Bryant Q674 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

•

Adapter plate available for mounting Q674 on wall or

horizontal outlet box.

These thermostats and subbases provide low voltage control

of multistage heating and cooling systems, including heat

pump systems.

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

SPECIFICATIONS

T874 THERMOSTAT

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

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.

applications.

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 set-

point protection.

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.

Q674

System

Heat-Auto-Cool

Heat-Off-Cool

Fan

Auto-On

None

Off-Auto

None

Off-Heat-Auto-Cool

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

Off-Auto

Auto-On

None

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.

Includes adjustable temperature locking stops.

T874D replaces T874A-F TRADELINE or standard models.

•

TRADELINE package with cross reference label and

special instruction sheet.

•

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.

0-2485—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.

93121A 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:

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

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.

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

and screws.

68°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

72°F (22°C) maximum and cooling setpoint to 78°F (26°C)

minimum.

OEM customer personalization.

Locking cover and locking lever (see Thermostat Accesso-

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.

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 set-

tling 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.

94559R 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.

T872B

T872C

Standard

—

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

—

T874C—Standard and TRADELINE.

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

—

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.

T872E

T872F

2-Stage Cool

2-Stage Heat

—

0-1.2A 0-1.0A

T874F—Standard and TRADELINE.

—

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

0-2485—8

T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

—

Fast cycling.

Electric Heat

0.12-

.6A

0.12-

0.6A

T874G—Heat pump, cool

changeover, with fast cycling.

T872G Heat Pump or

Standard

₁b

₀_-₁_.₀_Ac

0.1-1.2A 0-1.0A

—

Fixed anticipator for H2.

Fast cycling.

0.1-1.5A

0.12-

.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_.₅_Ac ₀_-₁_.₀_Ac

—

0.1-1.2A

₀_-₁_.₅_Ac

0-1.5A

None

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

changeover.

T872N

₁b

T874P—Heat pump.

None

Heat Pump

Standard

—

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

—

T874Q—Night setback heating.

T872Q

₁e

—

0.1-1.2A

—

T874R—Heat pump.

T872R

None

_H_e_a_t_P_u_m_pf

—

₀_-₁_.₅_Ac

0.1-1.2A 0-1.5A

—

T874S—Two-speed compressor heat

pump.

Heat Pump

₁g

₀_-₁_.₂_Ac

0-1.2A 0-1.0A

T874V—Standard.

None

Standard

—

0-1.5A

—

T874W—Heat pump and standard.

T872W Heat Pump or

Standard

_-₁_.₂_Ab ₀_-₁_.₂_Ac,d

0-1.2A 0-1.0A

—

Night setback heating.

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:

See Table 4.

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).

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

HEAT-AUTO-COOL AUTO-ON

Models and Options

Replaces Application

Fan

Use With

Q674A—Standard and TRADELINE®.

Q672A

Q672B

Standard

T874A-D

—

Indicator LEDs.

Q674B—Standard and TRADELINE.

Standard

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.

Heat Pump

OVERRIDE-HEAT-

OFF-COOL

OFF-AUTO

OVERRIDE-AUTO-

OFF

Q674C—Standard and TRADELINE.

Indicator LEDs.

Q672C

Q672D

Standard

AUTO-ON

T874A-H,W

T874K,L,N

T874P

—

Heat Pump

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-COOL

—

EM.HT. light.

—

Provision for AUTO fan operation in

EM.HT.

OFF-COOL-AUTO-

HEAT-EM.HT.

AUTO-ON

Q674G—O and B terminals.

Q674J—Provision for AUTO fan operation

in EM.HT.

Q672G

Q672J

Standard

OFF-AUTO

None

T874A-F

Heat Pump

EM.HT.-AUTO-OFF

EM.HT.-ON-OFF

SUPL.HT.-ON-OFF

AUTO-ON

None

T874A,D,G,J

—

Provision for outdoor thermistor.

Q674K—Standard.

Q672K

Q672L

Standard

OFF-HEAT-AUTO-

COOL

T874F

OFF-WOOD-WOOD/

OIL-OIL

None

Q674L—Provision for AUTO fan operation in

EM.HT.

Heat Pump EM.HT.-HEAT-OFF- AUTO-ON

COOL

T874R,W

—

Indicator LEDs.

SUPL.HT.-HEAT-

OFF-COOL

AUTO-ON

Q674N—Standard.

Q674P—Standard.

Q674Q—Standard.

Q674R—Standard.

Q672N

Q672P

None

Evaporative

Cooler

EVAP-COOL-OFF-

HEAT

T874C

T874G

T874A

T874V

T874C

Heat Pump

SUPL.HT.-HEAT-

COOL

Fan Coil

HEAT-OFF-COOL LO-MED-HI-

None

Fan Coil

OFF-COOL

LO-HI-

CONT.

—

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.

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.

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.

T874 Thermostat with

One Setpoint Lever

T874 Thermostat with

Separate

Q674 Subbase

with Four LEDs

Heating and Cooling Levers

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

0-2485—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

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.

If this control is replacing a control that contains

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

in the trash.

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

careful that the wires do not fall back into the wall

opening. 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.

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

screws provided. Do not fully tighten the subbase

screws.

Read these instructions carefully. Failure to follow them

could damage the product or cause a hazardous condi-

tion.

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

BOX

MOUNTING

SCREWS (2)

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

perature control to deviate from setpoint. It is not a

calibration problem.

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.

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.

radiant heat from the sun, fireplace, or appliances.

Fig. 4. Installing Q674 Subbase on outlet box.

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.

0-2485—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

H1, R3

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

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

Alternate Designations or

Standard Terminal

Designation

Customer Specials

Typical Connection

R1, R2

External temperature readout, T relay

LO and HI speed fan relays

Cooling contactor

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.

When installed, the stop brackets limit the movement of the

T874 HEAT and COOL levers.

Outdoor Disconnect

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.

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.

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

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.

0-2485—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.

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

ALLEN

at a specific temperature, use the two insulated head

screws supplied instead of the two adjustable lever stop

brackets.

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

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).

Equipment Damage Hazard.

Carefully remove and discard the polystyrene packing

insert that protects the mercury switches during ship-

ment.

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.

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.

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:

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

60-2485—8

T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

anticipator at 140 percent of the actual primary con-

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

11.

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.

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

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.

HEAT—heating system is automatically controlled by the

thermostat. Cooling system is off.

AUTO—thermostat automatically changes between heat-

ing and cooling system operation, depending on the

indoor temperature.

STAGE ONE

ANTICIPATOR

HEATING

COOL—cooling system is automatically controlled by the

CONTROL

thermostat. Heating system is off.

EM.HT.—emergency heat relay is automatically controlled

by the thermostat. Cooling system is off. Compressor is

de-energized.

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.

.2.8

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

STAGE TWO

ANTICIPATOR

HEATING CONTROL

stage.

MOVE INDICATOR TO

MATCH CURRENT RATING

OF PRIMARY CONTROL

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.

M5069

Fig. 11. Adjustable heat anticipator scales.

Temperature Setting

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

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

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

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.

°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.

LO—fan operates constantly at low speed.

MED—fan operates constantly at medium speed.

HI—fan operates constantly at high speed.

STAGE 1

HEATING

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.

HEATING

LEVER

COOLING

STAGE 3

LEVER

STAGE 1

COOLING

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.

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).

0-2485—8

T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

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

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.

Verify the Adjustment

Heating

HEATING SET-

POINT LEVER

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.

TENSION

HEAT

SCREW

50 60 70 80

FIRST

STAGE

SWITCH

Cooling

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

SCALE

SECOND STAGE

ADJUSTABLE)

SWITCH

temperatures is the interstage differential, which should match

the number set on the scale with the tension screw.

(

HEATING

TENSION

SCREW

CHECKOUT

ALIGN LOWER

EDGE WITH

SCALE

Heating

SLIDE LEVER

NOTCH

WIDER APART

FOR LARGER

DIFFERENTIAL

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.

10°F

SCALE

COOLING

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.

TENSION

SCREW

ALIGN LOWER

EDGE WITH

SCALE

SLIDE LEVER

CLOSER TOGETHER

FOR LARGER

NOTCH

4°F

DIFFERENTIAL

SCALE

Cooling

CAUTION

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

M937

Equipment Damage Hazard.

Do not operate cooling if outdoor temperature is below

Fig. 13. Set adjustable interstage differential.

0°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)

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

not used, thermostat performance deviates radically from

proper operation.

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.

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.

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

Fig. 14. Thermistor resistance chart.

CALIBRATION

Thermostat

T874 Thermostats are accurately calibrated at the factory.

They do not have provision for field calibration.

0-2485—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.

H2—Stage 2 Heating.

H3—Stage 3 Heating.

C1—Stage 1 Cooling.

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.

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:

(

LOW AMBIENT CUTOFF

heat or cool).

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.

0-2485—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.

0-2485—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.

0-2485—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

The addition of anticipator heat to the bimetal causes a control

factor called droop.

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

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.

Equipment Damage Hazard.

•

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.

AUXILIARY HEAT LED lights when the auxiliary heat is

Disconnect wire from T terminal on subbase.

Measure resistance with ohmmeter across the T sub-

base wire and X subbase terminal.

Take outdoor temperature measurement at thermistor

location and find correct thermistor resistance on the

chart in Fig. 29.

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.

operating.

SERVICE or MALFUNCTION LED can have several

meanings. Consult heating system instructions.

LEDs cannot be replaced or added in the field.

LEDS

EM.

HEAT FILTER SERV. HEAT

AUX

600

400

200

000

800

600

400

200

000

800

600

400

200

000

800

600

400

200

000

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.

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

In a standard residential heating-cooling circuit, changeover

between heating and cooling can be done either automatically

or manually.

LED Indicators

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

light on command when something specific happens in the

system. See Fig. 30.

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.

0-2485—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

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.

Commercial Rooftop Application

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

Outdoor Reset

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

Changeover between heating and cooing can be

accomplished either automatically or manually, depending on

the application. Changeover can occur in heating or cooling.

°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 cooling.

This is done by calibrating the thermostat high by 5°F (3°C)—

the amount of expected offset. This offset compensates for

differential and droop at high load factors so the thermostat

switches at the setpoint. At warm temperatures, heat is added

to the thermostat to compensate for this offset; at cold

temperatures, the extra heat is automatically removed. The

effect is the same as raising the setpoint as the outdoor

temperature goes down.

Whether automatic or manual changeover is used and

whether the reversing valve is energized with cooling or with

heating, a heat pump system can be controlled by the

appropriate T874 Multistage Thermostat.

0-2485—8

T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

Low Temperature Lockouts

THERMOSTAT

HEAT 2

Some manufacturers of heat pumps have previously controlled

their units so the compressor was locked out below a specified

outdoor temperature. This was done because of the stress

placed on the compressor by the very cold temperatures, and

the fact that efficiency drops off at low temperatures.

HEAT 1

COMP.

STAGE 1

Other manufacturers say that although efficiency is low at cold

temperatures, the problem of starting the cold compressor is

the most critical point. It is more important to keep the

compressor running than to shut it off and try to restart it when

the temperature warms up to +10°F (-12°C). Also the

crankcase heater, which can run only when the compressor is

off, compensates for the lower compressor efficiency. So, it is

equitable to let the heat pump run even with a coefficient of

performance (COP) slightly less than one, rather than to turn it

off and have to run the crankcase heater.

COMP.

STAGE 2

OUTDOOR

THERMOSTAT

AUXILIARY

HEAT

M5838

Fig. 35. Two-stage heat pump with two-stage thermostat.

The outdoor thermostat is set at the balance point with both

heat pump stages running. So if the heat pump can control the

heating load down to +20°F (-7°C), the auxiliary heat comes

on with the second stage of the compressor.

NOTE: To determine the COP of a heat pump, use the fol-

lowing formula:

Btu Out/Btu we pay for or Btuh Capacity

COP =

Unit Wattage x 3.413 Btu/Watt

In effect, this control strategy changes the heat input of the

second stage, depending on the amount of heat needed.

Additional stages of auxiliary heat can be added with or

without the control of more outdoor thermostats, See Auxiliary

Heat section.

Another factor favoring this control strategy is that newer heat

pump designs maintain a level of efficiency even at outdoor

temperatures well below zero. Some brands do not reach

1.0 COP until -25°F (-32°C).

Virtually all manufacturers now let the heat pump run

continuously rather than lock it out at cold temperatures.

Another way to control a two stage heat pump is with a three-

stage thermostat. The T874W is designed for this application

as well as others. See Fig. 36.

Heat Pumps with Dual Compressors

Staging residential sized heat pumps is a requirement that

developed from the need to make heat pumps as efficient as

possible. One of the methods used to improve overall

seasonal efficiency is to reduce the amount of time the

machine is operating in a transitional mode. Transitional mode

is starting and stopping in addition to recovering from defrost.

Under light loads, when the heat pump is cycling on and off,

this can be a significant amount of time and can result in a

significant efficiency reduction. Capacity control is one way to

reduce the cycling rate and improve the efficiency.

The advantage of this system is that no auxiliary heat is used

until there is an actual demand from the controlled space.

THERMOSTAT

HEAT 3 HEAT 2 HEAT 1

COMP.

STAGE 1

COMP.

STAGE 2

Additional capacity control methods used on other

refrigeration systems (unloading, multiple compressors) are

not as likely to be seen on residential heat pumps.

AUXILIARY

HEAT

With a single-stage heat pump, the compressor is normally

controlled by the first stage and the auxiliary heat by the

second stage. The T874G, N and R Thermostats are used for

this purpose.

M5837

Fig. 36. Two-stage heat pump with three-stage

thermostat.

With a two-stage compressor, the two thermostat switches

control the individual compressor stages. Auxiliary heat is

controlled along with the compressor stage-two, by the

thermostat second stage. It comes on with the compressor

high speed if an outdoor thermostat—wired in series with it—

is made. See Fig. 35.

Two speed Heat Pump

One way to control capacity is to use a two-speed

compressor. The compressor runs at low speed for stage-one

heating and cooling. The compressor runs at the low speed

instead of cycling the heat pump on and off under some light

load conditions.

The high speed is stage-two cooling. The compressor runs on

high speed when loads increase so the low speed can no

longer handle the cooling load. This reduces cycling rate and

improves capacity.

60-2485—8

T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

Defrost Control

(HOT)

In some regions, during half or more of the heating season,

the outdoor coil operates below 32°F (0°C) Frost or ice builds

up on the outdoor coil of a heat pump similarly to the frost

buildup in a household refrigerator. Eventually, this

TRANSFORMER

accumulation of ice interferes with efficient heat transfer from

the outdoor air to the coil and refrigerant. Defrosting is

occasionally required to remove this ice, and restore the heat

pump ability to absorb heat from the air.

THERMOSTAT

HEAT 2

DEFROST

RELAY

CONTACT

HEAT 1

AUXILIARY

HEAT RELAY

Defrosting

A heat pump defrosts its outdoor coil by temporarily switching

to the cooling mode, which causes hot gas from the

compressor to be directed to the outdoor coil instead of to the

indoor coil so the heat pump is taking heat from the home to

warm up the outdoor coil. Defrosting is the greatest detriment

to heat pump efficiency.

M5835

Fig. 38. Auxiliary heat with defrost control.

WHEN TO DEFROST

Timely defrosting is an essential component of effective heat

pump operation. Failure to defrost often enough permits too

much ice to accumulate on the coil. At the very least, this

hurts efficiency; at worst, it results in compressor damage.

Insufficient defrosting is a condition the heat pump

manufacturer wants very much to avoid.

Defrost Control Functions

Besides changing over to the cooling mode, defrosting

requires several more control initiated actions that follow.

OUTDOOR FAN

A 50 percent reduction in outdoor airflow is the maximum that

would be tolerated. So the designer of a heat pump would

select a control point that puts the system into defrost when

airflow through the outdoor coil approaches half its normal

level. Restricted airflow causes a greater load on the

compressor; the outdoor coil runs colder, suction pressure is

lower and the motor runs hotter.

When defrosting has begun, it is standard practice to turn off

the outdoor fan to speed up the melting process. A separate

defrost relay is required to control that fan. See Fig. 37. Other

contacts can be needed on the defrost relay to power the

changeover valve or power part of the auxiliary heat.

COMPRESSOR

CONTACTOR

MOTOR

At the other end of the scale, defrosting too often hurts the

overall energy efficiency of the system. Consider that in terms

of heating the building, defrosting is a big loss. Not only does

the system stop heating, but it actually moves heat out of the

building. If electric strip heat is used, it is a further waste

because its COP is 1.0 and not the 2.0 or more COP realized

if the heat is provided by the heat pump.

CONTROLLED BY

OUTDOOR

So, concern for equipment safety suggests fairly frequent

defrosting while economy of operation argues for fewer

defrost cycles. Since the equipment manufacturer chooses, the

balance is usually tipped in favor of more frequent defrosting to

avoid the possibility of damaging the compressor.

DEFROST RELAY

FAN MOTOR

M5839

Fig. 37. Outdoor fan with defrost control.

AUXILIARY HEAT DURING DEFROST

Auxiliary Heat

Most pump manufacturers bring on some auxiliary heat during

defrost, although this is not a universal practice. Some rely on

the second stage of the thermostat to call for auxiliary heat as

needed. Since the heat pump is operating in the cooling mode

to defrost, it is delivering cold air to the living space. Auxiliary

heat is used to offset this cooling. It requires another normally

open contact on the defrost relay. See Fig. 38.

Except in warm climates, all air-to-air heat pump installations

require auxiliary heat capability. Electric resistance heaters

can provide this auxiliary heat.

The electric heaters (sometimes called strip heaters) usually

are supplied in 5 kW units or strips (about 17,000 Btu). The

indoor unit of the heat pump is designed to accommodate

various electric heat units so it can be used in variously sized

buildings in different parts of the country.

Two-Stage Thermostat

The most common heat pump control strategy is for the

thermostat first stage to switch the compressor and the

second stage to switch the auxiliary heat. See Fig. 39.

0-2485—8

T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

The disadvantage of this system is that it can require several

outdoor thermostats. Also, since the unit is operating most

often at nearly full capacity, the thermostat is on longer which

results in more droop, with a little more offset from the

thermostat setpoint.

THERMOSTAT

HEAT 2 HEAT 1

In addition, the capacity is tied to theoretical load, based on heat

loss calculations. Normally this is very close to the actual load,

but there are times where the second stage is switched on too

soon (no harm done) or too late (the building gets cold).

COMPRESSOR

CONTRACTOR

CONTROLS

AT 65°F (18°C)

AUXILIARY

CONTROLS

HEAT RELAY

AT 63°F (17°C)

The T874 Thermostat produces time modulated control, which

is the on time of the thermostat changing during each cycle as

the load changes. The on time of the heating system is

directly proportional to the heating load of the building. As an

example of the quality of time modulated control, consider the

typical gas furnace. It can have a capacity of 100,000 Btu

M5836

Fig. 39. Auxiliary heat in two-stage thermostat.

This method keeps the auxiliary heat off until the heating load

is large enough to demand 100 percent heat pump operation.

That demand is measured by the thermostat and is the actual

heat requirement of the space. The two-stage thermostat

requires about a 2°F (1.1°C) room temperature drop to bring

on the second stage.

(

293 kW), which is controlled On-Off. The output is 100,000 Btu

(293 kW) or nothing. A quality thermostat controls so closely that

the occupants rarely detect changes in room temperature.

The same is true with 50,000 or 60,000 Btu (146.5 or

78.8 kW) of heat pump auxiliary electric heat. The T874 can

provide the same high quality control.

Stage or Time Modulated Control

There are two different ways to control auxiliary heat, staged

or time modulated control.

This scheme also eliminates the need for outdoor thermostats

and thus offers the manufacturer an opportunity to reduce unit

cost. One disadvantage is that a thermostat jiggler can be

demanding more kW of strip heat when chilly.

Outdoor thermostats are used to switch additional increments

of electric heat into the thermostat circuit as the outdoor

temperature gets lower. All increments of auxiliary heat are

still controlled by stage-two of the thermostat but only if the

outdoor temperature requires the additional capacity. See

Fig. 40.

The method any particular manufacturer selects depends on

which arguments it finds persuasive and the opinions of its

distributors and dealers.

Three-Stage Thermostat

A definite improvement in quality of control and economy can

be achieved with a three-stage thermostat (T874W), which is

especially true with heat pumps using dual compressors.

THERMOSTAT

HEAT 2 HEAT 1

Three-stage control permits both the heat pump and the

auxiliary heat to be tied directly to the demand of the

controlled space. With a two-stage compressor, maximum

COMPRESSOR

CONTACTOR

(

100 percent) heat pump operation is required before auxiliary

AUXILIARY

HEAT RELAY 1

heat is brought on.

20°F

(

-7°C)

Emergency Heat

OUTDOOR

THERMOSTATS

AUXILIARY

HEAT RELAY 2

Some provision is generally required to back up the compressor

in the event of a failure during the heating season. In fact, it is

quite common to find that local building codes or electric utilities

require that a specified percent of the building heating

°F

(-15°C)

AUXILIARY

HEAT RELAY 3

requirements be available from emergency heat—a source

other than the heat pump compressor. Almost universally, the

source is electric resistance heaters. Typically, the requirement

is for 70 or 80 percent of the building heating needs.

M5834

Fig. 40. Outdoor thermostats for auxiliary heat.

The rationale for this system is that it very closely matches the

system capacity to the heating load of the building. By

calculating the heat loss of a building, the installing contractor

can pick the theoretical outdoor temperature at which to

permit each additional unit of electric heat to be operated by

the second stage of the room thermostat.

Switching to Emergency Heat

Conventional heat pump control (thermostat logic) usually

includes a manual subbase switch to bring on the emergency

heat.

In one control strategy, the emergency heat relay is turned on

by the EM.HT. selector switch on the thermostat subbase. The

compressor is prevented from running. The electric heaters

are cycled as they normally would be by the second stage of

60-2485—8

T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

the thermostat. A set of contacts on the emergency heat relay

is used to bypass each outdoor thermostat so that all electric

heat is under immediate thermostat stage-two control. So the

emergency heat relay simply bypasses the outdoor

thermostats. See Fig. 41.

Crankcase Heat

In any mechanical refrigeration system, refrigerant tends to

migrate to the coldest part of the equipment and condense

there. In a heat pump, this coldest spot could be the compressor

because it is usually outdoors. In addition, the oil in the com-

pressor crankcase is capable of absorbing a high concentration

of refrigerant. On compressor startup, there are two risks: (1) the

liquid refrigerant, which is noncompressible, can be drawn into

the cylinder, and (2) considerable oil can be entrained in the

refrigerant and swept out of the crankcase through the

In the second method, the emergency heat relay is cycled

directly by the thermostat mercury bulb. In this hookup, the

relay is controlled by the thermostat first stage. The

emergency heat relay energizes all of the auxiliary heat relays

on a call for heat. See Fig. 42.

changeover valve and dispersed in the piping and coils.

So the emergency heat is controlled by the first stage of the

thermostat. The emergency heat relay energizes all of the

electric strip heaters so the heat and the indoor fan come on

together as soon as the first stage calls for heat.

So the migration of refrigerant to the crankcase is undesirable

because it can interfere with system lubrication and because the

liquid refrigerant must be kept from going through the compressor.

Adding a crankcase heater creates enough temperature

differential to prevent this migration because the compressor

is no longer the coldest part of the system so refrigerant does

not migrate to it.

With the first method, the fan remains on (being controlled by

the first stage of the thermostat) and the electric heaters cycle

on and off with the second stage.

In either case, a subbase indicator light is turned on whenever

the function switch is in the emergency heat (EM.HT.) position.

Compressor Fault Relay

Another very simple method of detecting a compressor failure

has been developed. It uses the voltage developed across the

capacitor in a permanent split capacitor compressor motor.

THERMOSTAT

FUNCTION

HEAT 2 HEAT 1

SWITCH

COMPRESSOR

The principle involved is that virtually all serious problems

reduce the voltage across the motor capacitor. By carefully

selecting the correct coil voltage, the normally closed relay

does not pull in unless the proper voltage builds up across the

capacitor, which brings on the SERVICE LED. See Fig. 43.

AUXILIARY

HEAT RELAY 1

OUTDOOR

THERMOSTAT

AUXILIARY

HEAT RELAY 2

Here is how it works:

•

On a call for cooling, the thermostat makes R to Y, the

contactor coil is powered, and the compressor motor turns on.

At the same time, the indicator light is energized.

As the motor gets up to speed—about one second, the

fault relay sees enough voltage to pull in. Its normally

closed contact opens and the indicator light goes out.

Anything that prevents the motor from running prevents the

fault relay from pulling in.

EMERGENCY HEAT

•

RELAY CONTACT

EMERGENCY

HEAT RELAY

M5833

•

Fig. 41. Emergency heat relay that bypasses

the outdoor thermostats.

COMPRESSOR MOTOR

FAULT RELAY

24 V

RUN

CAP.

THERMOSTAT

FUNCTION

HEAT 2 HEAT 1

SWITCH

EMERGENCY

HEAT RELAY

CONTACT

START

COMPRESSOR

AUXILIARY

HEAT RELAY

(

HOT)

LED

EMERGENCY

HEAT RELAY

RISE

M5832

CONTACTOR

THERMOSTAT-

SUBBASE

Fig. 42. Emergency heat relay that energizes all of the

auxiliary heat relays on a call for heat.

M5831

Fig. 43. Compressor fault relay schematic.

0-2485—8

T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

The Description column identifies certain characteristics

of the existing device.

The Trade Replacement column identifies the correct

replacement device.

The Remarks column lists any characteristics or adjust-

ments to be made on the Trade replacement model.

The Fig. No. column lists the wiring hookup used for the

specific model number. The wiring diagrams follow the

Cross Reference section.

CROSS REFERENCE

The following section lists the current T874 Trade replace-

ments for T872 and T874 Multistage Thermostats. It also lists

the Q674 Trade replacements for Q672 and Q674 Subbases.

All thermostats listed carry the Honeywell logo unless

otherwise noted.

Refer to the circuit illustration that corresponds with the

appropriate number. This figure illustrates the internal

electric circuit for the desired model number.

When using the Cross Reference Charts, refer to the following

instructions:

Locate and identify the existing thermostat and subbase

model.

Refer to the Model Number column to find the model

that matches the existing thermostat and subbase.

Table 6. T872-T874 Thermostat Cross Reference.

T874 Trade

Model Number

T872A

Description

Replacement^a

Remarks

1-stage heat, 1-stage cool

A1006

A1014

Standard OEM

T874A1036

T874A1010

72-78°F (22-26°C) stop; locking cover; no

thermometer

Use T874A1176 for Dept. of

Defense (DoD) application.

A1022

A1030

A1048

A1055

A1063

A1071

A1089

A1097

A1105

A1113

A1121

A1139

A1147

A1154

A1162

A1170

A1188

A1196

A1204

A1212

Heat anticipator set at 0.45A

0.4A heat anticipator setting

TRADELINE

T874A1036

T874A1010

AAF logo; locking cover

Climatrol logo

Lennox logo; 0.5A heat anticipator setting

Melco logo

Westinghouse logo

Amana logo; 1.2A heat anticipator setting

Trane logo

Airtemp (Chrysler) logo

Singer logo; no. 02005384

American Standard logo

Carrier logo and color; no. HH07AT074

General Electric logo; no. AY28X097

Sears logo; no. 4291441

Command-Aire logo

Singer logo; Celsius scale; no. 02005521

Carteret logo

General Electric; no. AY28X114; 72-78°F

Use T874A1176 for Dept. of

Defense (DoD) application.

(

22-26°C) stop

A1220

A1238

A1246

Singer logo; no. 050240-01

Celsius scale

T874A1036

TRADELINE; locking cover and adjustable locking T874A1150

lever stops

A1253

A1261

Trane Comfort Corp

T874A1036

T874A1176

TRADELINE; 72-78°F (22-26°C) stop; locking

cover; no thermometer; meets DoD specs

When replacing a T872 Thermostat and Q672 Subbase, always replace both the thermostat and the subbase.

60-2485—8

T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

Table 6. T872-T874 Thermostat Cross Reference.

T874 Trade

Model Number

Description

Replacement^a

Remarks

A1279

General Electric logo; no. AY28X114A; 75°F stop; T874A1010

locking cover; no thermometer.

Use T874A1176 for Dept. of

Defense (DoD) application.

A1287

General Electric; no. AY28X097A; fan current

shunt for auto fan in heating; heat anticipator set at Q674B1018

.4A.

T874A1036/

Set thermostat stage 1 heat

anticipator for total of fan and stage

1 primary control current draw.

A1295

A1303

A1311

A1329

A1337

A1345

A1352

A1360

A1378

Lennox; Celsius scale

T874A1036

T874A1176

T874A1010

T874A1036

Lennox logo; no. 64A0601; meets DoD specs

Envirotron special; Celsius scale

Carrier logo; Celsius scale; no. HH07AT074C

Square D; no. HCD-3; heat anticipator set at 0.2A T874A1036

TRADELINE; Celsius scale

Heat anticipator set 1.2A

Friedrich logo; heat anticipator set at 0.2A

Wesco logo; heat anticipator set at 0.8A

1-stage heat, 2-stage cool

Standard OEM

T874A1010

T874A1036

T872B

B1005

B1013

B1021

B1039

B1047

B1054

B1062

B1070

B1088

B1096

B1104

B1112

B1120

B1138

T874B1019

0.4A heat anticipator setting

TRADELINE

Westinghouse logo

American Standard logo

Climatrol logo

Carrier logo

Trane logo

Fedders logo

Climatrol logo

Carteret logo

Singer logo; no. 050240-03

Celsius scale

Singer logo; no. 02005711; locking cover and

lever.

B1146

B1153

Lennox logo; no. P-8-8895

Lennox logo; Celsius scale

2-stage heat, 1-stage cool

Standard OEM

T874B1019

T872C

C1004

C1012

C1020

T874C1018

T874C1141

Heat anticipator set at 0.4A

72-78°F (22-26°C) stop; locking cover; no

Use T874C1141 for Dept. of

Defense (DoD) application.

thermometer; 42-88°F (6-31°C) setpoint range.

C1038

C1046

C1053

C1061

C1079

TRADELINE

T874C1018

American Standard logo

Friedrich logo

Lennox logo; no. P-8-8896

Westinghouse logo; stage 1 heat anticipator set at T874C1018

.75A; stage 2 anticipator set at 1.2A

C1087

Fast cycle performance on both heat stages;

anticipator range 0.12-0.6A

T874C1018

Adjust anticipator faster for correct

performance.

C1095

Payne logo; no. 0.1-0.175

T874C1018

When replacing a T872 Thermostat and Q672 Subbase, always replace both the thermostat and the subbase.

0-2485—8

T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

Table 6. T872-T874 Thermostat Cross Reference.

T874 Trade

Model Number

C1103

Description

Amana logo; anticipator set at 1.2A

Airtemp (Chrysler) logo

Replacement^a

Remarks

T874C1018

C1111

C1129

Day and Night logo; no. 0.1-0.174

Fast cycle performance on both heat stages

C1137

Adjust anticipator faster for correct

performance.

C1145

C1152

C1160

C1178

C1186

C1194

C1202

C1210

C1228

C1236

C1244

C1251

C1269

C1277

C1285

C1293

C1301

C1319

C1327

C1335

C1343

C1350

C1368

Rheem logo

T874C1018

T874C1125

Ruud logo

Climatrol logo

Wesco logo

Sears logo

First Co. logo

Carrier logo

Carrier logo; Celsius scale

0.4A heat anticipator setting

Carrier logo; no. HH07AT070

Fraser-Johnson logo

Trane logo

Fedders logo

Climatrol special

Autogas logo

Fedders logo

Climatrol logo

Command-Aire logo

Melco logo; lock cover and levers

Singer logo (Carteret)

Singer logo; no. 050240-04

Celsius scale

12°F (67°C) differential between heat 1 and

heat 2

C1376

C1384

C1392

AAF logo; locking cover; 72-78°F (22-26°C) stop T874C1018

Servel logo T874C1018

TRADELINE; fast cycle performance on both heat T874C1018

stages.

Adjust anticipator faster for correct

performance.

C1400

72-78°F (22-26°C) stop; locking cover; no

thermometer; meets DoD specs

T874C1141

C1418

C1426

C1434

C1442

C1459

C1467

C1475

C1483

C1491

Solaron logo

T874C1018

None

Lennox logo; no. 42C04; four LEDs

Carrier logo; no. HH07AT070C; Celsius scale

Goettl logo

T874C1018

None

Goettl logo; used with Q672N1001, N1019

Lennox logo; Celsius scale

Tappan logo; no. 080-13708-37

Fasco logo; no. 1230-8161

T874C1018

T874C1141

Lennox logo; no. 64A0501; locking cover; no

thermometer; meets DoD specs

When replacing a T872 Thermostat and Q672 Subbase, always replace both the thermostat and the subbase.

60-2485—8

T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

Table 6. T872-T874 Thermostat Cross Reference.

T874 Trade

Model Number

Description

Replacement^a

Remarks

C1509

Fedders logo; used with Q672L1102

Y594R1425

Wiring differences (old=new): C=X,

K=E; remove Y-W1 jumper.

C1517

Climatrol logo; used with Q672L1102

Y594R1425

Wiring differences (old=new): C=X,

K=E; remove Y-W1 jumper.

C1525

C1533

C1541

Square D logo; no. HCD-2

Airtemp logo

T874C1018

Y594R1425

Airtemp logo; used with Q672L1102

Wiring differences (old=new): C=X,

K=E; remove Y-W1 jumper.

C1566

C1574

C1582

C1590

TRADELINE; Celsius scale

T874C1018

None

Replacement has °F scaleplate.

Whalen special; used with Q672B1269

Lennox; meets DoD

None

Singer; 8°F (4.4°C) differential between stage

T874C1125

Replacement has 2°F (1° C)

differential between H1 and H2.

and stage 2

C1608

Singer logo; no. 02005784 8°F (4.4°C) differential T874C1125

between stage 1 and stage 2

Replacement has 2°F (1°C)

differential between H1 and H2.

C1616

C1624

C1632

C1640

C1665

C1673

Celsius scale; stop set at 23°C

None

Add 4074ECK adjustable stop.

Heat Wave logo; no. 0360-116

T874C1018

York logo; heat anticipator set at 0.2A

Heat anticipator set at 0.4A

Wesco logo; heat anticipator set at 0.8A

Wards logo; used with Q672B1285, B1293

Gas systems:

T874C1018/

Q674B1034

Electric systems: Adjust anticipator faster for correct

T874C1018/

Q674B1075

performance.

C1681

C1699

Lechauffage Urbain logo; Celsius scale

T874C1018

Duotherm logo; used with Q672E1068 or

Q672F1000

See subbase description for

replacement information.

C1707

C1715

Sigler logo; used with Q672N1027

Westinghouse logo

2-stage heat, 2-stage cool

Standard OEM

None

T874C1018

T872D

D1003

D1011

D1029

D1037

D1045

D1052

D1060

D1078

D1086

D1094

D1102

D1110

D1128

D1136

T874D1165

TRADELINE

Westinghouse logo

Climatrol logo

Lennox logo; no. P-8-8897; used with Q672E1191. T874D1165

York logo

T874D1165

Singer logo; no. 02005385

Rheem logo

Rudd logo

York logo; locking cover

American Standard logo

ITT logo

Carrier logo

No anticipator on stage 1 cool; used with

Q672C1029

When replacing a T872 Thermostat and Q672 Subbase, always replace both the thermostat and the subbase.

0-2485—8

T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

Table 6. T872-T874 Thermostat Cross Reference.

T874 Trade

Model Number

D1144

Description

Replacement^a

Remarks

Carrier logo; no. HH07AT072

T874D1165

D1151

General Electric logo; no. AY28X098; 0.4 heat

anticipator setting

D1169

Airtemp (Chrysler) logo; used with Q672E1118

T874D1165/

Q674E1049

Wiring differences (old=new):

A=W1; set H1 anticipator to 1.2A.

D1177

D1185

D1193

D1201

D1219

D1227

D1235

D1243

D1250

D1268

D1276

D1284

D1292

Trane logo

T874D1165

Dunham-Bush logo

Fedders logo

Climatrol logo

Command-Aire logo

Singer logo; no. 02005522; Celsius scale

Singer logo; (Carteret)

Friedrich logo

Singer logo; no. 050240-02

Airtemp logo

Celsius scale

Solaron

Tokyo Carrier logo; no. HH07AT072C; Celsius

scale

D1300

D1318

SUPER TRADELINE

T874D1165

General Electric logo; no. AY28X098A; fan current T874D1165/

shunt for auto fan in heating.

Q674B1075

D1326

General Electric logo; locking cover 72-78°F

T874D1165

(

22-26°C) stop; fan current shunt for auto fan in

heating.

D1334

D1342

D1359

Lennox logo; Celsius scale

Tappan logo; no. 080-13708-32

T874D1165

Singer logo; no. 02005740; Celsius scale; 6.6°C T874D1165

differential between H1 and H2.

Only 2°F (1°C) differential between

H1 and H2.

D1367

D1375

Friedrich logo; no. 600-580-23 T874D1165

Singer logo; no. 02005713; Celsius scale; controls None

separate heat/cool circuits; used with

Q674G1017

D1383

D1391

TRADELINE; Celsius scale

T874D1165

Singer logo; no. 02005712; controls two separate None

heat/cool circuits; used with Q674G1017

D1409

D1417

D1425

D1433

12°F (7°C) differential between H1 and H2

Fedders logo

T874D1165

Climatrol logo

Singer logo; no. 02005785; 8°F (4.4°C) differential T874D1165

between H1 and H2

D1441

D1474

American Air Filter

Carrier; no. HH07AT072-03; Celsius scale

2-stage cool

T874D1165

T872E

E1002

E1010

E1028

Standard OEM

T874E1016

TRADELINE

Westinghouse logo

When replacing a T872 Thermostat and Q672 Subbase, always replace both the thermostat and the subbase.

60-2485—8

T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

Table 6. T872-T874 Thermostat Cross Reference.

T874 Trade

Model Number

E1036

E1044

E1051

E1069

E1077

E1085

E1093

E1101

E1119

Description

American Standard logo

Replacement^a

Remarks

T874E1016

York logo; locking cover

ITT logo

Climatrol logo

Airtemp (Chrysler) logo

Lennox logo

Trane logo

Dunham-Bush logo

Fedders logo

E1127

E1135

E1143

E1150

E1168

T872F

F1001

F1019

F1027

F1035

Climatrol logo

General Electric logo; no. AY28X108

Lennox logo; Celsius scale

Celsius scale

Airtemp logo

2-stage heat

Standard OEM

T874F1015

T874C1018

TRADELINE

Lennox logo; fast cycle heat anticipator

Luxaire logo; fast cycle performance on both heat T874C1018

stages.

M839 draws 0.72A; set adjustable

heat anticipator to setting of 0.9A

for proper cycle rate.

F1043

F1050

F1068

F1076

F1084

F1092

F1100

F1118

F1126

York logo; locking cover; used with M839.

Raehco logo

T874C1018

T874F1015

T874C1018

T874F1015

Fast cycle performance on both heat stages.

Wesco logo

Lennox logo

Auto Gas logo

Celsius scale

Lennox logo; Celsius scale

Stages 1 and 2 heat electrically isolated; used with None

Q672D1002

F1134

F1159

F1167

F1175

Square D logo; HCD-4

Wesco logo; heat anticipator set at 0.8A

York logo

T874F1015

Oneida Heater logo; no. 30A0287-01; used with

Q672K1012.

T874F1015/

Q674K1026

T872G

Heat pump models; 2-stage heat,

-stage cool, 1-stage changeover.

G1000

G1018

Fast cycle performance stage 2 heat; fixed

anticipator stage 1 heat and cool; changeover with

cooling; used with Q672F1018.

Y594G1252

Y594G1393

Wiring differences (old=new):

W=W2, X1=X.

Adjustable anticipator stage 2 heat; fixed

anticipator stage 1 heat and cool; changeover with

cooling; used with Q672E1027.

Wiring differences (old=new):

Y1=O, Y2=Y.

When replacing a T872 Thermostat and Q672 Subbase, always replace both the thermostat and the subbase.

0-2485—8

T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

Table 6. T872-T874 Thermostat Cross Reference.

T874 Trade

Model Number

Description

Replacement^a

Remarks

G1026

Lennox logo; fast cycle performance; stage 2 heat; None

fixed anticipator stage 1 heat and cool;

changeover with cooling; used with Q672E1019,

F1067.

See subbase description for

replacement information.

G1034

Westinghouse logo; adjustable anticipator stage 2 Y594G1252

heat; fixed anticipator stage 1 heat and cool;

changeover with cooling; used with Q672F1091,

F1141, F1257 or Q672J1013, J1062.

Wiring differences (old=new):

W=W2.

G1042

G1059

G1067

York logo; fixed anticipator on all stages heat and Y594G1567

cool; changeover with cooling; used with

Q672J1021.

TRADELINE; no anticipator stage 2 heat; fixed

anticipator stage 1 heat and cool; changeover with

cooling; used with Q672J1039 outdoor reset.

Y594G1161

Designed for use on General

Electric heat pumps.

Lennox logo; no. P-8-11222; fast cycle

performance; stage 2 heat; fixed anticipator stage

See subbase description for

replacement information.

heat and cool; changeover with cooling; used

with Q672E1142, F1083.

G1075

G1083

Carrier logo; adjustable anticipator stage 2 heat;

fixed anticipator stage 1 heat and cool;

changeover with cooling; used with Q672J1054,

L1052.

See subbase description for

replacement information.

Adjustable anticipator stage 2 heat; fixed

anticipator stage 1 heat and cool; changeover with

cooling; includes shunt to bypass stage 2

anticipator with auxiliary heat relays; used with

Q672F1158.

Y594G1252

G1091

G1109

Lennox logo; same as T872G1026 except meets

DoD specs.

See subbase description for

replacement information.

Tappan logo; no. 080-13708-52; adjustable

anticipator stage 1 heat and cool; changeover with

cooling; used with Q672F1117.

Y594G1252

Wiring differences (old=new): C=X.

G1117

G1125

Changeover; used with Q672F1125.

Wiring differences (old=new): C=X.

Lennox logo; no. 63A0401; Celsius scale; isolated None

auxiliary heat power circuit; fast cycle anticipator

stage 2 heat.

G1133

G1141

G1158

Rheem logo; no. RXMT-B71; adjustable

Y594G1252

anticipator stage 2 heat; fixed anticipator stage 1

heat and cool; changeover with cooling; includes

shunt to bypass stage 2 anticipator with auxiliary

heat relays; used with Q672F1158, F1232.

Ruud logo; adjustable anticipator stage 2 heat;

fixed anticipator stage 1 heat and cool;

changeover with cooling; includes shunt to bypass

stage 2 anticipator with auxiliary heat relays; used

with Q672F1158.

Y594G1252

Carrier logo; same as T872G1075. Except DoD

styling.

See subbase description for

replacement information.

G1166

G1174

G1182

BDP used with Q672F1125.

Y594G1252

Wiring differences (old=new): C=X.

Carrier; no. HH07AT071C; Celsius scale; used

with Q672L1052, J1054.

See subbase description for

replacement information.

G1190

Tappan; no. B13708-57; used with Q672L1078 for None

and 10 ton heat pumps with 2 compressors.

When replacing a T872 Thermostat and Q672 Subbase, always replace both the thermostat and the subbase.

60-2485—8

T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

Table 6. T872-T874 Thermostat Cross Reference.

T874 Trade

Model Number

Description

Replacement^a

Remarks

G1208

Marvair logo; special for Crispaire; used with

Q672F1166, F1323.

Y594G1393

G1216

G1224

Janitrol logo; no. 26G05; stage 2 heat anticipator Y594G1252

set at 1.2A; used with Q672F1117.

Wiring differences (old=new): C=X.

TRADELINE; adjustable anticipator stage 2 heat; Y594G1419

fixed anticipator stage 1 heat and cool;

changeover in cool; used with Q672F1208.

G1232

G1240

G1257

Heat Wave (Southwest Mfg.) logo; stage 2 heat

anticipator set at 1.2A; used with Q672F1224.

Lennox; fast cycle anticipator stage 2 heat; used None

with Q672F1240.

Carrier logo; no. HH07AT073; used with

Q672L1169 (HH93AZ071).

None

G1265

G1273

Addison logo; used with Q672F1265.

Y594G1252

Wiring differences (old=new): C=X.

Carrier logo; (DoD model); 68°F (20°C) maximum

heating; 78°F (26°C) minimum cooling; used with

Q672L1052, J1054

See subbase description for

replacement information.

G1281

G1299

Wesco logo; used with Q672F1265.

Y594G1252

Y594G1567

Wiring differences (old=new): C=X.

York logo; meets DoD specs; used with

Q672L1177.

G1307

G1315

G1323

G1331

Carrier; no. 38EQ1900; Celsius; used with

Q672J1054, L1052.

See subbase description for

replacement information.

Luxaire special; stage 2 heat anticipator set to

Y594G1252

None

.2A; used with Q672F1281.

Lennox special; used with Q672L1201 and

C815A1013.

York; used with Q672J1021.

Y594G1567

Wiring differences (old=new):

X=X1, L=X2, W=E; Y1-W1 and Y2-

W2 jumpers required.

G1356

Westinghouse; used with Q672L1219.

T874D1165/

Q674F1022

Wiring differences (old=new):

X=X1, L=X2, W=E; Y1-W1 and Y2-

W2 jumpers required.

G1364

G1372

York; used with Q672D1077.

None

Tappan/Janitrol logo; no. B-13708-521;

used with Q672F1117.

Y594G1252

Wiring differences (old=new): C=X.

G1398

York logo; no. 025-21454C; used with Q672J1021. Y594G1567

Heat pump model; 1 stage heat,

T872H

stage cool, 1 stage changeover.

H1009

Command-Aire logo; fixed anticipator stage 1 heat Y594H1012

and cool; changeover with cooling; used with

Q672C1086.

H1017

H1025

T872M

Friedrich logo.

Y594H1012

Friedrich logo; Celsius.

1 stage heat, 1 stage cool with stage 2 heat factory

set at 60°F (16°C).

M1003

Standard OEM; used with Q672D and external

switching means.

T874C1125

T874C1125 has 12°F (6.7°C)

differential between stage 1 and

stage 2 heat.

T872N

N1002

Heat pump model; 2-stage heat, 1-stage cool plus

changeover, changeover in heat mode.

None

When replacing a T872 Thermostat and Q672 Subbase, always replace both the thermostat and the subbase.

0-2485—8

T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

Table 6. T872-T874 Thermostat Cross Reference.

T874 Trade

Model Number

Description

Replacement^a

Remarks

N1010

Williamson logo; adjustable anticipator stage 1

and 2 heat; fixed anticipator stage 1 cool; used

with Q672F1133 or F1315.

T874N1016/

Q674F1220

N1028

N1036

N1044

TRADELINE heat mode changeover; used with

Q672F1216.

T874N1016

Bard; used with Q672F1299.

T874N1016/

Q674F1220

Thermotron logo; 1st stage changeover heat

mode; used with Q672F1299.

T874N1016/

Q674F1220

T872Q

1-stage heat; used with another T872 for heating

setback.

Q1009

Q1017

Standard OEM

York logo

T874C1125

Replacement only good for

1-stage heat and 1-stage cool

applications.

Q1025

Q1033

Q1041

ITT logo

None

Lennox logo; no. P-8-8899.

Lennox logo; no. 63A6401.

T872R

Heat pump model; 2-stage heat, 1-stage cool,

manual changeover.

R1008

Fixed anticipator stage 1 heat and cool; no

anticipator stage 2 heat; changeover with cooling;

used with Q672B1046.

Y594R1425

Wiring differences (old=new):

W=W2; W1-Y1 jumper required.

R1016

R1024

Addison same as T872R1008; used with

Q672B1103.

Wiring differences (old=new):

W=W2; W1-Y1 jumper required.

Lennox logo; fast cycle stage 2 heat; fixed

anticipator stage 1 heat and cool; changeover with

See subbase description for

replacement information.

cooling; used with Q672B1160 and Q672L1003.

R1032

R1040

York logo; fixed anticipator stage 1 and 2 heat and Y594R1664

stage 1 cool; changeover with cooling; used with

Q672L1011.

Lennox logo; fast cycle stage 2 heat; fixed

anticipator stage 1 heat and cool; isolated stage 2

heat switch; changeover with cooling; used with

Q672B1202 no. P-8-11221, Q672L1029

See subbase description for

replacement information.

R1057

R1065

R1073

TRADELINE; fixed anticipator stage 1 heat and

cool; no anticipator stage 2 heat; changeover on

cooling; used with Q672L1037; outdoor reset.

Y594R1763

Y594R1243

None

Designed for use on General

Electric heat pumps. Wiring

differences (old=new): A=T.

Adjustable anticipator stage 2 heat; fixed

anticipator stage 1 heat and cool; changeover on

cooling; used with Q672L1045.

Lennox logo; no. 63A0301; Celsius scale;

separate auxiliary heat power circuit; fast cycle

stage 2 heat antic.

R1081

R1099

R1107

R1115

Ruud logo; same circuit as T872R1065; used with Y594R1243

Q672L1045, L1136.

Dunham Bush special; used with Q672L1060,

L1383.

Y594R1243

Rheem; no. RXMT-B70; used with Q672L1045,

L1136.

Y594R1243

Friedrich logo; fixed anticipator; used with

Q672B1244, L1086.

See subbase description for

replacement information.

When replacing a T872 Thermostat and Q672 Subbase, always replace both the thermostat and the subbase.

60-2485—8

T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

Table 6. T872-T874 Thermostat Cross Reference.

T874 Trade

Model Number

R1123

Description

Replacement^a

Remarks

York logo; no. 025-21453C; Celsius scale.

Y594R1664

R1131

Lennox logo; separate auxiliary heat power circuit; None

fast cycle stage 2 heat anticipator; outdoor reset;

used with Q672L1144, C815A1013.

R1149

Westinghouse logo; adjustable stage 2 heat

anticipator; cooling changeover; fixed anticipator

stage 1 heat and cool; used with Q672L1110,

L1128, L1151.

Y594R1243

Wiring differences (old=new):

W=W2.

R1156

Marvair logo; used with Q672L1094, L1250.

Y594R1243

Replacement has auto fan in

Emergency Heat mode.

R1164

R1172

Bard; used with Q672L1185.

Y594R1243

Heil Quaker; used with Q672L1193.

Wiring differences (old=new): B=X,

W=W2.

R1180

R1198

R1206

R1214

York; Celsius; used with Q672L1011.

TRADELINE; used with Q672L1227

For use with W89; used with Q672L1235.

Intertherm logo; used with Q672L1276.

Y594R1664

Y594R1243

None

Y594R1300

Wiring differences (old=new):

W1=B, Z=L, K=E.

R1222

R1230

York logo; used with C815A1039; adjustable

anticipator stage-2 heat; used with Q672L1284

Y594R1672

None

Amana logo; no. C62397-1; used with

Q672L1300, W89.

R1255

R1271

Singer logo; used with Q672L1326 and W89.

None

Westinghouse logo; used with Q672L1375 and

W89.

Y594R1664

R1297

R1305

R1321

R1339

York; Celsius scale; used with Q672L1011.

Used with Q672L1235 and W89.

None

Duotherm logo; used with Q672L1409, and W89. None

Williamson; used with Q672L1391.

Y594R1243

T872S

Heat pump model; 2-stage heat plus changeover,

-stage cool.

S1005

Westinghouse logo; adjustable anticipator stage 1 T874C1018/

Wiring differences (old=new):

V=RC-RH, F=G, C=Y1, Z=B,

H2=W2; W1-Y1 jumper required.

and 2 heat; fixed anticipator stage 1 heat;

changeover with heating: used with Q672C1045

Q674C1058

None

T872T

T1004

T872W

1-heat stage, 1-vent and 2-cool stages.

Trane logo; used with Q672B1129.

3-heat stages, 2-cool stages, manual changeover

W1019

W1027

W1035

Lennox logo; no. 52C7501; used with Q672L1342 None

and C815A1013.

Westinghouse logo; used with Q672L1268 and

C815A1021.

None

TRADELINE; used with C815A1043 and

Q672B1251.

Y594W1014

When replacing a T872 Thermostat and Q672 Subbase, always replace both the thermostat and the subbase.

0-2485—8

T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

Table 7. Q672-Q674 Subbase Cross Reference.

Q674 Trade

Model Number

Q672A

Description

System: HEAT-AUTO-COOL, Fan: AUTO-ON

Standard OEM

Replacement^a

Remarks

A1005

A1013

A1021

A1039

Q674A1019

TRADELINE

RC-RH jumper

RC-RH jumper required.

RC-RH jumper; Chek Lite

System: HEAT-OFF-COOL, Fan: AUTO-ON

Standard OEM

Q672B

B1004

B1012

B1020

B1038

B1046

Q674B1034

Q674B1018

Q674B1034

Y594R1425

Isolated G terminal.

System: HEAT-OFF-RESET-COOL

TRADELINE

Used system off as reset position.

Heat pump circuit; used with T872R1008.

Wiring differences (old=new):

W1=Y1, W=W2.

B1053

B1061

B1079

B1087

B1095

B1103

RC-RH jumper

Q674B1034

Q674B1075

Q674B1034

Y594R1425

RC-RH jumper required.

Chek Lite

Auto fan operation in heat and cool.

Carrier no. HH93AZ078; RC-RH jumper.

Carrier no. HH93AZ084; RC-RH jumper.

Addison; heat pump circuit; used with

T872R1016.

Wiring differences (old=new):

W=W2; W1-Y1 jumper required.

B1111

B1129

B1137

B1145

B1152

Carrier no. HH93AZ080; isolated G terminal.

Trane special circuit; used with T872T1004.

RC-RH jumper; isolated G terminal.

Fedders special

Q674B1018

None

Q674B1018

Q674B1034

Q674B1075

RC-RH jumper required.

Fedders special; auto fan operation in heat

and cool.

B1160

B1178

Lennox; heat pump circuit; used with

T872R1024; includes compressor lockout in

EM. HT.

Y594R1243

Q674B1075

Wiring differences (old=new):

V-VR=R, M=Y, Y=W2, F=G, R=O.

General Electric no. AY28X107; auto fan

operation in heat and cool.

B1186

B1194

Trane; isolated G terminal; Filter LED.

Q674B1018

Q674B1075

General Electric no. AY28X124; auto fan

operation in heat and cool.

Set T874 stage-1 anticipator for

combined load of heat relay and fan

relay.

B1202

Lennox no. P-8-11223; heat pump circuit; used Y594R1243

with T872R1040.

Wiring differences (old=new):

V-VR=R, M=Y, Y=W2, F=G, R=O.

B1210

B1228

None

General Electric no. BAY28X137; isolated

G terminal.

Q674B1018

B1236

B1244

B1251

Carrier no. HH-93AZ079; RC-RH jumper;

LOCKOUT LED indicates compressor lockout

switch activated.

Q674B1034

RC-RH jumper; LED cannot be

provided in TRADELINE model.

Friedrich; heat mode changeover; used with

T872R1115.

T874C1018/

Q674B1034

Wiring differences (old=new): C=no

connection; RC-RH and W1-Y1

jumpers required.

Used with T872W1035 and C815A1043.

Y594W1014

When replacing a T872 Thermostat and Q672 Subbase, always replace both the thermostat and subbase.

60-2485—8

T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

Table 7. Q672-Q674 Subbase Cross Reference.

Q674 Trade

Model Number

Description

Replacement^a

Remarks

B1269

Whalen; Hi-Lo fan switch; constant fan in heat None

and cool system switch positions; used with

T872C1574.

B1277

B1285

Square D no. HCB-5.

Q674B1034

Wards; RC-RH and F-W3 jumpers; used with

T872C1673.

Gas Furnace:

T874C1018/

Q674B1034

Electric Furnace:

T874C1018/

Adjust anticipator faster for correct

performance.

Q674B1075

B1293

Lennox no. 41C96; used with T872C1673.

Same as

Q672B1285

Q672C

System: OFF-AUTO, Fan: AUTO-ON

Standard OEM

C1003

C1011

C1029

Q674C1058

Q674E1049

Westinghouse; RC-RH jumper.

RC-RH jumper required.

W1-Y2 jumper; for heat pump; used with

T872D1136.

Replacement has heat/cool system

switching.

C1037

C1045

Chrysler Airtemp; System: COOL-OFF-AUTO. Q674C1058

Westinghouse; heat pump; used with

T872S1005.

T874C1018/

Q674C1058

Wiring differences (old=new):

V=RC-RH, F=G, C=Y1, Z=B,

H2=W2; W1-Y1 jumper required.

C1052

Isolated stage 2 heat circuit for dc electric heat Recontrol

relay.

Use T874G1246/Q674F1212 which

adds HEAT-COOL-EM.HT. system

switch.

C1060

C1078

C1086

Fedders special.

Q674C1058

Y594H1012

General Electric no. AY28X109.

Command-Aire; heat pump; used with

T872H1009.

C1102

TRADELINE

Q674C1058

Q672D

System: None. Fan: None.

Standard OEM; used with T872F1126.

Lennox special.

D1002

D1010

D1028

D1036

D1044

D1069

D1077

D1085

Q674D1040

None

Carrier; RC-RH jumper.

Carrier no. HH93AZ070; RC-RH jumper.

General Electric no. AY-28X100.

Square D no. HCB-4.

None

Q674D1040

None

RC-RH jumper required.

None

York; used with T872G1364.

TRADELINE

None

Q674D1040

Q672E

System: OFF-HEAT-AUTO-COOL

Fan: AUTO-ON

E1001

E1019

Standard OEM

Q674E1049

Lennox no. P-8-8892; heat pump; cool mode

changeover; used with T872G1026.

T874D1165/

Q674B1075

E1027

E1035

E1043

Heat pump circuit; W1-Y2 jumper; cool mode

changeover; used with T872G1018.

Y594G1393

Q674E1049

Wiring differences (old=new): Y1=O,

Y2=Y.

Heat pump circuit; W1-Y2 jumper; cool mode

changeover.

Wiring differences (old=new): Y1=O,

Y2=Y.

Heat pump circuit; heat mode changeover.

Wiring differences (old=new):

W1=B, W2=W1; W1-Y1 jumper.

When replacing a T872 Thermostat and Q672 Subbase, always replace both the thermostat and subbase.

0-2485—8

T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

Table 7. Q672-Q674 Subbase Cross Reference.

Q674 Trade

Model Number

E1050

Description

Replacement^a

Remarks

TRADELINE

Q674E1049

E1068

RC-RH jumper; used with T872C1699.

RC-RH internal jumper; EM. HT. LED.

Carrier no. HH93AZ076; RC-RH jumper.

RC-RH jumper required.

E1076

E1084

E1092

Carrier no. HH93AZ082; RC-RH jumper; B

terminal; isolated G terminal.

E1100

E1118

General Electric no. AY28X099.

Q674E1049

Airtemp; heat pump; used with T872D1169.

T874D1165/

Q674E1049

Wiring differences (old=new):

A=W1; set H1 anticipator to 1.2A.

E1126

E1134

Common R terminal.

Q674E1049

RC-RH jumper required.

ITT Nesbitt; RC-RH jumper; O and B terminals; Q674E1049

isolated G terminal.

E1142

E1159

Lennox no. P-8-11224; heat pump;

V-VR jumper; cool mode changeover; used with

T872G1067.

Y594G1252

Wiring differences (old=new):

V-VR=R, Y=W2, R=O, M=Y, F=G.

Carrier no. HH93AZ077; RC-RH jumper;

LOCKOUT LED indicates activation of

compressor lockout switch.

Q674E1049

RC-RH jumper required; no LED in

TRADELINE model.

E1167

E1183

E1191

Square D no. HCB-2; Chek Lite.

Square D no. HCB-3.

Q674E1049

Lennox; HIGH and LOW LEDs; used with

T872D1045.

No LEDs on replacement.

Q672F

System: EM. HT.-HEAT-AUTO-COOL-OFF

Fan: AUTO-ON.

F1000

F1018

With EM. HT. LED; used with T872C1699.

Q674F1022

Remove W3-X2 jumper to E-X2.

Heat pump; heat mode changeover; Chek Lite; Y594G1252

used with T872G1000.

Wiring differences (old=new):

W=W2, X1=X.

F1026

F1034

Chek Lite; W2-X2 jumper.

Q674F1022

Airtemp; EM. HT. LED; RC-RH and X2-E

jumpers.

If RC-RH jumper removed on

Q672F1034, Q674F1022 does not

replace.

F1042

F1059

W2-X2 jumper; Chek Lite.

Q674F1022

Carrier no. HH93AZ074; EM. HT. LED.

Remove W2-X2 jumper to E-X2; X

terminal same as C.

F1067

F1075

F1083

Lennox; heat pump; cool mode changeover;

EM. HT. LED; used with T872G1026.

None

Heat pump circuit; EM. HT. LED; 2 terminal; RC- None

RH jumper.

Lennox no. P-8-11225; heat pump;

V-VR jumper; cool mode changeover; EM. HT.

LED; used with T872G1067.

None

F1091

Westinghouse; heat pump; EM. HT. LED; cool Y594G1252

mode changeover; compressor lockout in

EM. HT.; used with T872G1034.

Wiring differences (old=new):

W=W2.

F1109

F1117

Airtemp no. 3778567ACC; R-X2 jumper;

EM. HT. LED.

Q674F1022

R-X2 jumper required.

Tappan no. 080-13708-53; heat pump; EM. HT. Y594G1252

LED; cool mode changeover; compressor

lockout and auto fan in EM. HT.; used with

T872G1109, G1216, G1372.

Wiring differences (old=new): C=X.

When replacing a T872 Thermostat and Q672 Subbase, always replace both the thermostat and subbase.

60-2485—8

T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

Table 7. Q672-Q674 Subbase Cross Reference.

Q674 Trade

Model Number

Description

Replacement^a

Remarks

F1125

BDP no. 3442DP90; used with T872G1117,

G1166, G1174.

Y594G1252

Wiring differences (old=new): C=X.

F1133

Williamson; heat pump; EM. HT. LED; heat

mode changeover; compressor lockout in

EM. HT.; used with T872N1010.

T874N1016/

Q674F1220

F1141

F1158

Westinghouse; EM. HT. LED; auto fan in

EM. HT.; used with T872G1034.

Y594G1252

Wiring differences (old=new):

W=W2.

Rheem Ruud; heat pump; EM. HT. LED; cool

mode changeover; compressor lockout in

EM. HT.; used with T872G1083, G1133, G1141;

auto fan in EM. HT.

F1166

F1174

F1182

Crispaire; EM. HT. LED; used with T872G1208. Y594G1252

Carrier no. HH93AZ081; EM. HT. LED. Q674F1022

Square D no. HCB-1; Chek Lite; R-X2 jumper; Q674F1022

System: Aux. Heat replaces EM. HT.

R-X2 jumper; add monitor switch

between X1 and low side of power.

F1190

F1208

EM. HT. LED; X2-E and RC-RH jumpers.

None

TRADELINE; cool mode changeover; EM. HT. Y594G1419

LED; used with T872G1224.

F1216

F1224

TRADELINE; heat mode changeover; EM. HT. T874N1016/

LED; used with T872N1028.

Q674F1220

Heatwave (Southwest Mfg.); compressor

lockout in EM. HT.; EM. HT. LED; used with

T872G1232.

Y594G1252

F1232

Rheem; cool mode changeover; EM. HT. and

AUX. HEAT LEDs; compressor lockout in

EM. HT.; used with T872G1133.

Y594G1252

F1240

F1257

Lennox; cool mode changeover; EM. HT. and

SERVICE LEDs; used with T872G1240.

None

Westinghouse; cool mode changeover;

EM. HT. and AUX. HEAT LEDs; auto fan in

EM. HT; used with T872G1034.

Y594G1252

Wiring differences (old=new):

W=W2.

F1265

F1273

Wesco Addison; Chek and EM. HT. LEDs; used Y594G1252

with T872G1281, G1265.

Wiring differences (old=new): C=X.

Southwest special; no. 0361-136; W2-X2

jumper.

None

F1281

F1299

Luxaire special; used with T872G1315.

Y594G1252

Bard special; used with T872N1036, N1044.

T874N1016/

Q674F1220

F1315

Williamson special; used with T872N1010.

T874N1016/

Q674F1220

F1323

Crispaire special; used with T872G1208.

System: OFF-AUTO. Fan: None.

Standard OEM

Y594G1252

Q672G

G1009

G1017

G1025

G1033

G1041

Q674G1070

Singer; O-B jumper; used with T872D1375.

Includes O and B terminals.

General Electric no. AY28X102.

Singer no. 02005649; French wording; O-B

jumper.

Less French wording.

G1066

SLI-Mammoth; heat pump; heat mode

changeover.

Q674G1070

Connect fan (G) wire to Y1 terminal;

B-O and RC-RH jumpers.

When replacing a T872 Thermostat and Q672 Subbase, always replace both the thermostat and subbase.

0-2485—8

T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

Table 7. Q672-Q674 Subbase Cross Reference.

Q674 Trade

Model Number

Description

Replacement^a

Remarks

G1074

Singer no. 02005754; X1 and X2 terminals for None

field-added indicator lights; used with

T872D1375.

G1082

G1090

Singer no. 02005792; X1 and X2 terminals for None

field-added indicator lights; used with

T872D1375.

TRADELINE; B and O terminals.

Q674G1070

Q672H

System: OFF-ON

Fan: NIGHT INT.-DAY CONT.

H1008

Q672J

Used with T872.

None

System: EM. HT.-AUTO-OFF

Fan: AUTO-ON.

J1005

J1013

With EM. HT. LED; used with T872C or D.

Westinghouse; heat pump; EM. HT. LED; cool Y594G1252

mode changeover; compressor lockout in

EM. HT.; used with T872G1034.

J1021

J1039

York; heat pump; EM. HT. LED; cool mode

changeover; compressor lockout and auto fan in

EM. HT.; E-W jumper; used with T872G1042,

G1331, G1398.

Y594G1567

Heat pump; EM. HT. and AUX. HEAT LEDs;

cool mode changeover; compressor lockout and

auto fan in EM. HT.; used with T872G1059.

Y594G1161

Designed for use on General

Electric heat pumps.

J1047

J1054

Singer no. 02005699; heat pump; EM. HT. LED; Q674F1022

heat mode changeover.

Carrier no. HH93AZ073A; heat pump; EM. HT. Y594G1252

LED; cool mode changeover; compressor

lockout in EM. HT.; used with T872G1075,

G1182, G1307.

Wiring differences (old=new): C=X.

J1062

Westinghouse; EM. HT. and AUX. HEAT LEDs; Y594G1252

used with T872G1034.

Wiring differences (old=new):

W=W2.

Q672K

System: OFF-HEAT-AUTO-COOL. Fan: None.

K1004

K1012

Carrier no. HH93AZ072.

Q674E1049

Fan switch does not function.

Oneida Heater no. 30A0288-01; System: OFF- T874F1155/

WOOD-WOOD/OIL-OIL; used with T872F1175. Q674K1026

Q672L

System: EM. HT.-HEAT-OFF-COOL.

Fan: ON-AUTO.

L1003

L1011

Lennox; heat pump; EM. HT. LED; cool mode

changeover; used with T872R1024.

Y594R1425

Y594R1664

York; heat pump; EM. HT. LED; cool mode

changeover; compressor lockout and auto fan in

EM. HT.; used with T872R1032, R1180, R1297.

L1029

Lennox no. P-8-11226; heat pump; EM. HT.

LED; V-VR jumper; cool mode changeover;

compressor lockout in EM. HT.; used with

T872R1040.

Y594R1425

Wiring differences (old=new):

V-VR=R, M=Y, Y=W2, F=G, R=O.

L1037

L1045

Heat pump; EM. HT. and AUX. HT. LEDs; cool Y594R1763

mode changeover; compressor lockout and auto

fan in EM. HT.; used with T872R1057.

Wiring differences (old=new): A=T;

designed for used on General

Electric heat pumps.

Rheem; heat pump; EM. HT. LED; cool mode

changeover; compressor lockout in EM. HT.;

auto fan in EM. HT; used with T872R1065,

R1081, R1107.

Y594R1243

When replacing a T872 Thermostat and Q672 Subbase, always replace both the thermostat and subbase.

60-2485—8

T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

Table 7. Q672-Q674 Subbase Cross Reference.

Q674 Trade

Model Number

Description

Replacement^a

Remarks

L1052

Carrier no. HH93AZ075; heat pump; EM. HT.

LED; cool mode changeover; compressor

lockout in EM. HT.; used with T872G1075,

G1182, G1273, G1307.

Y594R1615

Wiring differences (old=new): C=X.

L1060

L1078

Dunham Bush; EM. HT. LED; used with

T872R1099.

Y594R1243

None

Tappan no. B13708-58; heat pump; EM. HT.

LED; cool mode changeover; compressor

lockout in EM. HT.; EM. HT. relay provides fan

operation in EM. HT.; used with T872G1190.

L1086

L1094

L1102

L1110

L1128

L1136

Friedrich; heat mode changeover; EM. HT. LED; Y594R1243

used T872R1115 with L terminal.

Wiring differences (old=new): X2=E,

C=X.

Crispaire; W2-H jumper; EM. HT. LED; used

with T872R1065, R1156.

Y594R1243

Replacement has auto fan in

Emergency Heat mode.

Fedders; heat mode changeover; EM. HT. LED; Y594R1425

used with T872C1509, C1517, C1541.

Wiring differences (old=new): K=E,

C=X; remove Y-W1 jumper.

Westinghouse; EM. HT. LED; used with

T872R1149.

Y594R1243

Wiring differences (old=new):

W=W2.

Westinghouse; EM. HT. LED; used with

T872R1149.

Wiring differences (old=new):

W=W2.

Rheem no. 41-20796-07; cool mode

changeover; EM. HT. and AUX. HT. LEDs; used

with T872R1081, T872R1107.

L1144

Lennox; V-VR jumper; EM. HT. and SERVICE None

LEDs; isolated stage 2 heat; used with

T872R1131, C815A1013.

L1151

L1169

Westinghouse; AUX. HT. and EM. HT. LEDs;

used with T872R1149.

Y594R1243

Wiring differences (old=new):

W=W2.

Carrier no. HH93AZ071; EM. HT. LED; used

with T872G1257 (HH07AT073).

None

L1177

L1185

L1193

York; DoD; EM. HT. LED; used with T872G1299. Y594G1567

Bard; used with T872R1164.

Y594R1243

Heil Quaker no. NAXA001TS01; used with

T872R1172.

Wiring differences (old=new): B=X,

W=W2.

L1201

L1219

Lennox special; no. 1485701; used with

T872G1323.

None

Westinghouse special; used with T872G1356. T874D1165/

Q674F1022

Wiring differences (old=new): X=X1,

L=X2, W=E; Y1-W1 and

Y2-W2 jumpers required.

L1227

L1235

L1250

Heat pump; used with T872R1198.

Y594R1243

None

Intertherm; used with T872R1206, R1305.

Crispaire special; used with T872R1156 and

C815A1021.

Y594R1243

L1268

L1276

Westinghouse special; used with T872W1027

and C815A1021.

None

Intertherm special; used with T872R1214.

Y594R1300

Wiring differences (old=new):

W1=B, Z=L, K=E.

L1284

L1300

York special; used with T872R1222.

Y594R1672

None

Amana special; no. C62345-1; used with

T872R1230.

L1326

Singer; used with T872R1255.

None

When replacing a T872 Thermostat and Q672 Subbase, always replace both the thermostat and subbase.

0-2485—8

T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

Table 7. Q672-Q674 Subbase Cross Reference.

Q674 Trade

Model Number

Description

Replacement^a

Remarks

L1342

Lennox no. 52C7601; used with T872W1019

and C815A1013.

None

L1375

L1383

L1391

L1409

Westinghouse; used with T872R1271, W89.

Dunham Bush; used with T872R1099.

Williamson; used with T872R1339.

None

Y594R1243

Duotherm special; used with T872R1321, W89. None

System: OFF-HEAT-COOL-EVAP.

Q672N

N1001

N1019

N1027

Goettl; heat pump; heat mode changeover;

evaporative cooler circuit; Fan: AUTO-ON;

System: AUTO; used with T872C1459.

None

Goettl; heat pump; heat mode changeover;

evaporative cooler circuit; W1-Y1 jumper; used

with T872C1459.

None

Siegler special; used with T872C1707.

System: EM. HT.-HEAT-COOL.

None

Q672P

P1009

Carrier special; no. HH93AZ069; used with

T872G1075 (HH07AT071).

Y594R1243

Replacement has OFF system

switch position

When replacing a T872 Thermostat and Q672 Subbase, always replace both the thermostat and subbase.

60-2485—8