Honeywell T874 Thermostat User Manual

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T874 Multistage Thermostats  
and Q674 Subbases  
PRODUCT DATA  
FEATURES  
T874 Thermostat has silent, dust-free mercury  
switches operated by coiled bimetal elements.  
Heat anticipator(s) are adjustable or fixed cooling  
anticipator(s) are fixed.  
Individual heat and cool levers and scales (most  
models) for temperature setting located on top of  
thermostat case.  
Cover thermometer on most T874 Multistage  
Thermostat models.  
Locking cover and locking lever screws available for  
T874 Multistage Thermostats.  
Versaguard™ Thermostat Guard or custom key lock  
thermostat guards available for T874 Multistage  
Thermostats.  
T874 Thermostat requires a Q674 Subbase.  
Q674 Subbase provides system and fan switching,  
wiring terminals and mounting base for T874  
Multistage Thermostat.  
APPLICATION  
These thermostats and subbases provide low voltage control  
of multistage heating and cooling systems, including heat  
pump systems.  
Adapter plate available for mounting Q674 on wall or  
horizontal outlet box.  
Light-emitting diodes (LEDs) located on subbase for  
easy reference.  
Up to three stages each of heating and cooling control  
possible.  
Models with setpoint restrictions and locking cover  
with no thermometer available for Department of  
Defense (DoD) and other special applications.  
Outdoor reset used on some models to improve  
thermal performance.  
Contents  
Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1  
Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1  
Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2  
Ordering Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2  
Installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7  
Settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11  
Checkout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13  
Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14  
Understanding Circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14  
Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21  
Cross Reference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29  
Wiring Diagrams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96  
®
U.S. Registered Trademark  
60- 2485- 8  
Copyright © 2001 Honeywell • All Rights Reserved  
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T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES  
Tradeline Models  
T874 THERMOSTAT  
SPECIFICATIONS  
IMPORTANT  
T874 TRADELINE models provide staged heat and/or cool  
operation. See Table 1.  
The specifications given in this publication do not  
include normal manufacturing tolerances. Therefore,  
this unit may not exactly match the listed specifica-  
tions. This product is tested and calibrated under  
closely controlled conditions, and some minor differ-  
ences in performance can be expected if those con-  
ditions are changed.  
Table 1. Heating and Cooling Stages.  
Models  
Heating Stages  
Cooling Stages  
A
1
1
B
1
2
C
2
1
D
2
2
E
2
F
2
T874A-F are standard models.  
Super Tradeline®/Tradeline® Models  
SUPER TRADELINE controls offer features not available on  
TRADELINE or standard models, and are designed to replace  
a wide range of Honeywell and competitive controls.  
TRADELINE FEATURES:  
TRADELINE package with cross reference label and  
special instruction sheet.  
T874A,C model available with factory stops for DoD  
applications.  
TRADELINE models are selected and packaged to provide  
ease of stocking, ease of handling, and maximum  
replacement value. Specifications of SUPER TRADELINE  
and TRADELINE controls are the same as those of standard  
models except as noted below.  
T874A,C model available with adjustable temperature  
locking stops.  
Q674 SUBBASE  
Q674 switching subbases provide system and fan switching.  
See Table 2.  
Super Tradeline Models  
T874 THERMOSTAT  
Table 2. System and Fan Switching.  
T874D Thermostat. Provides two stages of heating and two  
stages of cooling. Use with Q674A-F Subbases.  
Y594D (T874D/Q674E/TG504A) Thermostat/Subbase/key  
lock cover package. Provides two stages of heating and  
two stages of cooling. Includes a key lock cover for setpoint  
protection.  
Q674  
System  
Heat-Auto-Cool  
Heat-Off-Cool  
Fan  
A
B
C
D
E
F
G
J
Auto-On  
Auto-On  
Auto-On  
None  
Auto-On  
Auto-On  
None  
Off-Auto  
None  
Off-Heat-Auto-Cool  
Em. Ht.-Off-Heat-Auto-Cool  
Off-Auto  
Y594G (T874G/Q674F) Thermostat/Subbase package for  
heat pump. Provides two stages of heating and one stage  
of cooling. Automatic changeover in heat or cool mode.  
Available in beige or Premier White® color.  
Y594R (T874R/Q674L) Thermostat/Subbase package for  
heat pump. Provides two stages of heating and one stage  
of cooling. Manual changeover in heat or cool mode. Avail-  
able in beige or Premier White® color.  
Em. Ht.-Auto-Off  
Em. Ht.-Heat-Off-Cool  
Auto-On  
Auto-On  
L
SUPER TRADELINE FEATURES:  
SUPER TRADELINE package with cross reference label  
and special instruction sheet.  
TRADELINE FEATURE:  
SUPER TRADELINE model supplied with locking lever and  
locking cover accessories.  
TRADELINE package with cross reference label and  
special instruction sheet.  
Includes adjustable temperature locking stops.  
T874D replaces T874A-F TRADELINE or standard models.  
Q674A-E,G are standard (non-heat pump) models.  
Q674F,J,L are heat pump models.  
ORDERING INFORMATION  
When purchasing replacement and modernization products from your TRADELINE® wholesaler or distributor, refer to the  
TRADELINE® Catalog or price sheets for complete ordering number.  
If you have additional questions, need further information, or would like to comment on our products or services, please write or  
phone:  
1
.
.
Your local Home and Building Control Sales Office (check white pages of your phone directory).  
Home and Building Control Customer Relations  
2
Honeywell, 1885 Douglas Drive North  
Minneapolis, Minnesota 55422-4386 (800) 328-5111  
In Canada—Honeywell Limited/Honeywell Limitée, 35 Dynamic Drive, Scarborough, Ontario M1V 4Z9.  
International Sales and Service Offices in all principal cities of the world. Manufacturing in Australia, Canada, Finland, France,  
Germany, Japan, Mexico, Netherlands, Spain, Taiwan, United Kingdom, U.S.A.  
60-2485—8  
2
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T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES  
Thermometer Range: 42° to 88°F (6 to 31°C)  
Standard Models  
Changeover Differential: 4°F (2°C) minimum between heat-  
ing and cooling (5°F [3°C] on T874W). Levers can be set  
apart for greater separation.  
T874 THERMOSTATS  
Models: See Table 3.  
Electrical Rating: 24 to 30 Vac.  
Interstage Differential:  
Standard Models: Mechanical differential is 1°F (0.6°C)  
between heating or cooling stages; operating differential is  
approximately 1.9°F (1°C) between stages in heating or  
cooling.  
Switching: Coiled bimetal elements operate mercury  
switches.  
Special Models: See Table 3.  
Temperature Adjustment: Heating and cooling setting  
levers, with separate scales located on top of thermostat  
base. Common lever for heating and cooling on T874R;  
one cooling lever on T874E,V; and one heating lever on  
T874F,Q.  
Finish: Beige or Premier White® finish.  
Mounting Means: T874 Multistage Thermostat mounts on  
Q674 Subbase. Subbase mounts horizontally on wall or  
outlet box. Mounts on vertical outlet box with optional  
Dimensions: See Fig. 1.  
193121A Adapter Plate Assembly.  
Temperature: Scale Range: 42° to 88°F (6° to 31°C) stan-  
dard; optional ranges available.  
THERMOSTAT MOUNTED  
ON SUBBASE  
SUBBASE  
FRONT  
SIDE  
3/8 (10)  
5-5/8 (143)  
3-9/32 (83)  
HEAT  
50 60 70 80  
COOL  
50 60 70 80  
1-1/2  
(38)  
50 60 70 80  
3
-7/16  
3
(
-1/2  
89)  
(87)  
FAN  
FAN  
AUTO  
ON  
OFF EM. HT. HEAT AUTO COOL  
AUTO  
ON  
OFF EM. HT. HEAT AUTO COOL  
1-7/8 (48)  
5-1/8 (130)  
2-3/16 (56)  
M5849  
Fig. 1. T874 Thermostat and Q674 Subbase dimensions in in. (mm).  
Optional Specifications (T874 Only):  
Thermostat Accessories:  
Locking Cover and Locking Lever Assembly: Part no.  
194559R with thermometer; 194559S without thermome-  
ter. See Fig. 2. Includes cover, screws, and Allen wrench  
for locking cover. The screws must be used to assure  
proper operation.  
Temperature scale ranges are 40° to 75°F (4° to 24°C) heat-  
ing and 75° to 90°F (24° to 32°C) cooling with stop; 44° to  
6
8°F (7° to 20°C) heating, 80° to 86°F (27° to 30°C) cool-  
ing; 6° to 29°C (43° to 85°F) Celsius scale; 3° to 22°C (38°  
to 72°F) and 26° to 32°C (78° to 90°F) cooling with stop.  
Nonadjustable factory-added stop limits heating setpoint to  
Adjustable Lever Stop: Part no. 4074ECK; includes lever stop  
and screws.  
7
2°F (22°C) maximum and cooling setpoint to 78°F (26°C)  
minimum.  
Universal Versaguard™ Thermostat Guard: Includes wall-  
plate, ring base, guard cover, tumbler lock, two keys and  
optional Honeywell logo insert. Double-wall construction  
provides extra measure of tamper-resistance. Tamper-  
resistant lock; key cannot be removed without being in  
locked position. Vents in guard base allow airflow for opti-  
mum thermostat performance. See form 68-0104 for more  
information.  
OEM customer personalization.  
Locking cover and locking lever (see Thermostat Accesso-  
ries).  
Thermostat cover without thermometer.  
Adjustable locking temperature stops.  
Voltage heat anticipation for first or second stage heat or both.  
See Table 3.  
Fast cycling on heating stage(s) for electric heat applications.  
C815A Outdoor Thermistor for improved performance on  
specified models.  
TG511A1000: Clear cover.  
TG511B1008: Opaque cover.  
TG511D1004: Painted steel (off-white) cover. See  
Fig. 2.  
3
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T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES  
Key Lock Cover: Part no. TG504A replaces existing T874  
cover. Mounts on T874 base and covers thermostat settling  
levers and subbase switches. Includes LED window and  
two keys. Should not be used with 193121A Adapter Plate.  
TG504A1025: Blank face, internal thermometer.  
TG504A1033: External thermometer. See Fig. 2.  
194559R Locking Cover  
with Thermometer  
Allen Wrench  
TG504A1033 Key Lock  
Cover with External  
Thermometer  
TG511D1004 Includes  
Painted Steel Cover,  
Opaque Base and Wallplate  
Fig. 2. T874 Thermostat accessories.  
Table 3. T874 Thermostat Specifications.  
Anticipation  
Heating (Adj) Cooling (Fixed)  
System Stages  
Models and Options  
Replaces Applications Heat Cool Other Stage 1 Stage 2 Stage 1 Stage 2  
T874A—Standard and TRADELINE®.  
T872A  
Standard  
1
1
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  
a
thermometer (for DoD ).  
T874B—Standard and TRADELINE.  
T874C—Standard and TRADELINE.  
T872B  
T872C  
Standard  
Standard  
1
2
2
1
0.1-1.2A  
0.1-1.2A 0.1-1.2A 0-1.5A  
0-1.2A 0-1.0A  
72°F/78°F (22°C/26°C)set stops  
with locking cover, no thermometer  
a
(
for DoD ).  
12°F (7°C) differential between  
H1 and H2 stages (T874C1125).  
Fast cycling.  
0.12-  
.6A  
0.12-  
0.6A  
0
T874D—Standard and SUPER  
TRADELINE®.  
T872D  
Standard  
2
2
0.1-1.2A 0.1-1.2A 0-1.2A 0-1.0A  
Adjustable locking temperature  
stops (SUPER TRADELINE).  
T874E—Standard and TRADELINE.  
T874F—Standard and TRADELINE.  
T872E  
T872F  
2-Stage Cool  
2-Stage Heat  
2
2
0-1.2A 0-1.0A  
0.1-1.2A 0.1-1.2A  
Locking cover.  
a
Department of Defense.  
b
c
d
e
f
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.  
g
Changeover stage operates with cooling.Q674 Subbases  
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T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES  
Fast cycling.  
Electric Heat  
0.12-  
.6A  
0.12-  
0.6A  
0
T874G—Heat pump, cool  
T872G Heat Pump or  
Standard  
2
1
1
1
1b  
0-1.0Ac  
0.1-1.2A 0-1.0A  
changeover, with fast cycling.  
Fixed anticipator for H2.  
Fast cycling.  
0.1-1.5A  
0.12-  
0.6A  
T874H—Use with Q674C.  
T872H Heat Pump or  
Standard  
1b  
0-1.0Ac  
0-1.0A  
T874J—Heat pump.  
T874K—Heat pump.  
T874L—Heat pump.  
None  
Heat Pump  
2
2
1
1
2d  
1b  
0-1.0Ae  
0.1-1.2A  
0-1.5Ac 0-1.0Ac  
None  
Heat Pump  
0-1.5Ac  
0-1.5A  
None  
T872N  
Heat Pump  
Heat Pump  
2
2
1
1
0.1-1.2A 0.1-1.2A 0-1.5A  
0.1-1.2A 0.1-1.2A 0-1.0A  
T874N—Heat pump, heat  
1b  
changeover.  
T874P—Heat pump.  
T874Q—Night setback heating.  
None  
T872Q  
Heat Pump  
Standard  
Heat Pumpf  
2
1e  
2
1
0.1-1.2A 0.1-1.2A 0-1.5A  
0.1-1.2A  
0-1.5Ac  
0-1.2Ac  
T874R—Heat pump.  
T872R  
None  
1
2
1g  
0.1-1.2A 0-1.5A  
0-1.2Ac  
T874S—Two-speed compressor heat  
Heat Pump  
2
0-1.2A 0-1.0A  
pump.  
T874V—Standard.  
T874W—Heat pump and standard.  
None  
Standard  
3
1
2
0-1.5A  
0-1.2A 0-1.0A  
T872W Heat Pump or  
-1.2Ab 0-1.2Ac,d  
0
Night setback heating.  
Standard  
a
Department of Defense.  
b
c
d
e
f
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.  
g
Changeover stage operates with cooling.Q674 Subbases  
Models:  
Mounting: Designed to mount horizontally on an outlet box  
or the wall. Adapter plate assembly available for mounting  
on a vertical outlet box (see Subbase Accessory).  
See Table 4.  
Electrical Ratings:  
Switch Contacts: 2.5A at 30 Vac (7.5A inrush).  
LED Lights (Optional): 30 Vac.  
Finish: Dark brown or gray.  
Dimensions in in. (mm): 3-1/2 (89)height; 5-5/8 (143) width;  
Switches: Two slide switches (one switch on Q674G and K;  
no switches on Q674D) operated by levers. Switch position  
is shown on scaleplate.  
5/16 (8) depth. See Fig. 1.  
Table 4. Q674 Subbase Specifications.  
Switch Positions  
System  
Models and Options  
Replaces Application  
Fan  
Use With  
Q674A—Standard and TRADELINE®.  
Q672A  
Standard  
HEAT-AUTO-COOL  
AUTO-ON  
T874A-D  
Indicator LEDs.  
Q674B—Standard and TRADELINE.  
Q672B  
Standard  
Heat Pump  
HEAT-OFF-COOL  
AUTO-ON  
AUTO-ON  
T874A-D  
T874G,H,L,R  
T874P  
Provision for fan relay operation from  
external fan switch (isolate G terminal).  
Indicator LEDs.  
OVERRIDE-HEAT-  
OFF-COOL  
OFF-AUTO  
OVERRIDE-AUTO-  
OFF  
Q674C—Standard and TRADELINE.  
Indicator LEDs.  
Q672C  
Q672D  
Standard  
Heat Pump  
AUTO-ON  
AUTO-ON  
T874A-H,W  
T874K,L,N  
T874P  
Q674D—Standard and TRADELINE.  
Standard  
5
None  
None  
T874A-F  
For use when subbase switching is not  
required.  
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T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES  
Q674E—Standard and TRADELINE.  
Q674F—Two LED models.  
Q672E  
Q672F  
Standard  
OFF-HEAT-AUTO-  
COOL  
AUTO-ON  
T874A-D  
Heat Pump OFF-EM.HT.-HEAT-  
AUTO-ON T874C,D,G,N,S  
AUTO-ON  
EM.HT. light.  
AUTO-COOL  
OFF-COOL-AUTO-  
HEAT-EM.HT.  
Provision for AUTO fan operation in  
EM.HT.  
Q674G—O and B terminals.  
Q674J—Provision for AUTO fan operation  
in EM.HT.  
Q672G  
Q672J  
Standard  
Heat Pump  
OFF-AUTO  
None  
T874A-F  
T874A,D,G,J  
EM.HT.-AUTO-OFF  
EM.HT.-ON-OFF  
SUPL.HT.-ON-OFF  
AUTO-ON  
AUTO-ON  
AUTO-ON  
None  
Provision for outdoor thermistor.  
Q674K—Standard.  
Q672K  
Q672L  
Standard  
OFF-HEAT-AUTO-  
T874F  
COOL  
OFF-WOOD-WOOD/  
OIL-OIL  
None  
Q674L—Provision for AUTO fan operation in  
EM.HT.  
Heat Pump EM.HT.-HEAT-OFF-  
AUTO-ON  
AUTO-ON  
AUTO-ON  
AUTO-ON  
T874R,W  
COOL  
SUPL.HT.-HEAT-  
OFF-COOL  
Indicator LEDs.  
Q674N—Standard.  
Q674P—Standard.  
Q674Q—Standard.  
Q674R—Standard.  
Q672N  
Q672P  
None  
Evaporative  
Cooler  
Heat Pump  
EVAP-COOL-OFF-  
T874C  
T874G  
T874A  
T874V  
T874C  
HEAT  
SUPL.HT.-HEAT-  
COOL  
HEAT-OFF-COOL LO-MED-HI-  
Fan Coil  
ON  
LO-HI-  
CONT.  
None  
Fan Coil  
OFF-COOL  
International symbols.  
Q674S—Indicator LED.  
None  
Standard  
HEAT-COOL  
None  
Optional Specifications (Q674 Only):  
L terminal is used for system monitoring devices.  
Common R terminal for heating/cooling.  
Changeover in cool or heat mode for heat pumps.  
Auto fan in EM.HT. for heat pumps.  
Models available with up to four LEDs; for example, LEDs can  
show EM. HT, AUX. HT, SERVICE, CHECK, FILTER, and  
LOCKOUT. See Fig. 3.  
System switching marked HEAT-OFF/RESET-COOL for sys-  
tems requiring impedance relay reset. Available on Q674B  
only.  
Subbase Accessory: 193121A Adapter Plate Assembly for  
mounting on vertical outlet box. Assembly includes adapter  
ring and cover plate. Use to cover wall marks from  
replaced thermostat.  
G terminal isolated on heating to provide fan relay operation  
from external low voltage fan switch (Q674B only).  
Auto fan operation on both heat and cool.  
T874 Thermostat with  
T874 Thermostat with  
Q674 Subbase  
with Four LEDs  
One Setpoint Lever  
Separate  
Heating and Cooling Levers  
Fig. 3. Heating, cooling levers and system LED indicators.  
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T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES  
concealed pipes and chimneys.  
unheated (uncooled) areas such as an outside wall behind  
the thermostat.  
MERCURY NOTICE  
This control contains mercury in a sealed tube. Do not  
place control in the trash at the end of its useful life.  
Mount Subbase  
If this control is replacing a control that contains  
mercury in a sealed tube, do not place your old control  
in the trash.  
The subbase can be mounted on a vertical outlet box,  
horizontal outlet box or directly on the wall.  
1. If the subbase is mounted on a vertical outlet box, order  
Honeywell part no. 193121A Adapter Assembly. See  
Fig. 4. The assembly includes an adapter ring, two  
screws and a cover plate to cover marks on the wall.  
Install the ring and cover plate on the vertical outlet box.  
Contact your local waste management authority for  
instructions regarding recycling and the proper  
disposal of this control, or of an old control containing  
mercury in a sealed tube. If you have questions, call  
the Honeywell Customer Response Center at  
NOTE: For a wall installation, hold subbase in position and  
mark holes for anchors. See Fig. 5. Wall anchors  
must be obtained from local hardware store. Be care-  
ful that the wires do not fall back into the wall open-  
ing. Set aside subbase. Drill four  
1-800-468-1502.  
INSTALLATION  
3
/16 in. (4.8 mm) holes and gently tap anchors into  
the holes until flush with the wall.  
When Installing this Product…  
2
.
.
Pull wires through the cover plate (if used) and subbase  
cable opening. See Fig. 6.  
1.  
Read these instructions carefully. Failure to follow them  
could damage the product or cause a hazardous condi-  
tion.  
3
Secure the cover plate (if used) and subbase with the  
screws provided. Do not fully tighten the subbase  
screws.  
2.  
3.  
4.  
Check the ratings given on the product to make sure the  
product is suitable for your application.  
Installer must be a trained, experienced service techni-  
cian.  
Level the subbase using a spirit level, see Fig. 7, and firmly  
tighten subbase mounting screws. The subbase mounting  
holes provide for minor out-of-level adjustments.  
After installation is complete, check out product opera-  
tion as provided in these instructions.  
VERTICAL  
OUTLET  
MOUNTING  
SCREWS (2)  
1
BOX  
COVER  
PLATE  
2
CAUTION  
ADAPTER  
RING  
2
Hazardous Voltage.  
Can damage heating/cooling system.  
1
.
Disconnect power supply before beginning instal-  
lation to prevent electrical shock or equipment  
damage.  
2.  
.
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.  
1
HORIZONTAL  
OUTLET  
BOX  
3
4.  
THERMOSTAT  
IMPORTANT  
An incorrectly leveled thermostat will cause the tem-  
THERMOSTAT  
COVER  
COOL  
perature control to deviate from setpoint. It is not a  
calibration problem.  
50  
60 70 80  
SUBBASE  
50  
60 70 80  
HEAT  
SUBBASE  
Location  
MOUNTING SCREWS (2)  
Install the thermostat about 5 ft (1.5m) above the floor in an  
area with good air circulation at average temperature.  
50  
60 70 80  
CAPTIVE  
MOUNTING SCREWS (2)  
Do not mount the thermostat where it can be affected by:  
1
2
NOT INCLUDED WITH UNIT.  
drafts or dead spots behind doors, in corners or under  
cabinets.  
ACCESSORY PART AVAILABLE (193121A).  
M6009  
hot or cold air from ducts.  
Fig. 4. Installing Q674 Subbase on outlet box.  
radiant heat from the sun, fireplace, or appliances.  
7
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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  
1
0 wires can be used. Do not use larger than  
8 gauge wire.  
1
Follow equipment manufacturer wiring instructions when  
available. To wire subbase, proceed as follows:  
1.  
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  
2)  
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  
2
.
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.  
4.  
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.  
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8
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T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES  
JUMPER WIRE  
(SUPPLIED WITH SOME MODELS)  
1
1
1
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 Designationsa  
Standard Terminal  
Alternate Designations or  
Customer Specials  
Designation  
Typical Connection  
Heating damper motor; changeover valve  
Emergency heat relay  
Fan relay coil  
System monitor  
B
E
G
L
K
F
O
R
R
V
Cooling damper motor; changeover valve  
Power connection to transformer (internally connected for heating  
and cooling)  
RC  
RH  
W1  
W2  
W3  
Power connection to cooling transformer  
Power connection to heating transformer  
Stage 1 heating control  
Stage 2 heating control  
Stage 3 heating controlb  
H1, R3  
H2, Y, R4  
Y1  
Y2  
Y3  
X
C1, M  
C2  
Stage 1 cooling control  
Stage 2 cooling control  
Stage 3 cooling control  
Clogged filter switch or common connection  
Outdoor thermistor  
X1,X2,C  
T
A
L, C, H  
HSII control panel  
P
Defrost  
O
Momentary circuit, changeover  
LEDs  
A, A1, A2, Z, C, L  
a
b
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.  
9
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T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES  
Table 5. Terminal Designationsa  
Standard Terminal  
Designation  
Alternate Designations or  
Customer Specials  
Typical Connection  
External temperature readout, T relay  
LO and HI speed fan relays  
Cooling contactor  
T
R1, R2  
RS  
Y
M
Compressor contactor  
a
b
Other terminal designations can be used that are not listed on this table. Refer to the hookup drawing and internal schematic  
for exact connections.  
W3 controls the auxiliary heat like W2, and allows adding additional stages of auxiliary heat with outdoor thermostats while  
maintaining the proper second stage anticipation.  
mounting screw and two locking screws with insulated heads.  
Outdoor Disconnect  
When installed, the stop brackets limit the movement of the  
T874 HEAT and COOL levers.  
The National Electrical Code requires the installation of a  
disconnect switch within sight of the outdoor unit of an air  
conditioner or heat pump. The switch is for the safety of any  
technician working on the unit. The technician can assure that  
the unit remains unpowered.  
TO INSTALL:  
1
.
Remove the thermostat cover by pulling the bottom  
edge of the cover upward until it snaps free of the  
mounting slots.  
2.  
Turn to the back of the T874 Thermostat. Locate the  
hole for the brass insert in the plastic base below the  
LED window.  
Install and Adjust Stop Brackets  
The stop brackets should be installed only if there is a need to  
restrict the adjustable range of the heating and cooling  
temperature setpoint levers. If adjustable lever stops are  
desired, order 4074ECK Envelope Assembly, which contains  
two adjustable lever stop brackets, one brass insert, one  
3.  
4.  
5.  
Push the brass insert into the hole with finger.  
Turn to the front of the T874 Thermostat.  
Place the two stop brackets in position with the tabs in  
the slot between the HEAT and COOL levers. See  
Fig. 9.  
HOLES FOR INSULATED  
LOCKING LEVER SCREWS  
COOL  
LEVER  
HOLE WITH  
BRASS INSERT  
HEAT  
LEVER  
BRACKET  
TABS  
ADJUSTABLE LOCKING LEVERS  
NONADJUSTABLE STOPS  
WITH LOCKING LEVER SCREWS  
75°F (24°C) MAX. HEAT  
ADJUSTABLE  
LEVER STOP  
BRACKETS  
MOUNTING  
SCREW  
BRACKET  
SLOTS  
75°F (24°C) MIN. COOL  
NONADJUSTABLE D.O.D. STOPS  
ADJUSTABLE STOPS  
72°F (22°C) MAX. HEAT  
8°F (26°C) MIN. COOL  
M7626  
7
Fig. 9. Range limiting and lever locking methods.  
6.  
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.  
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T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES  
8
.
.
Move the HEAT and COOL levers to the maximum tem-  
perature desired.  
9
Slide the stop brackets until one rests against the HEAT  
lever and the other against the COOL lever.  
1
0. Firmly tighten the mounting screw.  
1. If the HEAT and COOL levers are to be locked in place  
at a specific temperature, use the two insulated head  
screws supplied instead of the two adjustable lever stop  
brackets.  
1
ALLEN  
RETAINING  
SCREWS (2)  
CAUTION  
Equipment Damage Hazard.  
Do not use standard screws that provide metal-to-  
metal contact with the stop brackets. Short circuit and  
potential equipment damage can result.  
M956  
Fig. 10. Installation of locking cover assembly.  
Mount Thermostat  
1.  
Remove the thermostat cover by pulling the bottom  
edge of the cover away from the base until it snaps free  
of the cover clip.  
SETTINGS  
NOTE: The cover is hinged at the top and must be removed  
by pulling up at the bottom.  
CAUTION  
Equipment Damage Hazard.  
2
.
Carefully remove and discard the polystyrene packing  
insert that protects the mercury switches during ship-  
ment.  
On systems using a gas valve, never apply a jumper  
across the valve coil terminals, even temporarily. This  
can burn out thermostat heat anticipator(s).  
3.  
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.  
4.  
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.  
5.  
6.  
7.  
8.  
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.  
1
.
Connect an ac ammeter of appropriate range between  
the heating terminals of the subbase:  
a. Stage 1—between W1 and RH or R;  
b. Stage 2—between W2 and RH or R  
c. Stage 3—between W3 and RH or R.  
Move the system switch to HEAT or AUTO.  
After one minute, read the ammeter and record the  
reading:  
Note the two tabs along the top inside edge of the ther-  
mostat base. The tabs fit into corresponding slots on top  
of the subbase. Mount the thermostat on the subbase.  
Align the two captive mounting screws in the thermostat  
base with the posts on the subbase. Tighten both  
screws. Do not overtighten screws or damage to sub-  
base posts can result.  
2.  
.
3
a. Stage 1—__________A;  
b. Stage 2—__________A;  
c. Stage 3—__________A.  
NOTE: If equipment cycles too fast, set the indicator to a  
higher current rating, but not more than one-half divi-  
sion at a time, and recheck the cycle rate. Most con-  
ventional two-stage heating equipment is designed  
to operate at three cycles per hour per stage, and  
one-stage heating equipment at six cycles per hour,  
at 50 percent load conditions. When using the T874  
Thermostat in heat pump systems, set the heat antic-  
ipator at 140 percent of the actual primary control  
current draw to reduce the cycling rate. See Fig. 11.  
11  
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Most heat pump systems should cycle 2-1/2 to 3 times per hour.  
HEAT—heating system is automatically controlled by the  
thermostat. Cooling system is off.  
4.  
Hang the upper edge of the thermostat cover on top of  
the thermostat base and swing the cover downward  
until it engages with the cover clip.  
AUTO—thermostat automatically changes between heat-  
ing and cooling system operation, depending on the  
indoor temperature.  
COOL—cooling system is automatically controlled by the  
thermostat. Heating system is off.  
STAGE ONE  
ANTICIPATOR  
HEATING  
EM.HT.—emergency heat relay is automatically controlled  
by the thermostat. Cooling system is off. Compressor is  
de-energized.  
CONTROL  
SUPL.HT.—supplemental heat relay is energized. Cooling  
system is off. Compressor is de-energized.  
WOOD—heating system is operating with only the wood-  
burning stage.  
OIL—heating system is operating with only the oil-burning  
stage.  
WOOD/OIL—wood and oil stages operate sequentially;  
first the WOOD stage operates, then the OIL stage  
operates if the WOOD stage cannot handle the load.  
EVAP—controls cooling system by water evaporation; see  
equipment instructions for further information.  
OVERRIDE—night setback is disabled.  
1
.2.8  
STAGE TWO  
ANTICIPATOR  
HEATING CONTROL  
MOVE INDICATOR TO  
MATCH CURRENT RATING  
OF PRIMARY CONTROL  
M5069  
ON—heating system is controlled by the thermostat. EM.  
HT. or SUPL. HT. relay is not energized.  
Fig. 11. Adjustable heat anticipator scales.  
FAN SWITCH positions control fan operation as follows:  
ON or CONT.—fan operates continuously.  
AUTO—fan operates as controlled by the thermostat in  
heat pump systems or conventional cooling mode; fan  
operates as controlled by the plenum switch in conven-  
tional heating mode.  
Temperature Setting  
Move the heating and cooling levers to the desired comfort  
positions. See Fig. 12. On some models with two stages of  
heating or cooling, the same lever controls both stages. The  
minimum differential between heating and cooling setpoints is  
LO—fan operates constantly at low speed.  
MED—fan operates constantly at medium speed.  
HI—fan operates constantly at high speed.  
4°F (2°C) (5°F [3°C]) on T874W.  
If model has optional screws to lock temperature control  
levers, loosen these screws before making temperature  
adjustment; tighten the screws when levers are set at desired  
position.  
To move the subbase switches to the desired control  
positions, use thumb and index finger to slide the lever. The  
lever must stop over desired function indicator position for  
proper circuit operation.  
STAGE 1  
HEATING  
HEATING  
LEVER  
COOLING  
STAGE 3  
HEATING  
Spring return momentary position switching feature is  
available on selected subbase models. On these models, the  
fan switch is positioned to the right of the system switch. By  
moving the fan switch to the far right and releasing it, the ON  
position circuit makes. The lever springs back on release. This  
position is not marked on the subbase.  
LEVER  
STAGE 1  
COOLING  
Setting the Adjustable Differential  
The adjustable interstage differential feature, on a selected  
T874D model only, can be identified by the scale and tension  
screw near the heating and cooling mercury switches. See  
Fig. 13. On this model, the number of degrees between the  
making of the first and second stage mercury bulbs is  
adjustable. This feature is especially useful if the first stage  
controls the comfort temperature, and the second stage  
controls the energy savings temperature. Timers, such as the  
S6005, for insertion between the first and second stage  
control points must be ordered separately.  
STAGE 2  
HEATING  
CAPTIVE  
MOUNTING  
SCREWS (2)  
STAGE 2  
COOLING  
M7625  
Fig. 12. Internal view of T874W (three stages of  
heating, two stages of cooling).  
Each mark on the scale represents 1°F (0.6°C) The  
differential is factory set at 2°F (1°C) the differential can be set  
as high as 12°F (7°C) To set the adjustable interstage  
differential, loosen the tension screw. See Fig. 13. Slide the  
adjustable scale to align with the number of degrees desired  
between stages. Use the lower edge of the tension screw  
bracket as a guide for alignment. In heating, slide the lever  
wider apart for a larger differential, or closer together for a  
Subbase Setting  
The subbase switching positions control the system operation  
as described below.  
SYSTEM SWITCH (see subbase for positions):  
OFF—both the heating and cooling systems are off. If the  
fan switch is at the AUTO position, the cooling fan is  
also off.  
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T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES  
smaller differential. In cooling, slide the lever closer together  
for a larger differential, or wider apart for a smaller differential.  
While supporting the scale with hand, tighten the tension screw.  
CAUTION  
Equipment Damage Hazard.  
When the thermostat is used to control a two-stage  
heating or cooling system, the second stage mercury  
bulb must never make before the first stage bulb, or  
severe equipment damage could result. To prevent  
this problem, provide at least 2°F (1°C) differential  
between stage-one and stage-two make points.  
Example: in heating, if stage-one makes at 70°F (21°C)  
stage-two should make at 68°F (20°C) or lower.  
IMPORTANT  
Support the scale with hand while tightening tension  
screw. See Fig. 13. Failure to do so can result in  
twisting and damaging bimetal coil.  
HEATING SET-  
POINT LEVER  
TENSION  
SCREW  
HEAT  
50 60 70 80  
FIRST  
STAGE  
SWITCH  
Verify the Adjustment  
Heating  
Start with the heating setpoint lever all the way to the left.  
Slowly move the lever to the right, just until the first stage bulb  
makes (mercury rolls to the right side of the bulb). Note the  
setting on the temperature scale. Slowly move the lever to the  
right until the second stage bulb makes. Note the setting on  
the temperature scale. The difference between the two  
temperatures is the interstage differential, which should match  
the number set on the scale with the tension screw.  
1
.2  
SCALE  
SECOND STAGE  
ADJUSTABLE)  
SWITCH  
(
Cooling  
HEATING  
Start with the cooling setpoint lever all the way to the right.  
Slowly move the lever to the left, just until the first stage bulb  
makes (mercury rolls to the left side of the bulb). Note the  
setting on the temperature scale. Slowly move the lever to the  
left until the second stage bulb makes. Note the setting on the  
temperature scale. The difference between the two  
TENSION  
SCREW  
ALIGN LOWER  
EDGE WITH  
SCALE  
SLIDE LEVER  
temperatures is the interstage differential, which should match  
the number set on the scale with the tension screw.  
NOTCH  
0°F  
WIDER APART  
FOR LARGER  
DIFFERENTIAL  
1
SCALE  
1
CHECKOUT  
COOLING  
Heating  
Move the system switch on the Q674 Subbase to HEAT or  
AUTO. Move the heat lever on the T874 about 10°F (6°C)  
above room temperature. See Fig. 12. Heating system should  
start and the fan should run after a short delay. Move the heat  
lever about 10°F (6°C) below room temperature. The heating  
equipment should shut off, and the fan should run for a short  
time, then shut off.  
TENSION  
SCREW  
ALIGN LOWER  
EDGE WITH  
SCALE  
SLIDE LEVER  
CLOSER TOGETHER  
FOR LARGER  
NOTCH  
4°F  
DIFFERENTIAL  
SCALE  
In heat pump applications, sometimes time delays are  
involved before the compressor and auxiliary heat are  
activated. This is due to a minimum-off timer, which prevents  
the compressor from restarting for five minutes from when the  
thermostat last turned off the compressor, or from when the  
system first received power.  
1
1
EACH MARK ON THE SCALE REPRESENTS 1°F (0.6°C).  
M937  
Fig. 13. Set adjustable interstage differential.  
Cooling  
CAUTION  
Equipment Damage Hazard.  
Do not operate cooling if outdoor temperature is below  
50°F (10°C). Refer to manufacturer recommendations.  
13  
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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.  
1
.
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.  
2
.
.
Set the thermostat cover on a table near an accurate  
thermometer.  
3
Allow ten minutes for cover thermometer to sense area  
temperature; compare the readings. Be careful not to  
touch thermometer or breathe on it.  
4.  
If the readings are the same, replace cover and put the  
system into operation.  
Outdoor Reset Thermistor (Where  
Applicable)  
5. If the readings are different, insert a small screwdriver in  
the thermometer slot and turn it until the thermometers  
have the same reading. See Fig. 15.  
If the system is supplied with a thermistor, it must be used; if  
not used, thermostat performance deviates radically from  
proper operation.  
6. Replace thermostat cover and put the system into oper-  
ation.  
The proper thermistor operation must be verified to ensure the  
correct operation of the thermostat. Check thermistor  
operations as follows:  
1
.
.
Disconnect the T wire on the subbase.  
2
Use an ohmmeter to measure resistance between the  
T wire and the A subbase terminal.  
3.  
.
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.  
4
4
4
4
4
3
3
3
3
3
2
2
2
2
2
1
1
1
1
1
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.  
8
6
4
2
00  
00  
00  
00  
0
Circuit descriptions and terminology are defined as follows:  
For standard heating-cooling circuits:  
-20  
0
20  
40  
60  
80  
100  
120 140  
M1590A  
TEMPERATURE OF THERMISTOR (°F)  
Auto changeover—refers to the presence of an AUTO  
position in the system switching (EX: Q674E with OFF-  
HEAT-AUTO-COOL switching). The thermostat auto-  
matically changes between heat and cool modes as  
indoor temperature changes.  
Fig. 14. Thermistor resistance chart.  
CALIBRATION  
Manual changeover—requires a system switch movement  
to change mode (EX: Q674B with HEAT-OFF-COOL  
switching). T874D Multistage Thermostats with 2 heat  
or 2 cool switches are shown on most standard circuits.  
Most standard or TRADELINE® subbases (Q674A-E,G)  
can be used with T874A-F standard or TRADELINE  
Thermostat  
T874 Thermostats are accurately calibrated at the factory.  
They do not have provision for field calibration.  
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T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES  
thermostats. The schematics can be field-modified as  
required (EX: if T874C is being used, eliminate second  
stage of heat).  
perature rise. Fixed anticipation is represented by a zig-  
zag line and adjustable anticipation is a zigzag with an  
arrow. The resistance of the fixed anticipator is so large  
it limits current so that a system relay cannot be pulled  
in from a circuit path going through the fixed anticipator.  
The relay can be pulled in through an adjustable antici-  
pator because its resistance is generally 0 to 5 ohms.  
5. The center portion (the subbase) contains the switches.  
The fan switch is above the system switch. The small  
circles on the switch represent the maximum possible  
contacts available on the Q674 Subbase. The larger cir-  
cles represent the switch positions available on this par-  
ticular Q674, with the solid circle representing where it  
is actually switched on the diagram.  
For heat pump circuits:  
Cool changeover valve—operates on cooling. The revers-  
ing valve or relay is activated either by moving the sys-  
tem switch to COOL (manual changeover) or by a  
mercury switch that makes on a temperature rise (auto  
changeover).  
Heat changeover valve—operates on heating. The revers-  
ing valve or relay is activated either by moving the sys-  
tem switch to HEAT (manual changeover) or by a  
mercury switch that makes on a temperature fall (auto  
changeover).  
System monitor relay—optional equipment on some heat  
pumps includes an R4222P1065 or equivalent. This  
system monitor relay detects a malfunction in the com-  
pressor and indicates the malfunction by activating the  
EMERGENCY HEAT LED on the Q674 Switching Sub-  
base. The system monitor relay is usually wired into the  
L terminal on the Q674.  
NOTE: Solid circles are not interconnected electrically.  
At the right, the relays and contactors are shown, attached to  
the proper terminals. The terminals are represented by large  
circles with terminal designations in capital letters. See Table  
3 for the meaning of each lettered terminal.  
Each mercury switch is identified by function:  
H1—Stage 1 Heating.  
Sometimes power for a fixed anticipator is brought through an  
off system relay like the changeover relay shown in Fig. 20.  
This current is kept low by the high resistance of the fixed  
anticipator so that relay does not pull in.  
H2—Stage 2 Heating.  
H3—Stage 3 Heating.  
C1—Stage 1 Cooling.  
C2—Stage 2 Cooling.  
C3—Stage 3 Cooling.  
C/O—Changeover (heat pumps).  
KEY TO HOOKUP SYMBOLS  
Each anticipator is identified and each switch affected is  
named (EX: H1 anticipator, C1 anticipator).  
RELAY/CONTACTOR CONTACTS  
TRANSFORMER  
All T874 Multistage Thermostats use mercury switches. Each  
schematic indicates switch operation by being drawn in the  
open position with an arrow indicating operation with a  
temperature RISE or FALL.  
(24 VAC SECONDARY)  
B
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  
1.  
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)  
RD  
DEFROST RELAY  
CHP  
PRESSURE SWITCH  
ADJUSTABLE ANTICIPATOR  
LOW RESISTANCE  
TYPICALLY 0 TO 5 OHMS)  
LACO  
2.  
3.  
4.  
Completely trace only one circuit at a time (for example:  
heat or cool).  
(
LOW AMBIENT CUTOFF  
M5848  
Connections are indicated by small dots at the point of  
intersection. If there is no dot, there is no connection.  
The left portion of the circuit (the thermostat) contains  
the mercury switches. The heat switches make on a  
temperature fall, and the cool switches make on a tem-  
Fig. 16. Key to hookup symbols.  
15  
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T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES  
THERMOSTAT  
SUBBASE  
SYSTEM COMPONENTS  
HOT) L1 L2  
(
1
H1/C1  
ANTICIPATOR  
R
FALL  
2
ODT 1  
EHR 1  
ODT 2  
H1  
RTD 1  
RTD 2  
C1  
W3  
3
4
FAN SWITCH  
AUTO  
ON  
EHR 2  
RTD 3  
RTD 1  
H2  
ANTICIPATOR  
W2  
5
6
AUX. HT.  
LED  
GREEN)  
(
H2  
2
X
FALL  
EM. HT.  
LED (RED)  
SYSTEM  
MONITOR  
SYSTEM  
SWITCH  
RD  
L
FAN RELAY  
G
O
EM. HT.  
HEAT  
OFF  
EM. HT.  
RELAY  
COOL CHANGEOVER  
VALVE  
E
COOL  
LACO  
CHP  
Y
B
COMPRESSOR  
CONTACTOR  
1
2
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.  
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16  
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T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES  
THERMOSTAT  
SUBBASE  
SYSTEM COMPONENTS  
L1  
(HOT)  
H1/C1  
R
ANTICIPATOR  
ODT 1  
EHR 1  
ODT 2  
L2  
FALL  
C1  
2
1
RTD 1  
RTD 2  
H1  
FAN SWITCH  
W3  
W2  
3
4
AUTO  
ON  
EHR 2  
RTD 3  
H2  
ANTICIPATOR  
AUX. HT.  
LED  
GREEN)  
RTD 1  
5
6
(
X
H2  
EM. HT.  
LED (RED)  
SYSTEM  
MONITOR  
FALL  
RD  
SYSTEM  
SWITCH  
L
FAN RELAY  
G
O
EM. HT.  
HEAT  
OFF  
EM. HT.  
RELAY  
CHANGEOVER  
VALVE  
E
COOL  
LACO  
CHP  
1
POWER SUPPLY. PROVIDE  
DISCONNECT MEANS AND  
OVERLOAD PROTECTION  
AS REQUIRED.  
Y
B
COMPRESSOR  
CONTACTOR  
M5840  
Fig. 18. Tracing the changeover relay circuit.  
THERMOSTAT  
H1/ C1  
SUBBASE  
SYSTEM COMPONENTS  
L1  
HOT)  
(
R
ANTICIPATOR  
ODT 1  
EHR 1  
ODT 2  
L2  
2
FALL  
C1  
1
RTD 1  
RTD 2  
H1  
FAN SWITCH  
W3  
3
4
AUTO  
ON  
EHR 2  
RTD 3  
H2  
ANTICIPATOR  
W2  
AUX. HT.  
LED  
GREEN)  
RTD 1  
5
6
(
X
H2  
EM. HT.  
LED (RED)  
SYSTEM  
MONITOR  
FALL  
RD  
SYSTEM  
SWITCH  
L
FAN RELAY  
G
O
EM. HT.  
HEAT  
OFF  
EM. HT.  
RELAY  
CHANGEOVER  
VALVE  
E
1
POWER SUPPLY. PROVIDE  
DISCONNECT MEANS AND  
OVERLOAD PROTECTION  
AS REQUIRED.  
COOL  
LACO  
CHP  
Y
B
COMPRESSOR  
CONTACTOR  
M5841  
Fig. 19. Tracing the heat 1 and anticipation circuit.  
17  
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T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES  
THERMOSTAT  
H1/ C1  
SUBBASE  
SYSTEM COMPONENTS  
L1  
ANTICIPATOR  
(HOT)  
R
FALL  
2
ODT 1  
EHR 1  
L2  
1
H1  
C1  
RTD 1  
RTD 2  
3
4
FAN SWITCH  
W3  
AUTO  
ON  
ODT 2  
EHR 2  
H2  
ANTICIPATOR  
RTD 3  
W2  
5
6
AUX. HT.  
LED  
GREEN)  
RTD 1  
(
H2  
X
FALL  
EM. HT.  
LED (RED)  
SYSTEM  
MONITOR  
RD  
SYSTEM  
SWITCH  
L
FAN RELAY  
G
O
EM. HT.  
HEAT  
OFF  
EM. HT.  
RELAY  
CHANGEOVER  
VALVE  
E
1
POWER SUPPLY. PROVIDE  
DISCONNECT MEANS AND  
OVERLOAD PROTECTION  
AS REQUIRED.  
COOL  
LACO  
CHP  
Y
B
COMPRESSOR  
CONTACTOR  
M5842  
Fig. 20. Tracing the heat 1 anticipation circuit.  
THERMOSTAT  
SUBBASE  
SYSTEM COMPONENTS  
L1  
H1/C1  
ANTICIPATOR  
(HOT)  
R
ODT 1  
EHR 1  
L2  
1
FALL  
2
RTD 1  
RTD 2  
FAN SWITCH  
H1  
C1  
W3  
3
4
AUTO  
ON  
ODT 2  
EHR 2  
RTD 3  
H2  
ANTICIPATOR  
W2  
AUX. HT.  
LED  
GREEN)  
RTD 1  
5
6
(
X
H2  
EM. HT.  
LED (RED)  
SYSTEM  
MONITOR  
FALL  
RD  
SYSTEM  
SWITCH  
L
FAN RELAY  
G
O
EM. HT.  
HEAT  
OFF  
EM. HT.  
RELAY  
CHANGEOVER  
VALVE  
E
1
POWER SUPPLY. PROVIDE  
DISCONNECT MEANS AND  
OVERLOAD PROTECTION  
AS REQUIRED.  
COOL  
LACO  
CHP  
Y
B
COMPRESSOR  
CONTACTOR  
M5843  
Fig. 21. Tracing the heat 2, anticipation and AUX. HT. LED circuit.  
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18  
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T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES  
THERMOSTAT  
SUBBASE  
SYSTEM COMPONENTS  
L1  
(
HOT)  
H1/C1  
ANTICIPATOR  
R
ODT 1  
EHR 1  
L2  
FALL  
2
1
RTD 1  
RTD 2  
FAN SWITCH  
H1  
C1  
W3  
3
AUTO  
ON  
ODT 2  
EHR 2  
4
RTD 3  
H2  
ANTICIPATOR  
W2  
AUX. HT.  
LED  
GREEN)  
RTD 1  
5
6
(
X
H2  
EM. HT.  
LED (RED)  
SYSTEM  
MONITOR  
FALL  
RD  
SYSTEM  
SWITCH  
L
FAN RELAY  
G
O
EM. HT.  
HEAT  
OFF  
EM. HT.  
RELAY  
CHANGEOVER  
VALVE  
E
COOL  
1
POWER SUPPLY. PROVIDE  
DISCONNECT MEANS AND  
OVERLOAD PROTECTION  
AS REQUIRED.  
LACO  
CHP  
Y
B
COMPRESSOR  
CONTACTOR  
M5844  
Fig. 22. Tracing the emergency heat and EM. HT. LED circuit.  
THERMOSTAT  
SUBBASE  
SYSTEM COMPONENTS  
L1  
(HOT)  
R
H1/C1  
ANTICIPATOR  
ODT 1  
EHR 1  
L2  
FALL  
2
1
RTD 1  
RTD 2  
FAN SWITCH  
H1  
W3  
W2  
C1  
H2  
3
4
AUTO  
ON  
ODT 2  
EHR 2  
RTD 3  
ANTICIPATOR  
AUX. HT.  
LED  
GREEN)  
RTD 1  
5
6
(
X
H2  
EM. HT.  
LED (RED)  
SYSTEM  
MONITOR  
FALL  
RD  
SYSTEM  
SWITCH  
L
FAN RELAY  
G
O
EM. HT.  
HEAT  
OFF  
EM. HT.  
RELAY  
CHANGEOVER  
VALVE  
E
1
POWER SUPPLY. PROVIDE  
DISCONNECT MEANS AND  
OVERLOAD PROTECTION  
AS REQUIRED.  
COOL  
LACO  
CHP  
Y
B
COMPRESSOR  
CONTACTOR  
M5845  
Fig. 23. Tracing the cooling circuit.  
19  
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T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES  
THERMOSTAT  
SUBBASE  
SYSTEM COMPONENTS  
L1  
R
W3  
W2  
(HOT)  
H1/C1  
ANTICIPATOR  
ODT 1  
EHR 1  
L2  
FALL  
2
1
RTD 1  
RTD 2  
FAN SWITCH  
H1  
C1  
3
4
AUTO  
ON  
ODT 2  
EHR 2  
RTD 3  
H2  
ANTICIPATOR  
AUX. HT.  
LED  
GREEN)  
RTD 1  
5
6
(
X
H2  
EM. HT.  
LED (RED)  
SYSTEM  
MONITOR  
FALL  
RD  
SYSTEM  
SWITCH  
L
FAN RELAY  
G
O
EM. HT.  
HEAT  
OFF  
EM. HT.  
RELAY  
CHANGEOVER  
VALVE  
1
POWER SUPPLY. PROVIDE  
DISCONNECT MEANS AND  
OVERLOAD PROTECTION  
AS REQUIRED.  
E
COOL  
LACO  
CHP  
Y
B
COMPRESSOR  
CONTACTOR  
M5846  
Fig. 24. Tracing the auto fan circuit.  
THERMOSTAT  
SUBBASE  
SYSTEM COMPONENTS  
L1  
H1/C1  
R
(HOT)  
ANTICIPATOR  
ODT 1  
EHR 1  
ODT 2  
L2  
FALL  
2
1
RTD 1  
RTD 2  
FAN SWITCH  
H1  
C1  
W3  
3
4
AUTO  
ON  
EHR 2  
RTD 3  
H2  
W2  
ANTICIPATOR  
AUX. HT.  
LED  
RTD 1  
5
6
(GREEN)  
X
H2  
EM. HT.  
LED (RED)  
SYSTEM  
MONITOR  
FALL  
RD  
SYSTEM  
SWITCH  
L
FAN RELAY  
G
O
EM. HT.  
HEAT  
OFF  
EM. HT.  
RELAY  
CHANGEOVER  
VALVE  
E
1
POWER SUPPLY. PROVIDE  
DISCONNECT MEANS AND  
OVERLOAD PROTECTION  
AS REQUIRED.  
COOL  
LACO  
CHP  
Y
B
COMPRESSOR  
CONTACTOR  
M5847  
Fig. 25. Tracing the fan on circuit.  
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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.  
L1  
L2  
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  
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.  
L1  
HOT)  
L2  
(
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.  
H1  
STAGE 1 HEAT RELAY  
FALL  
M5823  
Fig. 26. Adjustable anticipation heater in series with load.  
Fixed anticipation is also called voltage anticipation. See  
Fig. 27. The heater is in parallel with the primary control and is  
not affected by the current draw of the primary. It establishes  
the proper cycle rate for any application.  
Droop  
factor called droop.  
The addition of anticipator heat to the bimetal causes a control  
As the weather grows colder, the heating appliance must  
operate longer and more frequently. More heat must be added  
to the bimetal. The bimetal now thinks it is warmer than it  
really is, and actual room temperature is controlled at a lower  
temperature than the setpoint indicates. At 100 percent heat  
load, this droop can be significant.  
L1  
HOT)  
L2  
(
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  
H1  
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.  
21  
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T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES  
A strip of four inserts is included with TRADELINE T874. Drop  
a strip into the recessed area behind the scaleplate so  
selected LEDs show. Make sure insert is completely seated in  
recessed area. Let scaleplate pop back; then set levers to  
desired position.  
CAUTION  
Equipment Damage Hazard.  
Due to calibration techniques used for T874  
Thermostats with outdoor reset, the C815A Thermistor  
must be wired into the system at all times. Failure to  
do so will result in serious degradation of performance.  
FILTER LED lights when the filter is clogged and needs  
replacement.  
CHECK LED lights when something needs to be checked  
or done to maintain efficient operation of system. See  
heating system instructions for CHECK LED meaning.  
EM. HT. LED lights when the emergency heat is operating.  
SUPL. HT. LED lights when the supplemental heat is  
operating.  
Service and Replacement of C815A Outdoor  
Thermistor  
To check and verify thermistor operation, perform the  
following steps:  
LOCKOUT LED lights when the system is shut down and  
needs maintenance.  
1
.
.
Disconnect wire from T terminal on subbase.  
Measure resistance with ohmmeter across the T sub-  
base wire and X subbase terminal.  
AUXILIARY HEAT LED lights when the auxiliary heat is  
operating.  
2
SERVICE or MALFUNCTION LED can have several  
meanings. Consult heating system instructions.  
3.  
Take outdoor temperature measurement at thermistor  
location and find correct thermistor resistance on the  
chart in Fig. 29.  
LEDs cannot be replaced or added in the field.  
4.  
Compare resistance on the chart with measured resis-  
tance. Replace C815A if resistance varies more than 15  
percent. Contact installing dealer for packaged replace-  
ment outdoor thermistor.  
LEDS  
EM.  
HEAT FILTER SERV. HEAT  
AUX  
4600  
4400  
4200  
4000  
3800  
3600  
3400  
3200  
3000  
2800  
2600  
2400  
2200  
2000  
1800  
1600  
1400  
1200  
1000  
COOL  
80  
50 60 70 80  
C815A THERMISTOR RESISTANCE  
R = 400 ohms ± 10% AT 77°F (25°C)  
M5830  
Fig. 30. T874/Q674 LED location.  
Restricted Setpoint (DoD)  
The Department of Defense (DoD) models are equipped with  
a restricted setpoint feature for fuel efficiency.  
800  
600  
400  
200  
0
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  
0
20  
40  
60  
80  
100  
120 140  
M1590A  
TEMPERATURE OF THERMISTOR (°F)  
Fig. 29. Thermistor resistance chart.  
Applications  
The T874/Q674 can be applied to standard residential  
systems for automatic or manual changeover, to commercial  
rooftop applications, or to heat pump applications.  
Features  
Two of the features of the T874/Q674 include LED indicators  
and restricted setpoint.  
Changeover on Standard Residential  
Systems  
LED Indicators  
The light-emitting diodes (LED) indicators on the subbase  
light on command when something specific happens in the  
system. See Fig. 30.  
In a standard residential heating-cooling circuit, changeover  
between heating and cooling can be done either automatically  
or manually.  
Up to four different LEDs are available. The thermostat has a  
clear lens window for viewing each LED. On TRADELINE  
models, a small insert is used so the LED function desired can  
be selected. This must be done during installation.  
Automatic changeover is done by an AUTO position on the  
subbase system switch. See Fig. 31. When the switch is in the  
AUTO position, the thermostat automatically changes between  
heat and cool modes, depending on the indoor temperature.  
A blank insert is factory-installed in some T874 models. To  
remove it, push both temperature setting levers to the far ends  
of the thermostat. Use index fingernail to gently pull out the  
scaleplate a fraction of an inch. Turn thermostat upside-down,  
and the blank insert falls out.  
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T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES  
When enthalpy is low enough, the thermostat and a sensor tell  
the economizer damper to open to let in cool air from the  
outside. When enthalpy is high, the thermostat and sensor tell  
the damper to close. No more outside air comes in, so the air  
conditioner is used instead.  
OFF EM. HT. HEAT AUTO COOL  
Heat Pump Application  
M5828  
A heat pump system operates much like an air conditioner.  
However, the heat pump is capable of moving heat in two  
directions; from inside the home to outdoors for cooling, or  
from outdoors to inside for heating.  
Fig. 31. System switching for automatic changeover.  
Automatic changeover is frequently used in areas where there  
are large temperature differences in 24 hours. Some uses for  
the automatic changeover feature are in the southern states  
The heat pump can be controlled by separate relay/contactor  
components, which are energized or de-energized by the T874.  
(
cool in the morning and hot in the afternoon) and in  
commercial applications.  
Changeover between heating and cooling for heat pump  
circuits can be accomplished either manually or automatically.  
Manual changeover requires a system switch movement to  
change the mode. See Fig. 32. Most manual changeover  
switches consist of HEAT-OFF-COOL choices. For heating,  
switch to HEAT. For cooling, switch to COOL. It is not automatic.  
Manual changeover control requires a system switch  
movement to change the mode. The reversing valve is  
activated by moving the system switch to COOL for cooling or  
to HEAT for heating operation.  
Manual changeover is often used in areas where the  
temperature is relatively stable between day and night.  
Automatic changeover is accomplished by one of the mercury  
bulb switches in the thermostat. On a system with automatic  
changeover on cooling, the changeover valve is energized by  
a mercury switch that makes on a temperature rise. On  
heating, the changeover valve is energized by a mercury  
switch that makes on a temperature fall.  
A description of the methods used to control heat pumps  
follows.  
OFF EM. HT. HEAT COOL  
M5829  
Fig. 32. System switching for manual changeover.  
Space Temperature Sensing Low Voltage  
Controls  
Commercial Rooftop Application  
Control systems for heat pumps usually utilize low voltage  
control circuits. The variety of control functions required and  
the relative complexity require a versatile and economical  
method of control. There are several interlocks and indication  
functions that would be more difficult and expensive to wire in  
line voltage circuits. Low voltage controls have the precision  
and flexibility needed.  
The T874 can be used as part of a commercial rooftop  
application to run mechanical cooling.  
An economizer is often used as part of a rooftop application to  
save on air conditioning. See Fig. 33.  
The thermostat used is a low voltage device that has good  
temperature sensitivity and several manual and automatic  
switching capabilities.  
OUTDOOR  
AIR  
Two-Stage Thermostat  
In most installations, the heat pump itself requires auxiliary  
heat, usually in the form of electric strip heaters, to carry the  
building heating load at design conditions. Since the heat  
pump is usually more efficient, the control strategy is to run  
the heat pump whenever possible and the auxiliary heat only  
when necessary—during very cold weather or if the heat  
pump fails. Usually the auxiliary heat is used only below the  
balance point (temperature at which the heat pump cannot  
handle heating load by itself).  
H205 OR H705  
ENTHALPY  
CONTROL  
RETURN  
AIR  
DISCHARGE AIR SENSOR  
DOWNSTREAM FROM THE  
EVAPORATOR COIL  
M5827A  
Fig. 33. Typical commercial rooftop economizer  
application.  
A two-stage thermostat makes this possible. The two stages  
make sequentially as the temperature drops. There is a  
degree or two between stages so that the second stage (the  
Enthalpy is the temperature and humidity of the air. A more  
accurate analysis of the suitability for free cooling can be  
made by monitoring the enthalpy.  
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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.  
L1  
HOT)  
L2  
(
Voltage Anticipation  
Although not a specific requirement for heat pumps, several  
models of the T874 have voltage heat anticipation instead of  
current anticipation.  
C815A  
RESET  
HEATER  
OUTDOOR  
THERMISTOR  
SENSOR  
With voltage anticipation, the amount of heat added to the  
thermostat is constant no matter which load is connected to it.  
The anticipator realizes a constant voltage and produces a  
constant amount of heat during the ON cycle. This allows the  
cycling rate to be designed into the thermostat. The heat  
pump manufacturer is assured of correct performance  
independent of later thermostat adjustments.  
M5826  
Fig. 34. Location of thermistor.  
Changeover  
accomplished either automatically or manually, depending on  
the application. Changeover can occur in heating or cooling.  
Outdoor Reset  
Changeover between heating and cooing can be  
Outdoor reset, which a C815A Outdoor Thermistor makes  
possible, is sometimes applied in heat pump systems to help  
minimize the effect of the differential between stages. It takes  
2
°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.  
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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.  
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Defrost Control  
L1  
(HOT)  
L2  
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.  
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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  
HEAT RELAY  
CONTROLS  
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  
1
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  
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.  
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  
5
(
°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  
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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  
SWITCH  
HEAT 2 HEAT 1  
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  
L1  
L3  
24 V  
RUN  
CAP.  
R
THERMOSTAT  
FUNCTION  
C
HEAT 2 HEAT 1  
SWITCH  
EMERGENCY  
HEAT RELAY  
CONTACT  
START  
S
COMPRESSOR  
Y
R
L
X
L2  
L1  
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.  
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3
4
5
6
.
.
.
.
The Description column identifies certain characteristics  
of the existing device.  
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.  
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.  
All thermostats listed carry the Honeywell logo unless  
otherwise noted.  
7.  
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:  
1
.
.
Locate and identify the existing thermostat and subbase  
model.  
2
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  
a
Model Number  
T872A  
Description  
1-stage heat, 1-stage cool  
Replacement  
Remarks  
A1006  
A1014  
Standard OEM  
T874A1036  
T874A1010  
72-78°F (22-26°C) stop; locking cover; no  
Use T874A1176 for Dept. of  
Defense (DoD) application.  
thermometer  
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  
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  
T874A1036  
T874A1036  
T874A1036  
T874A1036  
T874A1036  
T874A1036  
T874A1036  
T874A1036  
T874A1036  
T874A1036  
T874A1036  
T874A1036  
T874A1036  
T874A1036  
T874A1036  
T874A1036  
T874A1036  
T874A1036  
T874A1036  
T874A1010  
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  
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  
a
When replacing a T872 Thermostat and Q672 Subbase, always replace both the thermostat and the subbase.  
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Table 6. T872-T874 Thermostat Cross Reference.  
T874 Trade  
a
Model Number  
Description  
Replacement  
Remarks  
A1279  
General Electric logo; no. AY28X114A; 75°F stop; T874A1010  
Use T874A1176 for Dept. of  
locking cover; no thermometer.  
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.  
0
A1295  
A1303  
A1311  
A1329  
A1337  
A1345  
A1352  
A1360  
A1378  
Lennox; Celsius scale  
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  
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  
T874A1036  
T874A1176  
T874A1010  
T874A1036  
T874A1010  
T874A1036  
T874A1036  
T874A1036  
T872B  
B1005  
B1013  
B1021  
B1039  
B1047  
B1054  
B1062  
B1070  
B1088  
B1096  
B1104  
B1112  
B1120  
B1138  
T874B1019  
T874B1019  
T874B1019  
T874B1019  
T874B1019  
T874B1019  
T874B1019  
T874B1019  
T874B1019  
T874B1019  
T874B1019  
T874B1019  
T874B1019  
T874B1019  
Singer logo; no. 02005711; locking cover and  
lever.  
B1146  
B1153  
T872C  
Lennox logo; no. P-8-8895  
Lennox logo; Celsius scale  
2-stage heat, 1-stage cool  
Standard OEM  
T874B1019  
T874B1019  
C1004  
C1012  
C1020  
T874C1018  
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  
American Standard logo  
Friedrich logo  
T874C1018  
T874C1018  
T874C1018  
T874C1018  
Lennox logo; no. P-8-8896  
Westinghouse logo; stage 1 heat anticipator set at T874C1018  
0.75A; stage 2 anticipator set at 1.2A  
C1087  
C1095  
Fast cycle performance on both heat stages;  
T874C1018  
T874C1018  
Adjust anticipator faster for correct  
anticipator range 0.12-0.6A  
performance.  
Payne logo; no. 0.1-0.175  
a
When replacing a T872 Thermostat and Q672 Subbase, always replace both the thermostat and the subbase.  
60-2485—8  
30  
href="
T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES  
Table 6. T872-T874 Thermostat Cross Reference.  
T874 Trade  
a
Model Number  
C1103  
C1111  
C1129  
C1137  
Description  
Amana logo; anticipator set at 1.2A  
Airtemp (Chrysler) logo  
Replacement  
T874C1018  
T874C1018  
T874C1018  
T874C1018  
Remarks  
Day and Night logo; no. 0.1-0.174  
Fast cycle performance on both heat stages  
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  
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  
T874C1018  
T874C1018  
T874C1018  
T874C1018  
T874C1018  
T874C1018  
T874C1018  
T874C1018  
T874C1018  
T874C1018  
T874C1018  
T874C1018  
T874C1018  
T874C1018  
T874C1018  
T874C1018  
T874C1018  
T874C1018  
T874C1018  
T874C1018  
T874C1018  
T874C1018  
T874C1125  
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  
Adjust anticipator faster for correct  
stages.  
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  
T874C1018  
T874C1018  
None  
T874C1018  
T874C1018  
T874C1018  
T874C1141  
Lennox logo; no. 42C04; four LEDs  
Carrier logo; no. HH07AT070C; Celsius scale  
Goettl logo  
Goettl logo; used with Q672N1001, N1019  
Lennox logo; Celsius scale  
Tappan logo; no. 080-13708-37  
Fasco logo; no. 1230-8161  
Lennox logo; no. 64A0501; locking cover; no  
thermometer; meets DoD specs  
a
When replacing a T872 Thermostat and Q672 Subbase, always replace both the thermostat and the subbase.  
31  
60-2485—8  
href="
T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES  
Table 6. T872-T874 Thermostat Cross Reference.  
T874 Trade  
a
Model Number  
Description  
Replacement  
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  
Airtemp logo; used with Q672L1102  
T874C1018  
T874C1018  
Y594R1425  
Wiring differences (old=new): C=X,  
K=E; remove Y-W1 jumper.  
C1566  
C1574  
C1582  
C1590  
TRADELINE; Celsius scale  
Whalen special; used with Q672B1269  
Lennox; meets DoD  
T874C1018  
None  
None  
Replacement has °F scaleplate.  
Singer; 8°F (4.4°C) differential between stage  
T874C1125  
Replacement has 2°F (1° C)  
1
and stage 2  
differential between H1 and H2.  
C1608  
Singer logo; no. 02005784 8°F (4.4°C) differential T874C1125  
Replacement has 2°F (1°C)  
between stage 1 and stage 2  
differential between H1 and H2.  
C1616  
C1624  
C1632  
C1640  
C1665  
C1673  
Celsius scale; stop set at 23°C  
Heat Wave logo; no. 0360-116  
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  
None  
Add 4074ECK adjustable stop.  
T874C1018  
T874C1018  
T874C1018  
T874C1018  
Gas systems:  
T874C1018/  
Q674B1034  
Electric systems: Adjust anticipator faster for correct  
T874C1018/  
Q674B1075  
performance.  
C1681  
C1699  
Lechauffage Urbain logo; Celsius scale  
T874C1018  
T874C1018  
Duotherm logo; used with Q672E1068 or  
See subbase description for  
replacement information.  
Q672F1000  
C1707  
C1715  
T872D  
Sigler logo; used with Q672N1027  
Westinghouse logo  
2-stage heat, 2-stage cool  
Standard OEM  
TRADELINE  
Westinghouse logo  
Climatrol logo  
Lennox logo; no. P-8-8897; used with Q672E1191. T874D1165  
York logo  
Singer logo; no. 02005385  
Rheem logo  
Rudd logo  
None  
T874C1018  
D1003  
D1011  
D1029  
D1037  
D1045  
D1052  
D1060  
D1078  
D1086  
D1094  
D1102  
D1110  
D1128  
D1136  
T874D1165  
T874D1165  
T874D1165  
T874D1165  
T874D1165  
T874D1165  
T874D1165  
T874D1165  
T874D1165  
T874D1165  
T874D1165  
T874D1165  
T874D1165  
York logo; locking cover  
American Standard logo  
ITT logo  
Carrier logo  
No anticipator on stage 1 cool; used with  
Q672C1029  
a
When replacing a T872 Thermostat and Q672 Subbase, always replace both the thermostat and the subbase.  
60-2485—8  
32  
href="
T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES  
Table 6. T872-T874 Thermostat Cross Reference.  
T874 Trade  
a
Model Number  
D1144  
Description  
Carrier logo; no. HH07AT072  
Replacement  
T874D1165  
Remarks  
D1151  
General Electric logo; no. AY28X098; 0.4 heat  
T874D1165  
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  
Dunham-Bush logo  
Fedders logo  
T874D1165  
T874D1165  
T874D1165  
T874D1165  
T874D1165  
T874D1165  
T874D1165  
T874D1165  
T874D1165  
T874D1165  
T874D1165  
T874D1165  
T874D1165  
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  
T874D1165  
D1326  
General Electric logo; locking cover 72-78°F  
(
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  
T874D1165  
T874D1165  
Singer logo; no. 02005740; Celsius scale; 6.6°C  
Only 2°F (1°C) differential between  
H1 and H2.  
differential between H1 and H2.  
D1367  
D1375  
Friedrich logo; no. 600-580-23  
T874D1165  
Singer logo; no. 02005713; Celsius scale; controls None  
2
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  
Climatrol logo  
T874D1165  
T874D1165  
T874D1165  
Singer logo; no. 02005785; 8°F (4.4°C) differential T874D1165  
between H1 and H2  
D1441  
D1474  
T872E  
American Air Filter  
Carrier; no. HH07AT072-03; Celsius scale  
2-stage cool  
T874D1165  
T874D1165  
E1002  
E1010  
E1028  
Standard OEM  
TRADELINE  
Westinghouse logo  
T874E1016  
T874E1016  
T874E1016  
a
When replacing a T872 Thermostat and Q672 Subbase, always replace both the thermostat and the subbase.  
33  
60-2485—8  
href="
T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES  
Table 6. T872-T874 Thermostat Cross Reference.  
T874 Trade  
a
Model Number  
E1036  
E1044  
E1051  
E1069  
E1077  
E1085  
E1093  
E1101  
E1119  
E1127  
E1135  
E1143  
E1150  
E1168  
T872F  
F1001  
F1019  
F1027  
F1035  
Description  
American Standard logo  
Replacement  
T874E1016  
T874E1016  
T874E1016  
T874E1016  
T874E1016  
T874E1016  
T874E1016  
T874E1016  
T874E1016  
T874E1016  
T874E1016  
T874E1016  
T874E1016  
T874E1016  
Remarks  
York logo; locking cover  
ITT logo  
Climatrol logo  
Airtemp (Chrysler) logo  
Lennox logo  
Trane logo  
Dunham-Bush logo  
Fedders logo  
Climatrol logo  
General Electric logo; no. AY28X108  
Lennox logo; Celsius scale  
Celsius scale  
Airtemp logo  
2-stage heat  
Standard OEM  
TRADELINE  
T874F1015  
T874F1015  
T874C1018  
Lennox logo; fast cycle heat anticipator  
Luxaire logo; fast cycle performance on both heat T874C1018  
M839 draws 0.72A; set adjustable  
heat anticipator to setting of 0.9A  
for proper cycle rate.  
stages.  
F1043  
F1050  
F1068  
F1076  
F1084  
F1092  
F1100  
F1118  
F1126  
York logo; locking cover; used with M839.  
Raehco logo  
Fast cycle performance on both heat stages.  
Wesco logo  
Lennox logo  
Auto Gas logo  
T874C1018  
T874F1015  
T874C1018  
T874F1015  
T874F1015  
T874F1015  
T874F1015  
T874F1015  
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  
T874F1015  
T874F1015  
Oneida Heater logo; no. 30A0287-01; used with  
T874F1015/  
Q672K1012.  
Q674K1026  
T872G  
Heat pump models; 2-stage heat,  
1
-stage cool, 1-stage changeover.  
G1000  
Fast cycle performance stage 2 heat; fixed  
anticipator stage 1 heat and cool; changeover with  
cooling; used with Q672F1018.  
Adjustable anticipator stage 2 heat; fixed  
anticipator stage 1 heat and cool; changeover with  
cooling; used with Q672E1027.  
Y594G1252  
Y594G1393  
Wiring differences (old=new):  
W=W2, X1=X.  
G1018  
Wiring differences (old=new):  
Y1=O, Y2=Y.  
a
When replacing a T872 Thermostat and Q672 Subbase, always replace both the thermostat and the subbase.  
60-2485—8  
34  
href="
T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES  
Table 6. T872-T874 Thermostat Cross Reference.  
T874 Trade  
a
Model Number  
Description  
Replacement  
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  
See subbase description for  
replacement information.  
performance; stage 2 heat; fixed anticipator stage  
1
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.  
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.  
See subbase description for  
replacement information.  
Y594G1252  
G1091  
G1109  
Lennox logo; same as T872G1026 except meets  
See subbase description for  
replacement information.  
Wiring differences (old=new): C=X.  
DoD specs.  
Tappan logo; no. 080-13708-52; adjustable  
anticipator stage 1 heat and cool; changeover with  
cooling; used with Q672F1117.  
Changeover; used with Q672F1125.  
Lennox logo; no. 63A0401; Celsius scale; isolated None  
auxiliary heat power circuit; fast cycle anticipator  
stage 2 heat.  
Y594G1252  
Y594G1252  
G1117  
G1125  
Wiring differences (old=new): C=X.  
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  
See subbase description for  
replacement information.  
styling.  
G1166  
G1174  
G1182  
BDP used with Q672F1125.  
BDP used with Q672F1125.  
Y594G1252  
Y594G1252  
Wiring differences (old=new): C=X.  
Wiring differences (old=new): C=X.  
Carrier; no. HH07AT071C; Celsius scale; used  
See subbase description for  
with Q672L1052, J1054.  
replacement information.  
G1190  
Tappan; no. B13708-57; used with Q672L1078 for None  
and 10 ton heat pumps with 2 compressors.  
8
a
When replacing a T872 Thermostat and Q672 Subbase, always replace both the thermostat and the subbase.  
35  
60-2485—8  
href="
T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES  
Table 6. T872-T874 Thermostat Cross Reference.  
T874 Trade  
a
Model Number  
Description  
Replacement  
Remarks  
G1208  
Marvair logo; special for Crispaire; used with  
Y594G1393  
Q672F1166, F1323.  
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  
None  
Q672L1169 (HH93AZ071).  
G1265  
G1273  
Addison logo; used with Q672F1265.  
Carrier logo; (DoD model); 68°F (20°C) maximum  
heating; 78°F (26°C) minimum cooling; used with  
Q672L1052, J1054  
Y594G1252  
Wiring differences (old=new): C=X.  
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  
1.2A; 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.  
Tappan/Janitrol logo; no. B-13708-521;  
used with Q672F1117.  
None  
Y594G1252  
Wiring differences (old=new): C=X.  
G1398  
T872H  
York logo; no. 025-21454C; used with Q672J1021. Y594G1567  
Heat pump model; 1 stage heat,  
1
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.  
Friedrich logo; Celsius.  
Y594H1012  
Y594H1012  
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  
a
When replacing a T872 Thermostat and Q672 Subbase, always replace both the thermostat and the subbase.  
60-2485—8  
36  
href="
T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES  
Table 6. T872-T874 Thermostat Cross Reference.  
T874 Trade  
a
Model Number  
Description  
Replacement  
Remarks  
N1010  
Williamson logo; adjustable anticipator stage 1 and T874N1016/  
heat; fixed anticipator stage 1 cool; used with Q674F1220  
T874N1016  
2
Q672F1133 or F1315.  
N1028  
N1036  
N1044  
TRADELINE heat mode changeover; used with  
Q672F1216.  
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  
T874C1125  
Replacement only good for  
1
-stage heat and 1-stage cool  
applications.  
Q1025  
Q1033  
Q1041  
ITT logo  
Lennox logo; no. P-8-8899.  
Lennox logo; no. 63A6401.  
None  
None  
None  
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  
Y594R1425  
Wiring differences (old=new):  
W=W2; W1-Y1 jumper required.  
R1016  
R1024  
Addison same as T872R1008; used with  
Wiring differences (old=new):  
W=W2; W1-Y1 jumper required.  
Q672B1103.  
Lennox logo; fast cycle stage 2 heat; fixed  
See subbase description for  
anticipator stage 1 heat and cool; changeover with  
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.  
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.  
Y594R1763  
Y594R1243  
None  
Designed for use on General  
Electric heat pumps. Wiring  
differences (old=new): A=T.  
R1081  
R1099  
R1107  
R1115  
Ruud logo; same circuit as T872R1065; used with Y594R1243  
Q672L1045, L1136.  
Dunham Bush special; used with Q672L1060,  
L1383.  
Y594R1243  
Y594R1243  
Rheem; no. RXMT-B70; used with Q672L1045,  
L1136.  
Friedrich logo; fixed anticipator; used with  
Q672B1244, L1086.  
See subbase description for  
replacement information.  
a
When replacing a T872 Thermostat and Q672 Subbase, always replace both the thermostat and the subbase.  
37  
60-2485—8  
href="
T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES  
Table 6. T872-T874 Thermostat Cross Reference.  
T874 Trade  
a
Model Number  
R1123  
Description  
York logo; no. 025-21453C; Celsius scale.  
Replacement  
Remarks  
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.  
Heil Quaker; used with Q672L1193.  
Y594R1243  
Y594R1243  
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  
Y594R1664  
Westinghouse logo; used with Q672L1375 and  
W89.  
R1297  
R1305  
R1321  
R1339  
York; Celsius scale; used with Q672L1011.  
Used with Q672L1235 and W89.  
Duotherm logo; used with Q672L1409, and W89. None  
None  
None  
Williamson; used with Q672L1391.  
Y594R1243  
T872S  
Heat pump model; 2-stage heat plus changeover,  
1
-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;  
Q674C1058  
changeover with heating: used with Q672C1045  
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  
None  
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  
Y594W1014  
Q672B1251.  
a
When replacing a T872 Thermostat and Q672 Subbase, always replace both the thermostat and the subbase.  
60-2485—8  
38  
href="
T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES  
Table 7. Q672-Q674 Subbase Cross Reference.  
Q674 Trade  
a
Model Number  
Q672A  
Description  
System: HEAT-AUTO-COOL, Fan: AUTO-ON  
Standard OEM  
TRADELINE  
RC-RH jumper  
RC-RH jumper; Chek Lite  
System: HEAT-OFF-COOL, Fan: AUTO-ON  
Standard OEM  
Isolated G terminal.  
System: HEAT-OFF-RESET-COOL  
TRADELINE  
Replacement  
Remarks  
A1005  
A1013  
A1021  
A1039  
Q674A1019  
Q674A1019  
Q674A1019  
Q674A1019  
RC-RH jumper required.  
RC-RH jumper required.  
Q672B  
B1004  
B1012  
B1020  
B1038  
B1046  
Q674B1034  
Q674B1018  
Q674B1034  
Q674B1034  
Y594R1425  
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  
Chek Lite  
Auto fan operation in heat and cool.  
Carrier no. HH93AZ078; RC-RH jumper.  
Carrier no. HH93AZ084; RC-RH jumper.  
Q674B1034  
Q674B1034  
Q674B1075  
Q674B1075  
Q674B1034  
Y594R1425  
RC-RH jumper required.  
Addison; heat pump circuit; used with  
Wiring differences (old=new):  
W=W2; W1-Y1 jumper required.  
T872R1016.  
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.  
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  
Set T874 stage-1 anticipator for  
combined load of heat relay and fan  
relay.  
operation in heat and cool.  
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  
Q674B1018  
General Electric no. BAY28X137; isolated  
G terminal.  
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  
T874C1018/  
Q674B1034  
Wiring differences (old=new): C=no  
connection; RC-RH and W1-Y1  
jumpers required.  
T872R1115.  
Used with T872W1035 and C815A1043.  
Y594W1014  
a
When replacing a T872 Thermostat and Q672 Subbase, always replace both the thermostat and subbase.  
39  
60-2485—8  
href="
T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES  
Table 7. Q672-Q674 Subbase Cross Reference.  
Q674 Trade  
a
Model Number  
Description  
Replacement  
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  
Gas Furnace:  
T874C1018/  
Q674B1034  
Wards; RC-RH and F-W3 jumpers; used with  
T872C1673.  
Electric Furnace:  
Adjust anticipator faster for correct  
T874C1018/  
performance.  
Q674B1075  
B1293  
Lennox no. 41C96; used with T872C1673.  
Same as  
Q672B1285  
Q672C  
System: OFF-AUTO, Fan: AUTO-ON  
Standard OEM  
Westinghouse; RC-RH jumper.  
C1003  
C1011  
C1029  
Q674C1058  
Q674C1058  
Q674E1049  
RC-RH jumper required.  
W1-Y2 jumper; for heat pump; used with  
Replacement has heat/cool system  
T872D1136.  
switching.  
C1037  
C1045  
Chrysler Airtemp; System: COOL-OFF-AUTO. Q674C1058  
Westinghouse; heat pump; used with  
T874C1018/  
Wiring differences (old=new):  
V=RC-RH, F=G, C=Y1, Z=B,  
H2=W2; W1-Y1 jumper required.  
T872S1005.  
Q674C1058  
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.  
General Electric no. AY28X109.  
Q674C1058  
Q674C1058  
Y594H1012  
Command-Aire; heat pump; used with  
T872H1009.  
C1102  
TRADELINE  
Q674C1058  
Q672D  
System: None. Fan: None.  
Standard OEM; used with T872F1126.  
Lennox special.  
Carrier; RC-RH jumper.  
Carrier no. HH93AZ070; RC-RH jumper.  
General Electric no. AY-28X100.  
Square D no. HCB-4.  
D1002  
D1010  
D1028  
D1036  
D1044  
D1069  
D1077  
D1085  
Q674D1040  
None  
None  
Q674D1040  
None  
RC-RH jumper required.  
None  
None  
Q674D1040  
York; used with T872G1364.  
TRADELINE  
Q672E  
System: OFF-HEAT-AUTO-COOL  
Fan: AUTO-ON  
E1001  
E1019  
Standard OEM  
Q674E1049  
Lennox no. P-8-8892; heat pump; cool mode  
T874D1165/  
changeover; used with T872G1026.  
Q674B1075  
E1027  
E1035  
E1043  
Heat pump circuit; W1-Y2 jumper; cool mode  
changeover; used with T872G1018.  
Y594G1393  
Y594G1393  
Q674E1049  
Wiring differences (old=new): Y1=O,  
Y2=Y.  
Heat pump circuit; W1-Y2 jumper; cool mode  
Wiring differences (old=new): Y1=O,  
changeover.  
Y2=Y.  
Heat pump circuit; heat mode changeover.  
Wiring differences (old=new):  
W1=B, W2=W1; W1-Y1 jumper.  
a
When replacing a T872 Thermostat and Q672 Subbase, always replace both the thermostat and subbase.  
60-2485—8  
40  
href="
T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES  
Table 7. Q672-Q674 Subbase Cross Reference.  
Q674 Trade  
a
Model Number  
E1050  
E1068  
E1076  
E1084  
Description  
Replacement  
Q674E1049  
Q674E1049  
Q674E1049  
Q674E1049  
Q674E1049  
Remarks  
TRADELINE  
RC-RH jumper; used with T872C1699.  
RC-RH internal jumper; EM. HT. LED.  
Carrier no. HH93AZ076; RC-RH jumper.  
RC-RH jumper required.  
RC-RH jumper required.  
RC-RH jumper required.  
E1092  
Carrier no. HH93AZ082; RC-RH jumper; B  
terminal; isolated G terminal.  
E1100  
E1118  
General Electric no. AY28X099.  
Airtemp; heat pump; used with T872D1169.  
Q674E1049  
T874D1165/  
Wiring differences (old=new):  
Q674E1049  
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.  
Carrier no. HH93AZ077; RC-RH jumper;  
LOCKOUT LED indicates activation of  
compressor lockout switch.  
Y594G1252  
Q674E1049  
Wiring differences (old=new):  
V-VR=R, Y=W2, R=O, M=Y, F=G.  
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  
Q674E1049  
Q674E1049  
Lennox; HIGH and LOW LEDs; used with  
No LEDs on replacement.  
T872D1045.  
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  
Wiring differences (old=new):  
used with T872G1000.  
W=W2, X1=X.  
F1026  
F1034  
Chek Lite; W2-X2 jumper.  
Q674F1022  
Q674F1022  
Airtemp; EM. HT. LED; RC-RH and X2-E  
If RC-RH jumper removed on  
Q672F1034, Q674F1022 does not  
replace.  
jumpers.  
F1042  
F1059  
W2-X2 jumper; Chek Lite.  
Carrier no. HH93AZ074; EM. HT. LED.  
Q674F1022  
Q674F1022  
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;  
Q674F1022  
R-X2 jumper required.  
EM. HT. LED.  
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.  
a
When replacing a T872 Thermostat and Q672 Subbase, always replace both the thermostat and subbase.  
41  
60-2485—8  
href="
T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES  
Table 7. Q672-Q674 Subbase Cross Reference.  
Q674 Trade  
a
Model Number  
Description  
Replacement  
Remarks  
F1125  
BDP no. 3442DP90; used with T872G1117,  
Y594G1252  
Wiring differences (old=new): C=X.  
G1166, G1174.  
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  
Y594G1252  
Y594G1252  
Wiring differences (old=new):  
W=W2.  
EM. HT.; used with T872G1034.  
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  
R-X2 jumper; add monitor switch  
between X1 and low side of power.  
System: Aux. Heat replaces EM. HT.  
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  
Y594G1252  
Heatwave (Southwest Mfg.); compressor  
lockout in EM. HT.; EM. HT. LED; used with  
T872G1232.  
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  
None  
SERVICE LEDs; used with T872G1240.  
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  
Wiring differences (old=new): C=X.  
with T872G1281, G1265.  
Southwest special; no. 0361-136; W2-X2  
jumper.  
None  
F1281  
F1299  
Luxaire special; used with T872G1315.  
Bard special; used with T872N1036, N1044.  
Y594G1252  
T874N1016/  
Q674F1220  
F1315  
F1323  
Williamson special; used with T872N1010.  
T874N1016/  
Q674F1220  
Y594G1252  
Crispaire special; used with T872G1208.  
System: OFF-AUTO. Fan: None.  
Standard OEM  
Singer; O-B jumper; used with T872D1375.  
Includes O and B terminals.  
Q672G  
G1009  
G1017  
G1025  
G1033  
G1041  
Q674G1070  
Q674G1070  
Q674G1070  
Q674G1070  
Q674G1070  
General Electric no. AY28X102.  
Singer no. 02005649; French wording; O-B  
Less French wording.  
jumper.  
G1066  
SLI-Mammoth; heat pump; heat mode  
changeover.  
Q674G1070  
Connect fan (G) wire to Y1 terminal;  
B-O and RC-RH jumpers.  
a
When replacing a T872 Thermostat and Q672 Subbase, always replace both the thermostat and subbase.  
60-2485—8  
42  
href="
T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES  
Table 7. Q672-Q674 Subbase Cross Reference.  
Q674 Trade  
a
Model Number  
G1074  
Description  
Singer no. 02005754; X1 and X2 terminals for  
field-added indicator lights; used with  
T872D1375.  
Replacement  
None  
Remarks  
G1082  
Singer no. 02005792; X1 and X2 terminals for  
field-added indicator lights; used with  
T872D1375.  
None  
G1090  
TRADELINE; B and O terminals.  
Q674G1070  
Q672H  
System: OFF-ON  
Fan: NIGHT INT.-DAY CONT.  
H1008  
Q672J  
Used with T872.  
None  
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.  
Heat pump; EM. HT. and AUX. HEAT LEDs;  
cool mode changeover; compressor lockout and  
auto fan in EM. HT.; used with T872G1059.  
Y594G1567  
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  
Wiring differences (old=new):  
W=W2.  
used with T872G1034.  
Q672K  
System: OFF-HEAT-AUTO-COOL. Fan: None.  
K1004  
Carrier no. HH93AZ072.  
Q674E1049  
Fan switch does not function.  
K1012  
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  
Y594R1425  
Y594R1664  
changeover; used with T872R1024.  
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  
a
When replacing a T872 Thermostat and Q672 Subbase, always replace both the thermostat and subbase.  
43  
60-2485—8  
href="
T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES  
Table 7. Q672-Q674 Subbase Cross Reference.  
Q674 Trade  
a
Model Number  
Description  
Replacement  
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  
Y594R1243  
None  
T872R1099.  
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  
Wiring differences (old=new): X2=E,  
C=X.  
used T872R1115 with L terminal.  
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  
Wiring differences (old=new): K=E,  
C=X; remove Y-W1 jumper.  
used with T872C1509, C1517, C1541.  
Westinghouse; EM. HT. LED; used with  
T872R1149.  
Y594R1243  
Y594R1243  
Y594R1243  
Wiring differences (old=new):  
W=W2.  
Westinghouse; EM. HT. LED; used with  
Wiring differences (old=new):  
W=W2.  
T872R1149.  
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  
LEDs; isolated stage 2 heat; used with  
T872R1131, C815A1013.  
None  
L1151  
L1169  
Westinghouse; AUX. HT. and EM. HT. LEDs;  
Y594R1243  
None  
Wiring differences (old=new):  
W=W2.  
used with T872R1149.  
Carrier no. HH93AZ071; EM. HT. LED; used  
with T872G1257 (HH07AT073).  
L1177  
L1185  
L1193  
York; DoD; EM. HT. LED; used with T872G1299. Y594G1567  
Bard; used with T872R1164.  
Y594R1243  
Y594R1243  
Heil Quaker no. NAXA001TS01; used with  
Wiring differences (old=new): B=X,  
W=W2.  
T872R1172.  
L1201  
L1219  
Lennox special; no. 1485701; used with  
T872G1323.  
Westinghouse special; used with T872G1356. T874D1165/  
None  
Wiring differences (old=new): X=X1,  
L=X2, W=E; Y1-W1 and  
Q674F1022  
Y2-W2 jumpers required.  
L1227  
L1235  
L1250  
Heat pump; used with T872R1198.  
Intertherm; used with T872R1206, R1305.  
Y594R1243  
None  
Y594R1243  
Crispaire special; used with T872R1156 and  
C815A1021.  
L1268  
L1276  
Westinghouse special; used with T872W1027  
and C815A1021.  
Intertherm special; used with T872R1214.  
None  
Y594R1300  
Wiring differences (old=new):  
W1=B, Z=L, K=E.  
L1284  
L1300  
York special; used with T872R1222.  
Amana special; no. C62345-1; used with  
T872R1230.  
Y594R1672  
None  
L1326  
Singer; used with T872R1255.  
None  
a
When replacing a T872 Thermostat and Q672 Subbase, always replace both the thermostat and subbase.  
60-2485—8  
44  
href="
T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES  
Table 7. Q672-Q674 Subbase Cross Reference.  
Q674 Trade  
a
Model Number  
L1342  
Description  
Lennox no. 52C7601; used with T872W1019  
Replacement  
None  
Remarks  
and C815A1013.  
L1375  
L1383  
L1391  
L1409  
Westinghouse; used with T872R1271, W89.  
Dunham Bush; used with T872R1099.  
Williamson; used with T872R1339.  
Duotherm special; used with T872R1321, W89. None  
System: OFF-HEAT-COOL-EVAP.  
None  
Y594R1243  
Y594R1243  
Q672N  
N1001  
N1019  
N1027  
Goettl; heat pump; heat mode changeover;  
evaporative cooler circuit; Fan: AUTO-ON;  
System: AUTO; used with T872C1459.  
Goettl; heat pump; heat mode changeover;  
evaporative cooler circuit; W1-Y1 jumper; used  
with T872C1459.  
Siegler special; used with T872C1707.  
System: EM. HT.-HEAT-COOL.  
None  
None  
None  
Q672P  
P1009  
Carrier special; no. HH93AZ069; used with  
Y594R1243  
Replacement has OFF system  
switch position  
T872G1075 (HH07AT071).  
a
When replacing a T872 Thermostat and Q672 Subbase, always replace both the thermostat and subbase.  
45  
60-2485—8  
href="
T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES  
Table 8. T874 Cross Reference.  
Description  
Model  
Fig.  
No.  
Trade Model  
Replacement  
Number  
Remarks  
T874A  
A1002  
A1010  
1-stage heat, 1-stage cool.  
44, 65 Honeywell logo; Standard OEM; used with Q674B1331.  
T874A1036  
T874A1010  
44  
Honeywell logo; includes Allen wrench and screws to  
lock cover; non-adjustable range stops set at 72°F (max.  
heat) and 78°F (min. cool) (22-26°C); field adjustable  
locking levers.  
A1028  
A1036  
44  
44  
Honeywell logo; heat anticipator factory set at 0.4A.  
Honeywell logo; TRADELINE® model; includes Allen  
wrench and screws to lock cover; field adjustable locking  
levers.  
T874A1036  
T874A1036  
A1044  
A1051  
44  
44  
44  
Obsolete. AAF/Snyder General logo; part no. 4ELN-  
T874A1036 Replacement has °F  
scaleplate.  
3
006C; °F/°C scaleplate.  
Lennox logo; part no. 12F9901; °F/°C scaleplate; heat  
T874A1036 Replacement has °F  
anticipator set at 0.45A.  
scaleplate.  
A1069  
A1077  
Obsolete. Westinghouse logo; used with Q674A-E,G.  
T874A1036  
T874A1036  
46, 82 Obsolete. Amana Air Conditioning logo; part no.  
D68535-1; heat anticipator factory set at 1.2A; used with  
Q674B1075, E1056; used in Y594A1001, A1019.  
A1085  
A1093  
A1101  
44, 46, Obsolete. Trane logo; part no. 13510451; used with  
T874A1036  
5
5, 66 Q674B1018, B1075, D1008.  
44  
Obsolete. McQuay logo; part no. 02005935; °F/°C  
T874A1036 Replacement has °F/°C  
scaleplate.  
scaleplate.  
51, 53 Carrier logo; part no. HH07AT174; used with Q674B.  
Y594C1421 or When using the T874C1036,  
a
T874A1036  
set stage 1 heat anticipator for  
total current draw of fan and  
stage 1 heat primary.  
A1119  
82  
Obsolete. Sears logo; part no. 541. 91441/49. 9144; °F/  
T874A1036 Replacement has °F  
°
C scaleplate; used with Q674E1007; used in  
scaleplate.  
Y594A1068.  
A1127  
A1135  
A1143  
A1150  
44  
44  
44  
44  
Obsolete. Climate Control logo; part no. 02005936; °C  
T874A1036 Replacement has °F  
scaleplate.  
T874A1036  
scaleplate.  
Fedders multi-logo; part no. THTX-110-WCHA,  
88-58-00500-001.  
Honeywell logo; °C scaleplate.  
T874A1036 Replacement has °F  
scaleplate.  
T874A1150  
Honeywell logo; TRADELINE model; includes Allen  
wrench and screws to lock cover; field adjustable locking  
levers (68°F max. heat, 78°F min. cool).  
A1168  
A1176  
44  
44  
Obsolete. Trane Comfort Corps logo.  
T874A1036  
T874A1176  
Honeywell logo; meets DoD specs; includes Allen  
wrench and screws to lock cover; no thermometer; non-  
adjustable range stops set at 72°F (max. heat) and 78°F  
(min. cool) (22-26°C); field adjustable locking levers.  
A1184  
51, 53 Trane logo; part no. BAY28X182; °F/°C scaleplate;  
automatic fan in heating and cooling modes; heat  
anticipator set at 0.4A; used with Q674B1141, E1080.  
T874A1036a When used with Q674E1080,  
set stage 1 heat anticipator for  
total current draw of fan and  
stage 1 heat primary;  
replacement has °F  
scaleplate.  
A1192  
44  
Obsolete. Lennox logo; part no. 13F0001; °C scaleplate;  
heat anticipator factory set at .458A.  
T874A1036 Replacement has °F  
scaleplate.  
aBoth the thermostat and subbase must be replaced.  
60-2485—8  
46  
href="
T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES  
Table 8. T874 Cross Reference.  
Model  
Fig.  
No.  
Trade Model  
Replacement  
T874A1036  
Number  
Description  
Remarks  
a
A1200  
51, 53 Obsolete. Carrier logo; part no. HH07AT164; °C  
scaleplate; includes Allen wrench and screws to lock  
cover; used with Q674B.  
Set stage 1 heat anticipator for  
total current draw of fan and  
stage 1 heat primary;  
replacement has °F  
scaleplate.  
A1226  
A1234  
44  
44  
Honeywell logo; Honeywell Canada TRADELINE model;  
T874A1036 Replacement has °F  
°
C scaleplate; includes Allen wrench and screws to lock  
scaleplate.  
cover.  
Obsolete. Friedrich Climate Master logo; heat anticipator  
T874A1036  
factory set 1.2A.  
A1242  
A1259  
A1267  
A1275  
44  
44  
59  
44  
Obsolete. Friedrich Climate Master logo.  
Obsolete. Wesco logo; heat anticipator set at 0.8A.  
Obsolete. McQuay logo; used with Q674Q1004.  
T874A1036  
T874A1036  
T874A1036  
T874A1176  
Obsolete. Lennox logo; part no. 13F0101; meets DoD  
specifications.  
A1283  
75  
McQuay Air Conditioning logo; part no. 03005047; used  
with Q674C1132 and thermistor to display outdoor  
temperature.  
T874A1036 Replacement does not have  
ability to display external  
temperature or momentary  
system switch position.  
A1291  
A1309  
64  
44  
No logo; three-speed fan; one setpoint lever for heat and  
cool setpoints; used with Q674Q1012; used in  
Y594A1126.  
Obsolete. Arcoaire Air Conditioning and Heating logo;  
part no. AO2076; includes Allen wrench and screws to  
lock cover.  
T874A1036/ Replacement has two levers  
a
Q674B1075  
and 1 speed fan.  
T874A1036  
A1317  
A1325  
44  
75  
Obsolete. McQuay logo; part no. 60685301; no  
T874A1036  
scaleplate; used with Q674A-E,G.  
Obsolete. McQuay logo; part no. 60685401; no  
scaleplate; used with Q674C1306 and outdoor  
thermistor to display outdoor temperature.  
T874A1036 Replacement does not have  
ability to display external  
temperature or momentary  
system switch position.  
A1341  
44  
44  
Obsolete. Trane logo; part no. THT-0577; meets DoD  
specifications.  
York-Miraco logo.  
T874A1176  
A1358  
A1366  
T874A1036  
a
51, 53 Obsolete. BDP Multi-logo; part no. HH07AT182.  
T874A1036  
Set stage 1 heat anticipator for  
total current draw of fan and  
stage 1 heat primary.  
T874A1036a  
A1374  
A1390  
A1416  
A1424  
A1440  
44  
44  
44  
American Standard logo; part no. ASYSTAT610; heat  
anticipator factory set at 0.4A.  
Obsolete. Climate Control logo; part no. 654958804; °F/  
T874A1036 Replacement has °F  
scaleplate.  
°C scaleplate.  
Obsolete. Reznor logo; part no. 654758803; °F/°C  
T874A1036 Replacement has °F  
scaleplate.  
scaleplate.  
214 Obsolete. Carrier Olympia Series Split System logo; part See OEM for  
no. HH07AT194; used with Q674B1455 (HH93AZ208).  
Obsolete. Simplex System 2000 logo; heat anticipator  
factory set at 1.2A; used with Q674G1005; used in  
Y594A.  
replacement.  
T874A1036  
91  
A1465  
212 Honeywell Trol-A-Temp™ logo; Premier White® color;  
heat anticipator factory set at 0.10A; used with  
Q674E1379.  
None