temperature; compare the readings. Be careful not to

touch thermometer or breathe on it.

If the readings are the same, replace cover and put the

system into operation.

Outdoor Reset Thermistor (Where

Applicable)

5. If the readings are different, insert a small screwdriver in

the thermometer slot and turn it until the thermometers

have the same reading. See Fig. 15.

If the system is supplied with a thermistor, it must be used; if

not used, thermostat performance deviates radically from

proper operation.

6. Replace thermostat cover and put the system into oper-

ation.

The proper thermistor operation must be verified to ensure the

correct operation of the thermostat. Check thermistor

operations as follows:

Disconnect the T wire on the subbase.

Use an ohmmeter to measure resistance between the

T wire and the A subbase terminal.

Take outdoor temperature at thermistor location and find

the correct thermistor resistance on the Fig. 14 chart.

If the resistance measured in step 2 and the calculated

resistance in step 3 vary by more than 15 percent, the

thermistor requires replacement. Contact Honeywell or

installing dealer for replacement packaged outdoor ther-

mistor, part no. C815A1005.

600

400

200

000

800

600

400

200

000

800

600

400

200

000

800

600

400

200

000

C815A THERMISTOR RESISTANCE

R = 400 ohms ± 10% AT 77°F (25°C)

M5070

Fig. 15. Thermometer calibration.

UNDERSTANDING CIRCUITS

To understand wiring diagrams, it is important to know what all

the symbols mean and how to trace the path of the circuits

from the transformer. See Fig. 16 through 25.

Circuit descriptions and terminology are defined as follows:

For standard heating-cooling circuits:

-20

100

120 140

M1590A

TEMPERATURE OF THERMISTOR (°F)

Auto changeover—refers to the presence of an AUTO

position in the system switching (EX: Q674E with OFF-

HEAT-AUTO-COOL switching). The thermostat auto-

matically changes between heat and cool modes as

indoor temperature changes.

Fig. 14. Thermistor resistance chart.

CALIBRATION

Manual changeover—requires a system switch movement

to change mode (EX: Q674B with HEAT-OFF-COOL

switching). T874D Multistage Thermostats with 2 heat

or 2 cool switches are shown on most standard circuits.

Most standard or TRADELINE® subbases (Q674A-E,G)

can be used with T874A-F standard or TRADELINE

Thermostat

T874 Thermostats are accurately calibrated at the factory.

They do not have provision for field calibration.

60-2485—8

T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

thermostats. The schematics can be field-modified as

required (EX: if T874C is being used, eliminate second

stage of heat).

perature rise. Fixed anticipation is represented by a zig-

zag line and adjustable anticipation is a zigzag with an

arrow. The resistance of the fixed anticipator is so large

it limits current so that a system relay cannot be pulled

in from a circuit path going through the fixed anticipator.

The relay can be pulled in through an adjustable antici-

pator because its resistance is generally 0 to 5 ohms.

5. The center portion (the subbase) contains the switches.

The fan switch is above the system switch. The small

circles on the switch represent the maximum possible

contacts available on the Q674 Subbase. The larger cir-

cles represent the switch positions available on this par-

ticular Q674, with the solid circle representing where it

is actually switched on the diagram.

For heat pump circuits:

Cool changeover valve—operates on cooling. The revers-

ing valve or relay is activated either by moving the sys-

tem switch to COOL (manual changeover) or by a

mercury switch that makes on a temperature rise (auto

changeover).

Heat changeover valve—operates on heating. The revers-

ing valve or relay is activated either by moving the sys-

tem switch to HEAT (manual changeover) or by a

mercury switch that makes on a temperature fall (auto

changeover).

System monitor relay—optional equipment on some heat

pumps includes an R4222P1065 or equivalent. This

system monitor relay detects a malfunction in the com-

pressor and indicates the malfunction by activating the

EMERGENCY HEAT LED on the Q674 Switching Sub-

base. The system monitor relay is usually wired into the

L terminal on the Q674.

NOTE: Solid circles are not interconnected electrically.

At the right, the relays and contactors are shown, attached to

the proper terminals. The terminals are represented by large

circles with terminal designations in capital letters. See Table

3 for the meaning of each lettered terminal.

Each mercury switch is identified by function:

H1—Stage 1 Heating.

Sometimes power for a fixed anticipator is brought through an

off system relay like the changeover relay shown in Fig. 20.

This current is kept low by the high resistance of the fixed

anticipator so that relay does not pull in.

H2—Stage 2 Heating.

H3—Stage 3 Heating.

C1—Stage 1 Cooling.

C2—Stage 2 Cooling.

C3—Stage 3 Cooling.

C/O—Changeover (heat pumps).

KEY TO HOOKUP SYMBOLS

Each anticipator is identified and each switch affected is

named (EX: H1 anticipator, C1 anticipator).

RELAY/CONTACTOR CONTACTS

TRANSFORMER

All T874 Multistage Thermostats use mercury switches. Each

schematic indicates switch operation by being drawn in the

open position with an arrow indicating operation with a

temperature RISE or FALL.

(24 VAC SECONDARY)

TERMINAL

RELAY OR CONTACTOR COIL

MERCURY SWITCH

One circuit has been selected that is typical of various models

used with heat pumps. This circuit has been traced to illustrate

the functions performed by these control systems.

LED

ODT

OUTDOOR THERMOSTAT

EHR

Tracing Method:

EMERGENCY HEAT RELAY

Always begin at the system transformer or R terminal.

You may want to draw the switch contacts in each

switch position to aid in tracing. Colored pencils are

helpful when only one copy of the circuit is available.

See Fig. 16 for a description of the hookup symbols and

Fig. 17 through 25 for typical hookups.

FIXED ANTICIPATOR

HIGH RESISTANCE

RTD

TIME DELAY RELAY

(TYPICALLY 5 KILOHMS)

DEFROST RELAY

CHP

PRESSURE SWITCH

ADJUSTABLE ANTICIPATOR

LOW RESISTANCE

TYPICALLY 0 TO 5 OHMS)

LACO

Completely trace only one circuit at a time (for example:

heat or cool).

(

LOW AMBIENT CUTOFF

M5848

Connections are indicated by small dots at the point of

intersection. If there is no dot, there is no connection.

The left portion of the circuit (the thermostat) contains

the mercury switches. The heat switches make on a

temperature fall, and the cool switches make on a tem-

Fig. 16. Key to hookup symbols.

60-2485—8

T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

THERMOSTAT

SUBBASE

SYSTEM COMPONENTS

HOT) L1 L2

(

H1/C1

ANTICIPATOR

FALL

ODT 1

EHR 1

ODT 2

RTD 1

RTD 2

FAN SWITCH

AUTO

EHR 2

RTD 3

RTD 1

ANTICIPATOR

AUX. HT.

LED

GREEN)

(

FALL

EM. HT.

LED (RED)

SYSTEM

MONITOR

SYSTEM

SWITCH

FAN RELAY

EM. HT.

HEAT

OFF

EM. HT.

RELAY

COOL CHANGEOVER

VALVE

COOL

LACO

CHP

COMPRESSOR

CONTACTOR

POWER SUPPLY. PROVIDE DISCONNECT MEANS AND OVERLOAD PROTECTION AS REQUIRED.

AUXILIARY HEAT LED AVAILABLE ON SOME MODELS.

M5072A

Fig. 17. Internal schematic and typical hookup of a T874R Thermostat and Q674L Subbase in a heat pump application.

The thermostat provides two-stage heating and one-stage cooling manual changeover operates on cooling.

60-2485—8

T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

THERMOSTAT

SUBBASE

SYSTEM COMPONENTS

(HOT)

H1/C1

ANTICIPATOR

ODT 1

EHR 1

ODT 2

FALL

RTD 1

RTD 2

FAN SWITCH

AUTO

EHR 2

RTD 3

ANTICIPATOR

AUX. HT.

LED

GREEN)

RTD 1

(

EM. HT.

LED (RED)

SYSTEM

MONITOR

FALL

SYSTEM

SWITCH

FAN RELAY

EM. HT.

HEAT

OFF

EM. HT.

RELAY

CHANGEOVER

VALVE

COOL

LACO

CHP

POWER SUPPLY. PROVIDE

DISCONNECT MEANS AND

OVERLOAD PROTECTION

AS REQUIRED.

COMPRESSOR

CONTACTOR

M5840

Fig. 18. Tracing the changeover relay circuit.

THERMOSTAT

H1/ C1

SUBBASE

SYSTEM COMPONENTS

HOT)

(

ANTICIPATOR

ODT 1

EHR 1

ODT 2

FALL

RTD 1

RTD 2

FAN SWITCH

AUTO

EHR 2

RTD 3

ANTICIPATOR

AUX. HT.

LED

GREEN)

RTD 1

(

EM. HT.

LED (RED)

SYSTEM

MONITOR

FALL

SYSTEM

SWITCH

FAN RELAY

EM. HT.

HEAT

OFF

EM. HT.

RELAY

CHANGEOVER

VALVE

POWER SUPPLY. PROVIDE

DISCONNECT MEANS AND

OVERLOAD PROTECTION

AS REQUIRED.

COOL

LACO

CHP

COMPRESSOR

CONTACTOR

M5841

Fig. 19. Tracing the heat 1 and anticipation circuit.

60-2485—8

T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

THERMOSTAT

H1/ C1

SUBBASE

SYSTEM COMPONENTS

ANTICIPATOR

(HOT)

FALL

ODT 1

EHR 1

RTD 1

RTD 2

FAN SWITCH

AUTO

ODT 2

EHR 2

ANTICIPATOR

RTD 3

AUX. HT.

LED

GREEN)

RTD 1

(

FALL

EM. HT.

LED (RED)

SYSTEM

MONITOR

SYSTEM

SWITCH

FAN RELAY

EM. HT.

HEAT

OFF

EM. HT.

RELAY

CHANGEOVER

VALVE

POWER SUPPLY. PROVIDE

DISCONNECT MEANS AND

OVERLOAD PROTECTION

AS REQUIRED.

COOL

LACO

CHP

COMPRESSOR

CONTACTOR

M5842

Fig. 20. Tracing the heat 1 anticipation circuit.

THERMOSTAT

SUBBASE

SYSTEM COMPONENTS

H1/C1

ANTICIPATOR

(HOT)

ODT 1

EHR 1

FALL

RTD 1

RTD 2

FAN SWITCH

AUTO

ODT 2

EHR 2

RTD 3

ANTICIPATOR

AUX. HT.

LED

GREEN)

RTD 1

(

EM. HT.

LED (RED)

SYSTEM

MONITOR

FALL

SYSTEM

SWITCH

FAN RELAY

EM. HT.

HEAT

OFF

EM. HT.

RELAY

CHANGEOVER

VALVE

POWER SUPPLY. PROVIDE

DISCONNECT MEANS AND

OVERLOAD PROTECTION

AS REQUIRED.

COOL

LACO

CHP

COMPRESSOR

CONTACTOR

M5843

Fig. 21. Tracing the heat 2, anticipation and AUX. HT. LED circuit.

60-2485—8

T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

THERMOSTAT

SUBBASE

SYSTEM COMPONENTS

(

HOT)

H1/C1

ANTICIPATOR

ODT 1

EHR 1

FALL

RTD 1

RTD 2

FAN SWITCH

AUTO

ODT 2

EHR 2

RTD 3

ANTICIPATOR

AUX. HT.

LED

GREEN)

RTD 1

(

EM. HT.

LED (RED)

SYSTEM

MONITOR

FALL

SYSTEM

SWITCH

FAN RELAY

EM. HT.

HEAT

OFF

EM. HT.

RELAY

CHANGEOVER

VALVE

COOL

POWER SUPPLY. PROVIDE

DISCONNECT MEANS AND

OVERLOAD PROTECTION

AS REQUIRED.

LACO

CHP

COMPRESSOR

CONTACTOR

M5844

Fig. 22. Tracing the emergency heat and EM. HT. LED circuit.

THERMOSTAT

SUBBASE

SYSTEM COMPONENTS

(HOT)

H1/C1

ANTICIPATOR

ODT 1

EHR 1

FALL

RTD 1

RTD 2

FAN SWITCH

AUTO

ODT 2

EHR 2

RTD 3

ANTICIPATOR

AUX. HT.

LED

GREEN)

RTD 1

(

EM. HT.

LED (RED)

SYSTEM

MONITOR

FALL

SYSTEM

SWITCH

FAN RELAY

EM. HT.

HEAT

OFF

EM. HT.

RELAY

CHANGEOVER

VALVE

POWER SUPPLY. PROVIDE

DISCONNECT MEANS AND

OVERLOAD PROTECTION

AS REQUIRED.

COOL

LACO

CHP

COMPRESSOR

CONTACTOR

M5845

Fig. 23. Tracing the cooling circuit.

60-2485—8

T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

THERMOSTAT

SUBBASE

SYSTEM COMPONENTS

(HOT)

H1/C1

ANTICIPATOR

ODT 1

EHR 1

FALL

RTD 1

RTD 2

FAN SWITCH

AUTO

ODT 2

EHR 2

RTD 3

ANTICIPATOR

AUX. HT.

LED

GREEN)

RTD 1

(

EM. HT.

LED (RED)

SYSTEM

MONITOR

FALL

SYSTEM

SWITCH

FAN RELAY

EM. HT.

HEAT

OFF

EM. HT.

RELAY

CHANGEOVER

VALVE

POWER SUPPLY. PROVIDE

DISCONNECT MEANS AND

OVERLOAD PROTECTION

AS REQUIRED.

COOL

LACO

CHP

COMPRESSOR

CONTACTOR

M5846

Fig. 24. Tracing the auto fan circuit.

THERMOSTAT

SUBBASE

SYSTEM COMPONENTS

H1/C1

(HOT)

ANTICIPATOR

ODT 1

EHR 1

ODT 2

FALL

RTD 1

RTD 2

FAN SWITCH

AUTO

EHR 2

RTD 3

ANTICIPATOR

AUX. HT.

LED

RTD 1

(GREEN)

EM. HT.

LED (RED)

SYSTEM

MONITOR

FALL

SYSTEM

SWITCH

FAN RELAY

EM. HT.

HEAT

OFF

EM. HT.

RELAY

CHANGEOVER

VALVE

POWER SUPPLY. PROVIDE

DISCONNECT MEANS AND

OVERLOAD PROTECTION

AS REQUIRED.

COOL

LACO

CHP

COMPRESSOR

CONTACTOR

M5847

Fig. 25. Tracing the fan on circuit.

60-2485—8

T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

Heat anticipators add heat during the call for heat. Cooling

anticipators are activated while the air conditioner is off. See

Fig. 28. This heater makes the thermostat think it is warmer than

it really is, and brings it on sooner than the bimetal alone would.

OPERATION

Heat Anticipation/Cool Anticipation

When controlling a heating unit with a thermostat, the

temperature does not remain exactly at the thermostat

setpoint, but varies within a certain temperature range. Heat

anticipation is added to the thermostat to reduce this range.

HOT)

(

The anticipator is a small resistive heater in the thermostat

that heats when the system is on (heat mode) or off (cool

mode). The heat produced by the anticipator raises the

internal bimetal temperature slightly faster than the

surrounding room temperature. The thermostat anticipates the

need to shut off the heating system sooner than it would if

affected by room temperature only.

RISE

STAGE 1

COOL RELAY

C1 ANTICIPATOR

There are two types of heat anticipation, adjustable and fixed.

M5825

Fig. 28. Internal cooling anticipation schematic.

Adjustable anticipation is also called current anticipation. See

Fig. 26. The heater is in series with the mercury switch and

heating primary. The pointer is adjusted to match the current

draw of the primary control, and the correct amount of heat is

added for proper cycle rates.

Interstage Differential

The mercury switch of each stage of heat makes at a slightly

different temperature; that is, the mercury makes the contacts

of the first stage bulb at one temperature, and the second

stage bulb at another temperature. The difference between

these two temperatures is the interstage differential.

HOT)

(

Interstage differential is the difference between the two make

points when the bulbs are controlled by the heating of the

bimetal and the action of the heat anticipator. The interstage

differential is 1.9°F (1°C) between the stages of heating or the

stages of cooling for most models.

H1 ANTICIPATOR

Mechanical differential is the difference between the make

and break points of each switch. The mechanical differential

for the T874 is 1°F (0.6°C) between the stages of heating or

between the stages of cooling for most models.

STAGE 1 HEAT RELAY

FALL

M5823

Fig. 26. Adjustable anticipation heater in series with load.

Fixed anticipation is also called voltage anticipation. See

Fig. 27. The heater is in parallel with the primary control and is

not affected by the current draw of the primary. It establishes

the proper cycle rate for any application.

Droop

factor called droop.

The addition of anticipator heat to the bimetal causes a control

As the weather grows colder, the heating appliance must

operate longer and more frequently. More heat must be added

to the bimetal. The bimetal now thinks it is warmer than it

really is, and actual room temperature is controlled at a lower

temperature than the setpoint indicates. At 100 percent heat

load, this droop can be significant.

HOT)

(

The C815A Outdoor Reset Thermistor is used in heat pump

systems to minimize droop. Without some method of

countering this situation, the occupant could notice the

temperature offset.

STAGE 1

HEAT RELAY

FALL

H1 ANTICIPATOR

Outdoor Reset

M5824

Some T874/Q674 models are intended for use with the

C815A Outdoor Thermistor. The outdoor thermistor provides

significantly improved thermostat performance when

Fig. 27. Fixed anticipation heater in parallel with load.

compared to conventional multistage thermostats. The C815A

is usually located in the heat pump condensing unit outdoors.

60-2485—8

T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

A strip of four inserts is included with TRADELINE T874. Drop

a strip into the recessed area behind the scaleplate so

selected LEDs show. Make sure insert is completely seated in

recessed area. Let scaleplate pop back; then set levers to

desired position.

CAUTION

Equipment Damage Hazard.

Due to calibration techniques used for T874

Thermostats with outdoor reset, the C815A Thermistor

must be wired into the system at all times. Failure to

do so will result in serious degradation of performance.

•

FILTER LED lights when the filter is clogged and needs

replacement.

CHECK LED lights when something needs to be checked

or done to maintain efficient operation of system. See

heating system instructions for CHECK LED meaning.

EM. HT. LED lights when the emergency heat is operating.

SUPL. HT. LED lights when the supplemental heat is

operating.

Service and Replacement of C815A Outdoor

Thermistor

•

To check and verify thermistor operation, perform the

following steps:

•

LOCKOUT LED lights when the system is shut down and

needs maintenance.

Disconnect wire from T terminal on subbase.

Measure resistance with ohmmeter across the T sub-

base wire and X subbase terminal.

AUXILIARY HEAT LED lights when the auxiliary heat is

operating.

SERVICE or MALFUNCTION LED can have several

meanings. Consult heating system instructions.

Take outdoor temperature measurement at thermistor

location and find correct thermistor resistance on the

chart in Fig. 29.

LEDs cannot be replaced or added in the field.

Compare resistance on the chart with measured resis-

tance. Replace C815A if resistance varies more than 15

percent. Contact installing dealer for packaged replace-

ment outdoor thermistor.

LEDS

EM.

HEAT FILTER SERV. HEAT

AUX

4600

4400

4200

4000

3800

3600

3400

3200

3000

2800

2600

2400

2200

2000

1800

1600

1400

1200

1000

COOL

50 60 70 80

C815A THERMISTOR RESISTANCE

R = 400 ohms ± 10% AT 77°F (25°C)

M5830

Fig. 30. T874/Q674 LED location.

Restricted Setpoint (DoD)

The Department of Defense (DoD) models are equipped with

a restricted setpoint feature for fuel efficiency.

800

600

400

200

Fixed stops are factory-set so setpoint levers cannot be set

above 72°F (22°C) on heating, or below 78°F (26°C) on cooling.

-20

100

120 140

M1590A

TEMPERATURE OF THERMISTOR (°F)

Fig. 29. Thermistor resistance chart.

Applications

The T874/Q674 can be applied to standard residential

systems for automatic or manual changeover, to commercial

rooftop applications, or to heat pump applications.

Features

Two of the features of the T874/Q674 include LED indicators

and restricted setpoint.

Changeover on Standard Residential

Systems

LED Indicators

The light-emitting diodes (LED) indicators on the subbase

light on command when something specific happens in the

system. See Fig. 30.

In a standard residential heating-cooling circuit, changeover

between heating and cooling can be done either automatically

or manually.

Up to four different LEDs are available. The thermostat has a

clear lens window for viewing each LED. On TRADELINE

models, a small insert is used so the LED function desired can

be selected. This must be done during installation.

Automatic changeover is done by an AUTO position on the

subbase system switch. See Fig. 31. When the switch is in the

AUTO position, the thermostat automatically changes between

heat and cool modes, depending on the indoor temperature.

A blank insert is factory-installed in some T874 models. To

remove it, push both temperature setting levers to the far ends

of the thermostat. Use index fingernail to gently pull out the

scaleplate a fraction of an inch. Turn thermostat upside-down,

and the blank insert falls out.

60-2485—8

T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

When enthalpy is low enough, the thermostat and a sensor tell

the economizer damper to open to let in cool air from the

outside. When enthalpy is high, the thermostat and sensor tell

the damper to close. No more outside air comes in, so the air

conditioner is used instead.

OFF EM. HT. HEAT AUTO COOL

Heat Pump Application

M5828

A heat pump system operates much like an air conditioner.

However, the heat pump is capable of moving heat in two

directions; from inside the home to outdoors for cooling, or

from outdoors to inside for heating.

Fig. 31. System switching for automatic changeover.

Automatic changeover is frequently used in areas where there

are large temperature differences in 24 hours. Some uses for

the automatic changeover feature are in the southern states

The heat pump can be controlled by separate relay/contactor

components, which are energized or de-energized by the T874.

(

cool in the morning and hot in the afternoon) and in

commercial applications.

Changeover between heating and cooling for heat pump

circuits can be accomplished either manually or automatically.

Manual changeover requires a system switch movement to

change the mode. See Fig. 32. Most manual changeover

switches consist of HEAT-OFF-COOL choices. For heating,

switch to HEAT. For cooling, switch to COOL. It is not automatic.

Manual changeover control requires a system switch

movement to change the mode. The reversing valve is

activated by moving the system switch to COOL for cooling or

to HEAT for heating operation.

Manual changeover is often used in areas where the

temperature is relatively stable between day and night.

Automatic changeover is accomplished by one of the mercury

bulb switches in the thermostat. On a system with automatic

changeover on cooling, the changeover valve is energized by

a mercury switch that makes on a temperature rise. On

heating, the changeover valve is energized by a mercury

switch that makes on a temperature fall.

A description of the methods used to control heat pumps

follows.

OFF EM. HT. HEAT COOL

M5829

Fig. 32. System switching for manual changeover.

Space Temperature Sensing Low Voltage

Controls

Commercial Rooftop Application

Control systems for heat pumps usually utilize low voltage

control circuits. The variety of control functions required and

the relative complexity require a versatile and economical

method of control. There are several interlocks and indication

functions that would be more difficult and expensive to wire in

line voltage circuits. Low voltage controls have the precision

and flexibility needed.

The T874 can be used as part of a commercial rooftop

application to run mechanical cooling.

An economizer is often used as part of a rooftop application to

save on air conditioning. See Fig. 33.

The thermostat used is a low voltage device that has good

temperature sensitivity and several manual and automatic

switching capabilities.

OUTDOOR

AIR

Two-Stage Thermostat

In most installations, the heat pump itself requires auxiliary

heat, usually in the form of electric strip heaters, to carry the

building heating load at design conditions. Since the heat

pump is usually more efficient, the control strategy is to run

the heat pump whenever possible and the auxiliary heat only

when necessary—during very cold weather or if the heat

pump fails. Usually the auxiliary heat is used only below the

balance point (temperature at which the heat pump cannot

handle heating load by itself).

H205 OR H705

ENTHALPY

CONTROL

RETURN

AIR

DISCHARGE AIR SENSOR

DOWNSTREAM FROM THE

EVAPORATOR COIL

M5827A

Fig. 33. Typical commercial rooftop economizer

application.

A two-stage thermostat makes this possible. The two stages

make sequentially as the temperature drops. There is a

degree or two between stages so that the second stage (the

Enthalpy is the temperature and humidity of the air. A more

accurate analysis of the suitability for free cooling can be

made by monitoring the enthalpy.

60-2485—8

T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

one controlling the auxiliary heat) makes only when the heat

pump alone cannot handle the load. That is how the strategy

of using the most economical heat first is implemented.

This is particularly important on the 3-stage T874. The

additional stage means that more droop is introduced when

the system is in operation. That is the reason the TRADELINE

T874W is sold in a package with the outdoor sensor and the

correct subbase.

Cycling Rate

Normally, thermostat anticipation is set to cycle a fossil fuel

furnace about five or six times an hour (at 50 percent load).

This provides a reasonable balance between comfort, stability

and economy.

Outdoor Reset—How it Works

The heater in the thermostat is connected in series with a

thermistor sensor that measures outdoor temperature. See

Fig. 34. In warm weather, the thermistor has very low

resistance, permitting a large current flow in the heater, which

generates a relatively large amount of heat in the thermostat.

A heat pump, being a mechanical refrigeration system, should

be cycled at a rate of 2-1/2 to 3 times an hour. With a T874,

this is accomplished by setting the anticipator at the total

current draw of the controlled device (the contactor and

possibly the changeover relay or valve). Most likely, the

compressor operates from the first stage of the thermostat.

In cold weather, the outdoor sensor has high resistance,

which results in low current flow and little or no heat being

added to the thermostat. This raises the control point,

overcoming the effects of droop and interstage differential.

The second stage normally controls the auxiliary heaters. This

heat anticipator can be set for the current draw of the electric

heat primary, and produces good control.

HOT)

(

Voltage Anticipation

Although not a specific requirement for heat pumps, several

models of the T874 have voltage heat anticipation instead of

current anticipation.

C815A

RESET

HEATER

OUTDOOR

THERMISTOR

SENSOR

With voltage anticipation, the amount of heat added to the

thermostat is constant no matter which load is connected to it.

The anticipator realizes a constant voltage and produces a

constant amount of heat during the ON cycle. This allows the

cycling rate to be designed into the thermostat. The heat

pump manufacturer is assured of correct performance

independent of later thermostat adjustments.

M5826

Fig. 34. Location of thermistor.

Changeover

accomplished either automatically or manually, depending on

the application. Changeover can occur in heating or cooling.

Outdoor Reset

Changeover between heating and cooing can be

Outdoor reset, which a C815A Outdoor Thermistor makes

possible, is sometimes applied in heat pump systems to help

minimize the effect of the differential between stages. It takes

°F (1.1°C) change in temperature to bring on the second

The changeover valve is energized as long as the subbase

function switch is in the appropriate position when manual

changeover is used. If a system manually changes over on

cooling, the valve energizes whenever the subbase switch is

in the cooling mode.

stage and full heating. Added to a certain amount of droop

under high loads, there can be a large offset between the

setpoint and room temperature.

Without some method to counteract this situation, the

temperature offset from the setpoint could be as much as 6°F

One of the mercury bulb switches is used for control when the

thermostat has automatic changeover. When the automatic

changeover is in cooling, the changeover valve is energized

when the first stage cooling switch makes. The second stage

switch turns on the compressor.

(

3.3°C) under high load conditions. It is not likely that the

occupants accept this variation without making frequent

thermostat setting changes.

Outdoor reset has the effect of raising the setpoint at cold

outdoor temperatures. Then, even with a few degrees droop,

the space temperature stays near the desired setting.

It is more common with new equipment to find that

changeover occurs with cooling; that is, the changeover valve

is energized to produce