Bryant Q674 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  
Adapter plate available for mounting Q674 on wall or  
horizontal outlet box.  
These thermostats and subbases provide low voltage control  
of multistage heating and cooling systems, including heat  
pump systems.  
Light-emitting diodes (LEDs) located on subbase for  
easy reference.  
Up to three stages each of heating and cooling control  
possible.  
Models with setpoint restrictions and locking cover  
with no thermometer available for Department of  
Defense (DoD) and other special applications.  
Outdoor reset used on some models to improve  
thermal performance.  
Contents  
Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1  
Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1  
Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2  
Ordering Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2  
Installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7  
Settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11  
Checkout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13  
Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14  
Understanding Circuits . . . . . . . . . . . . . . . . . . . . . . . . . . 14  
Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21  
Cross Reference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29  
Wiring Diagrams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96  
®
U.S. Registered Trademark  
60- 2485-8  
Copyright © 2001 Honeywell • All Rights Reserved  
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T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES  
Tradeline Models  
SPECIFICATIONS  
T874 THERMOSTAT  
IMPORTANT  
T874 TRADELINE models provide staged heat and/or cool  
operation. See Table 1.  
The specifications given in this publication do not  
include normal manufacturing tolerances. Therefore,  
this unit may not exactly match the listed specifica-  
tions. This product is tested and calibrated under  
closely controlled conditions, and some minor differ-  
ences in performance can be expected if those con-  
ditions are changed.  
Table 1. Heating and Cooling Stages.  
Models  
A
1
1
B
1
2
C
2
1
D
2
2
E
2
F
2
Heating Stages  
Cooling Stages  
T874A-F are standard models.  
Super Tradeline®/Tradeline® Models  
SUPER TRADELINE controls offer features not available on  
TRADELINE or standard models, and are designed to replace  
a wide range of Honeywell and competitive controls.  
TRADELINE FEATURES:  
TRADELINE package with cross reference label and  
special instruction sheet.  
T874A,C model available with factory stops for DoD  
TRADELINE models are selected and packaged to provide  
ease of stocking, ease of handling, and maximum  
replacement value. Specifications of SUPER TRADELINE  
and TRADELINE controls are the same as those of standard  
models except as noted below.  
applications.  
T874A,C model available with adjustable temperature  
locking stops.  
Q674 SUBBASE  
Q674 switching subbases provide system and fan switching.  
See Table 2.  
Super Tradeline Models  
T874 THERMOSTAT  
Table 2. System and Fan Switching.  
T874D Thermostat. Provides two stages of heating and two  
stages of cooling. Use with Q674A-F Subbases.  
Y594D (T874D/Q674E/TG504A) Thermostat/Subbase/key  
lock cover package. Provides two stages of heating and  
two stages of cooling. Includes a key lock cover for set-  
point protection.  
Y594G (T874G/Q674F) Thermostat/Subbase package for  
heat pump. Provides two stages of heating and one stage  
of cooling. Automatic changeover in heat or cool mode.  
Available in beige or Premier White® color.  
Y594R (T874R/Q674L) Thermostat/Subbase package for  
heat pump. Provides two stages of heating and one stage  
of cooling. Manual changeover in heat or cool mode. Avail-  
able in beige or Premier White® color.  
Q674  
A
System  
Heat-Auto-Cool  
Heat-Off-Cool  
Fan  
Auto-On  
Auto-On  
Auto-On  
None  
B
C
Off-Auto  
D
None  
E
Off-Heat-Auto-Cool  
Em. Ht.-Off-Heat-Auto-Cool  
Off-Auto  
Auto-On  
Auto-On  
None  
F
G
J
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.  
Includes adjustable temperature locking stops.  
T874D replaces T874A-F TRADELINE or standard models.  
TRADELINE package with cross reference label and  
special instruction sheet.  
Q674A-E,G are standard (non-heat pump) models.  
Q674F,J,L are heat pump models.  
ORDERING INFORMATION  
When purchasing replacement and modernization products from your TRADELINE® wholesaler or distributor, refer to the  
TRADELINE® Catalog or price sheets for complete ordering number.  
If you have additional questions, need further information, or would like to comment on our products or services, please write or  
phone:  
1
2
.
.
Your local Home and Building Control Sales Office (check white pages of your phone directory).  
Home and Building Control Customer Relations  
Honeywell, 1885 Douglas Drive North  
Minneapolis, Minnesota 55422-4386 (800) 328-5111  
In CanadaHoneywell 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.  
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0-24858  
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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.  
1
93121A Adapter Plate Assembly.  
Temperature: Scale Range: 42° to 88°F (6° to 31°C) stan-  
dard; optional ranges available.  
THERMOSTAT MOUNTED  
ON SUBBASE  
SUBBASE  
FRONT  
SIDE  
3/8 (10)  
5-5/8 (143)  
3-9/32 (83)  
HEAT  
50 60 70 80  
COOL  
50 60 70 80  
1-1/2  
(38)  
50 60 70 80  
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:  
Temperature scale ranges are 40° to 75°F (4° to 24°C) heat-  
ing and 75° to 90°F (24° to 32°C) cooling with stop; 44° to  
Locking Cover and Locking Lever Assembly: Part no.  
194559R with thermometer; 194559S without thermome-  
ter. See Fig. 2. Includes cover, screws, and Allen wrench  
for locking cover. The screws must be used to assure  
proper operation.  
Adjustable Lever Stop: Part no. 4074ECK; includes lever stop  
and screws.  
68°F (7° to 20°C) heating, 80° to 86°F (27° to 30°C) cool-  
ing; 6° to 29°C (43° to 85°F) Celsius scale; 3° to 22°C (38°  
to 72°F) and 26° to 32°C (78° to 90°F) cooling with stop.  
Nonadjustable factory-added stop limits heating setpoint to  
72°F (22°C) maximum and cooling setpoint to 78°F (26°C)  
minimum.  
OEM customer personalization.  
Locking cover and locking lever (see Thermostat Accesso-  
Universal VersaguardThermostat Guard: Includes wall-  
plate, ring base, guard cover, tumbler lock, two keys and  
optional Honeywell logo insert. Double-wall construction  
provides extra measure of tamper-resistance. Tamper-  
resistant lock; key cannot be removed without being in  
locked position. Vents in guard base allow airflow for opti-  
mum thermostat performance. See form 68-0104 for more  
information.  
ries).  
Thermostat cover without thermometer.  
Adjustable locking temperature stops.  
Voltage heat anticipation for first or second stage heat or both.  
See Table 3.  
Fast cycling on heating stage(s) for electric heat applications.  
C815A Outdoor Thermistor for improved performance on  
specified models.  
TG511A1000: Clear cover.  
TG511B1008: Opaque cover.  
TG511D1004: Painted steel (off-white) cover. See  
Fig. 2.  
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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 set-  
tling levers and subbase switches. Includes LED window  
and two keys. Should not be used with 193121A Adapter  
Plate.  
TG504A1025: Blank face, internal thermometer.  
TG504A1033: External thermometer. See Fig. 2.  
1
94559R Locking Cover  
with Thermometer  
Allen Wrench  
TG504A1033 Key Lock  
Cover with External  
Thermometer  
TG511D1004 Includes  
Painted Steel Cover,  
Opaque Base and Wallplate  
Fig. 2. T874 Thermostat accessories.  
Table 3. T874 Thermostat Specifications.  
Anticipation  
Heating (Adj) Cooling (Fixed)  
System Stages  
Models and Options  
Replaces Applications Heat Cool Other Stage 1 Stage 2 Stage 1 Stage 2  
T874AStandard 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 ).  
T874BStandard and TRADELINE.  
T872B  
T872C  
Standard  
Standard  
1
2
2
1
0.1-1.2A 0-1.2A 0-1.0A  
T874CStandard and TRADELINE.  
0.1-1.2A 0.1-1.2A 0-1.5A  
72°F/78°F (22°C/26°C)set stops  
with locking cover, no thermometer  
a
(
for DoD ).  
12°F (7°C) differential between  
H1 and H2 stages (T874C1125).  
Fast cycling.  
0.12-  
.6A  
0.12-  
0.6A  
0
T874DStandard 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).  
T874EStandard and TRADELINE.  
T872E  
T872F  
2-Stage Cool  
2-Stage Heat  
2
0-1.2A 0-1.0A  
T874FStandard and TRADELINE.  
2
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 stageuse 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
T874GHeat pump, cool  
changeover, with fast cycling.  
T872G Heat Pump or  
Standard  
2
1
1
1
1b  
0-1.0Ac  
0.1-1.2A 0-1.0A  
Fixed anticipator for H2.  
Fast cycling.  
0.1-1.5A  
0.12-  
0
.6A  
T874HUse with Q674C.  
T872H Heat Pump or  
Standard  
1b  
0-1.0Ac  
0-1.0A  
T874JHeat pump.  
T874KHeat pump.  
T874LHeat pump.  
None  
None  
Heat Pump  
Heat Pump  
2
2
1
1
2d  
1b  
0-1.0Ae 0-1.5Ac 0-1.0Ac  
0.1-1.2A  
0-1.5Ac  
0-1.5A  
None  
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  
T874NHeat pump, heat  
changeover.  
T872N  
1b  
T874PHeat pump.  
None  
Heat Pump  
Standard  
2
1
0.1-1.2A 0.1-1.2A 0-1.5A  
T874QNight setback heating.  
T872Q  
1e  
0.1-1.2A  
T874RHeat pump.  
T872R  
None  
Heat Pumpf  
2
2
1
2
0-1.5Ac  
0.1-1.2A 0-1.5A  
T874STwo-speed compressor heat  
pump.  
Heat Pump  
1g  
0-1.2Ac  
0-1.2Ac  
0-1.2A 0-1.0A  
T874VStandard.  
None  
Standard  
1
2
0-1.5A  
T874WHeat pump and standard.  
T872W Heat Pump or  
Standard  
3
-1.2Ab 0-1.2Ac,d  
0-1.2A 0-1.0A  
0
Night setback heating.  
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 stageuse Q674B,L subbase.  
g
Changeover stage operates with cooling.Q674 Subbases  
Models:  
See Table 4.  
Mounting: Designed to mount horizontally on an outlet box  
or the wall. Adapter plate assembly available for mounting  
on a vertical outlet box (see Subbase Accessory).  
Electrical Ratings:  
Switch Contacts: 2.5A at 30 Vac (7.5A inrush).  
LED Lights (Optional): 30 Vac.  
Finish: Dark brown or gray.  
Dimensions in in. (mm): 3-1/2 (89)height; 5-5/8 (143) width;  
Switches: Two slide switches (one switch on Q674G and K;  
no switches on Q674D) operated by levers. Switch position  
is shown on scaleplate.  
5/16 (8) depth. See Fig. 1.  
Table 4. Q674 Subbase Specifications.  
Switch Positions  
System  
HEAT-AUTO-COOL AUTO-ON  
Models and Options  
Replaces Application  
Fan  
Use With  
Q674AStandard and TRADELINE®.  
Q672A  
Q672B  
Standard  
T874A-D  
Indicator LEDs.  
Q674BStandard and TRADELINE.  
Standard  
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.  
Heat Pump  
OVERRIDE-HEAT-  
OFF-COOL  
OFF-AUTO  
OVERRIDE-AUTO-  
OFF  
Q674CStandard and TRADELINE.  
Indicator LEDs.  
Q672C  
Q672D  
Standard  
AUTO-ON  
AUTO-ON  
T874A-H,W  
T874K,L,N  
T874P  
Heat Pump  
Q674DStandard 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  
Q674EStandard and TRADELINE.  
Q674FTwo LED models.  
Q672E  
Q672F  
Standard  
OFF-HEAT-AUTO-  
COOL  
AUTO-ON  
T874A-D  
Heat Pump OFF-EM.HT.-HEAT- AUTO-ON T874C,D,G,N,S  
AUTO-COOL  
EM.HT. light.  
Provision for AUTO fan operation in  
EM.HT.  
OFF-COOL-AUTO-  
HEAT-EM.HT.  
AUTO-ON  
Q674GO and B terminals.  
Q674JProvision for AUTO fan operation  
in EM.HT.  
Q672G  
Q672J  
Standard  
OFF-AUTO  
None  
T874A-F  
Heat Pump  
EM.HT.-AUTO-OFF  
EM.HT.-ON-OFF  
SUPL.HT.-ON-OFF  
AUTO-ON  
AUTO-ON  
AUTO-ON  
None  
T874A,D,G,J  
Provision for outdoor thermistor.  
Q674KStandard.  
Q672K  
Q672L  
Standard  
OFF-HEAT-AUTO-  
COOL  
T874F  
OFF-WOOD-WOOD/  
OIL-OIL  
None  
Q674LProvision for AUTO fan operation in  
EM.HT.  
Heat Pump EM.HT.-HEAT-OFF- AUTO-ON  
COOL  
T874R,W  
Indicator LEDs.  
SUPL.HT.-HEAT-  
OFF-COOL  
AUTO-ON  
AUTO-ON  
AUTO-ON  
Q674NStandard.  
Q674PStandard.  
Q674QStandard.  
Q674RStandard.  
Q672N  
Q672P  
None  
Evaporative  
Cooler  
EVAP-COOL-OFF-  
HEAT  
T874C  
T874G  
T874A  
T874V  
T874C  
Heat Pump  
SUPL.HT.-HEAT-  
COOL  
Fan Coil  
HEAT-OFF-COOL LO-MED-HI-  
ON  
None  
Fan Coil  
OFF-COOL  
LO-HI-  
CONT.  
International symbols.  
Q674SIndicator LED.  
None  
Standard  
HEAT-COOL  
None  
Optional Specifications (Q674 Only):  
L terminal is used for system monitoring devices.  
Common R terminal for heating/cooling.  
Changeover in cool or heat mode for heat pumps.  
Auto fan in EM.HT. for heat pumps.  
Models available with up to four LEDs; for example, LEDs can  
show EM. HT, AUX. HT, SERVICE, CHECK, FILTER, and  
LOCKOUT. See Fig. 3.  
System switching marked HEAT-OFF/RESET-COOL for sys-  
tems requiring impedance relay reset. Available on Q674B  
only.  
G terminal isolated on heating to provide fan relay operation  
from external low voltage fan switch (Q674B only).  
Auto fan operation on both heat and cool.  
Subbase Accessory: 193121A Adapter Plate Assembly for  
mounting on vertical outlet box. Assembly includes adapter  
ring and cover plate. Use to cover wall marks from  
replaced thermostat.  
T874 Thermostat with  
One Setpoint Lever  
T874 Thermostat with  
Separate  
Q674 Subbase  
with Four LEDs  
Heating and Cooling Levers  
Fig. 3. Heating, cooling levers and system LED indicators.  
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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  
The subbase can be mounted on a vertical outlet box,  
horizontal outlet box or directly on the wall.  
1. If the subbase is mounted on a vertical outlet box, order  
Honeywell part no. 193121A Adapter Assembly. See  
Fig. 4. The assembly includes an adapter ring, two  
screws and a cover plate to cover marks on the wall.  
Install the ring and cover plate on the vertical outlet box.  
If this control is replacing a control that contains  
mercury in a sealed tube, do not place your old control  
in the trash.  
Contact your local waste management authority for  
instructions regarding recycling and the proper  
disposal of this control, or of an old control containing  
mercury in a sealed tube. If you have questions, call  
the Honeywell Customer Response Center at  
NOTE: For a wall installation, hold subbase in position and  
mark holes for anchors. See Fig. 5. Wall anchors  
must be obtained from local hardware store. Be  
careful that the wires do not fall back into the wall  
opening. Set aside subbase. Drill four  
1-800-468-1502.  
INSTALLATION  
3
/16 in. (4.8 mm) holes and gently tap anchors into  
the holes until flush with the wall.  
When Installing this Product…  
2
3
.
.
Pull wires through the cover plate (if used) and subbase  
cable opening. See Fig. 6.  
Secure the cover plate (if used) and subbase with the  
screws provided. Do not fully tighten the subbase  
screws.  
1
.
Read these instructions carefully. Failure to follow them  
could damage the product or cause a hazardous condi-  
tion.  
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  
BOX  
MOUNTING  
SCREWS (2)  
1
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-  
OL  
THERMOSTAT  
COVER  
CO  
8
0
7
0
perature control to deviate from setpoint. It is not a  
calibration problem.  
60  
5
0
SUBBASE  
70  
8
0
6
0
HE  
AT  
50  
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.  
80  
0
7
0
6
5
0
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.  
radiant heat from the sun, fireplace, or appliances.  
Fig. 4. Installing Q674 Subbase on outlet box.  
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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
1
0 wires can be used. Do not use larger than  
8 gauge wire.  
Follow equipment manufacturer wiring instructions when  
available. To wire subbase, proceed as follows:  
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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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  
B
E
G
L
K
F
Fan relay coil  
System monitor  
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  
H1, R3  
Stage 1 heating control  
Stage 2 heating control  
Stage 3 heating controlb  
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.  
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Table 5. Terminal Designationsa  
Alternate Designations or  
Standard Terminal  
Designation  
Customer Specials  
Typical Connection  
T
R1, R2  
RS  
External temperature readout, T relay  
LO and HI speed fan relays  
Cooling contactor  
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.  
When installed, the stop brackets limit the movement of the  
T874 HEAT and COOL levers.  
Outdoor Disconnect  
The National Electrical Code requires the installation of a  
disconnect switch within sight of the outdoor unit of an air  
conditioner or heat pump. The switch is for the safety of any  
technician working on the unit. The technician can assure that  
the unit remains unpowered.  
TO INSTALL:  
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.  
Push the brass insert into the hole with finger.  
Turn to the front of the T874 Thermostat.  
Place the two stop brackets in position with the tabs in  
the slot between the HEAT and COOL levers. See  
Fig. 9.  
Install and Adjust Stop Brackets  
The stop brackets should be installed only if there is a need to  
restrict the adjustable range of the heating and cooling  
temperature setpoint levers. If adjustable lever stops are  
desired, order 4074ECK Envelope Assembly, which contains  
two adjustable lever stop brackets, one brass insert, one  
3
4
5
.
.
.
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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8
9
.
.
Move the HEAT and COOL levers to the maximum tem-  
perature desired.  
Slide the stop brackets until one rests against the HEAT  
lever and the other against the COOL lever.  
0. Firmly tighten the mounting screw.  
1
11. If the HEAT and COOL levers are to be locked in place  
ALLEN  
at a specific temperature, use the two insulated head  
screws supplied instead of the two adjustable lever stop  
brackets.  
RETAINING  
SCREWS (2)  
CAUTION  
Equipment Damage Hazard.  
Do not use standard screws that provide metal-to-  
metal contact with the stop brackets. Short circuit and  
potential equipment damage can result.  
M956  
Fig. 10. Installation of locking cover assembly.  
Mount Thermostat  
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  
On systems using a gas valve, never apply a jumper  
across the valve coil terminals, even temporarily. This  
can burn out thermostat heat anticipator(s).  
Equipment Damage Hazard.  
2
.
Carefully remove and discard the polystyrene packing  
insert that protects the mercury switches during ship-  
ment.  
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.  
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.  
1
.
Connect an ac ammeter of appropriate range between  
the heating terminals of the subbase:  
a. Stage 1between W1 and RH or R;  
b. Stage 2between W2 and RH or R  
c. Stage 3between W3 and RH or R.  
Move the system switch to HEAT or AUTO.  
After one minute, read the ammeter and record the  
reading:  
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  
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anticipator at 140 percent of the actual primary con-  
trol current draw to reduce the cycling rate. See Fig.  
11.  
Subbase Setting  
The subbase switching positions control the system operation  
as described below.  
SYSTEM SWITCH (see subbase for positions):  
OFFboth the heating and cooling systems are off. If the  
fan switch is at the AUTO position, the cooling fan is  
also off.  
Most heat pump systems should cycle 2-1/2 to 3 times per hour.  
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.  
HEATheating system is automatically controlled by the  
thermostat. Cooling system is off.  
AUTOthermostat automatically changes between heat-  
ing and cooling system operation, depending on the  
indoor temperature.  
STAGE ONE  
ANTICIPATOR  
HEATING  
COOLcooling system is automatically controlled by the  
CONTROL  
thermostat. Heating system is off.  
EM.HT.emergency heat relay is automatically controlled  
by the thermostat. Cooling system is off. Compressor is  
de-energized.  
SUPL.HT.supplemental heat relay is energized. Cooling  
system is off. Compressor is de-energized.  
WOODheating system is operating with only the wood-  
burning stage.  
1
.2.8  
OILheating system is operating with only the oil-burning  
STAGE TWO  
ANTICIPATOR  
HEATING CONTROL  
stage.  
MOVE INDICATOR TO  
MATCH CURRENT RATING  
OF PRIMARY CONTROL  
WOOD/OILwood and oil stages operate sequentially;  
first the WOOD stage operates, then the OIL stage  
operates if the WOOD stage cannot handle the load.  
EVAPcontrols cooling system by water evaporation; see  
equipment instructions for further information.  
OVERRIDEnight setback is disabled.  
M5069  
Fig. 11. Adjustable heat anticipator scales.  
Temperature Setting  
ONheating system is controlled by the thermostat. EM.  
Move the heating and cooling levers to the desired comfort  
positions. See Fig. 12. On some models with two stages of  
heating or cooling, the same lever controls both stages. The  
minimum differential between heating and cooling setpoints is  
HT. or SUPL. HT. relay is not energized.  
FAN SWITCH positions control fan operation as follows:  
ON or CONT.fan operates continuously.  
AUTOfan 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.  
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.  
LOfan operates constantly at low speed.  
MEDfan operates constantly at medium speed.  
HIfan operates constantly at high speed.  
STAGE 1  
HEATING  
To move the subbase switches to the desired control  
positions, use thumb and index finger to slide the lever. The  
lever must stop over desired function indicator position for  
proper circuit operation.  
HEATING  
LEVER  
COOLING  
STAGE 3  
LEVER  
STAGE 1  
COOLING  
HEATING  
Spring return momentary position switching feature is  
available on selected subbase models. On these models, the  
fan switch is positioned to the right of the system switch. By  
moving the fan switch to the far right and releasing it, the ON  
position circuit makes. The lever springs back on release. This  
position is not marked on the subbase.  
Setting the Adjustable Differential  
The adjustable interstage differential feature, on a selected  
T874D model only, can be identified by the scale and tension  
screw near the heating and cooling mercury switches. See  
Fig. 13. On this model, the number of degrees between the  
making of the first and second stage mercury bulbs is  
adjustable. This feature is especially useful if the first stage  
controls the comfort temperature, and the second stage  
controls the energy savings temperature. Timers, such as the  
S6005, for insertion between the first and second stage  
control points must be ordered separately.  
STAGE 2  
HEATING  
CAPTIVE  
MOUNTING  
SCREWS (2)  
STAGE 2  
COOLING  
M7625  
Fig. 12. Internal view of T874W (three stages of  
heating, two stages of cooling).  
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Each mark on the scale represents 1°F (0.6°C) The  
differential is factory set at 2°F (1°C) the differential can be set  
as high as 12°F (7°C) To set the adjustable interstage  
differential, loosen the tension screw. See Fig. 13. Slide the  
adjustable scale to align with the number of degrees desired  
between stages. Use the lower edge of the tension screw  
bracket as a guide for alignment. In heating, slide the lever  
wider apart for a larger differential, or closer together for a  
smaller differential. In cooling, slide the lever closer together  
for a larger differential, or wider apart for a smaller differential.  
While supporting the scale with hand, tighten the tension screw.  
CAUTION  
Equipment Damage Hazard.  
When the thermostat is used to control a two-stage  
heating or cooling system, the second stage mercury  
bulb must never make before the first stage bulb, or  
severe equipment damage could result. To prevent  
this problem, provide at least 2°F (1°C) differential  
between stage-one and stage-two make points.  
Example: in heating, if stage-one makes at 70°F (21°C)  
stage-two should make at 68°F (20°C) or lower.  
IMPORTANT  
Support the scale with hand while tightening tension  
screw. See Fig. 13. Failure to do so can result in  
twisting and damaging bimetal coil.  
Verify the Adjustment  
Heating  
HEATING SET-  
POINT LEVER  
Start with the heating setpoint lever all the way to the left.  
Slowly move the lever to the right, just until the first stage bulb  
makes (mercury rolls to the right side of the bulb). Note the  
setting on the temperature scale. Slowly move the lever to the  
right until the second stage bulb makes. Note the setting on  
the temperature scale. The difference between the two  
temperatures is the interstage differential, which should match  
the number set on the scale with the tension screw.  
TENSION  
HEAT  
SCREW  
50 60 70 80  
FIRST  
STAGE  
SWITCH  
Cooling  
Start with the cooling setpoint lever all the way to the right.  
Slowly move the lever to the left, just until the first stage bulb  
makes (mercury rolls to the left side of the bulb). Note the  
setting on the temperature scale. Slowly move the lever to the  
left until the second stage bulb makes. Note the setting on the  
temperature scale. The difference between the two  
1
.2  
SCALE  
SECOND STAGE  
ADJUSTABLE)  
SWITCH  
temperatures is the interstage differential, which should match  
the number set on the scale with the tension screw.  
(
HEATING  
TENSION  
SCREW  
CHECKOUT  
ALIGN LOWER  
EDGE WITH  
SCALE  
Heating  
SLIDE LEVER  
NOTCH  
WIDER APART  
FOR LARGER  
DIFFERENTIAL  
Move the system switch on the Q674 Subbase to HEAT or  
AUTO. Move the heat lever on the T874 about 10°F (6°C)  
above room temperature. See Fig. 12. Heating system should  
start and the fan should run after a short delay. Move the heat  
lever about 10°F (6°C) below room temperature. The heating  
equipment should shut off, and the fan should run for a short  
time, then shut off.  
10°F  
SCALE  
1
COOLING  
In heat pump applications, sometimes time delays are  
involved before the compressor and auxiliary heat are  
activated. This is due to a minimum-off timer, which prevents  
the compressor from restarting for five minutes from when the  
thermostat last turned off the compressor, or from when the  
system first received power.  
TENSION  
SCREW  
ALIGN LOWER  
EDGE WITH  
SCALE  
SLIDE LEVER  
CLOSER TOGETHER  
FOR LARGER  
NOTCH  
4°F  
DIFFERENTIAL  
SCALE  
Cooling  
1
CAUTION  
1
EACH MARK ON THE SCALE REPRESENTS 1°F (0.6°C).  
M937  
Equipment Damage Hazard.  
Do not operate cooling if outdoor temperature is below  
Fig. 13. Set adjustable interstage differential.  
5
0°F (10°C). Refer to manufacturer recommendations.  
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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
3
.
.
Set the thermostat cover on a table near an accurate  
thermometer.  
Allow ten minutes for cover thermometer to sense area  
temperature; compare the readings. Be careful not to  
touch thermometer or breathe on it.  
4
.
If the readings are the same, replace cover and put the  
system into operation.  
Outdoor Reset Thermistor (Where  
Applicable)  
If the system is supplied with a thermistor, it must be used; if  
not used, thermostat performance deviates radically from  
proper operation.  
5. If the readings are different, insert a small screwdriver in  
the thermometer slot and turn it until the thermometers  
have the same reading. See Fig. 15.  
6. Replace thermostat cover and put the system into oper-  
ation.  
The proper thermistor operation must be verified to ensure the  
correct operation of the thermostat. Check thermistor  
operations as follows:  
1
2
.
.
Disconnect the T wire on the subbase.  
Use an ohmmeter to measure resistance between the  
T wire and the A subbase terminal.  
3
4
.
.
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
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 changeoverrefers to the presence of an AUTO  
position in the system switching (EX: Q674E with OFF-  
HEAT-AUTO-COOL switching). The thermostat auto-  
matically changes between heat and cool modes as  
indoor temperature changes.  
Manual changeoverrequires a system switch movement  
to change mode (EX: Q674B with HEAT-OFF-COOL  
switching). T874D Multistage Thermostats with 2 heat  
or 2 cool switches are shown on most standard circuits.  
Most standard or TRADELINE® subbases (Q674A-E,G)  
can be used with T874A-F standard or TRADELINE  
Fig. 14. Thermistor resistance chart.  
CALIBRATION  
Thermostat  
T874 Thermostats are accurately calibrated at the factory.  
They do not have provision for field calibration.  
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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 valveoperates 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 valveoperates 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 relayoptional 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:  
H1Stage 1 Heating.  
H2Stage 2 Heating.  
H3Stage 3 Heating.  
C1Stage 1 Cooling.  
Sometimes power for a fixed anticipator is brought through an  
off system relay like the changeover relay shown in Fig. 20.  
This current is kept low by the high resistance of the fixed  
anticipator so that relay does not pull in.  
C2Stage 2 Cooling.  
C3Stage 3 Cooling.  
C/OChangeover (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:  
(
LOW AMBIENT CUTOFF  
heat or cool).  
M5848  
Connections are indicated by small dots at the point of  
intersection. If there is no dot, there is no connection.  
The left portion of the circuit (the thermostat) contains  
the mercury switches. The heat switches make on a  
temperature fall, and the cool switches make on a tem-  
Fig. 16. Key to hookup symbols.  
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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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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.  
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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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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.  
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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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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  
The addition of anticipator heat to the bimetal causes a control  
factor called droop.  
As the weather grows colder, the heating appliance must  
operate longer and more frequently. More heat must be added  
to the bimetal. The bimetal now thinks it is warmer than it  
really is, and actual room temperature is controlled at a lower  
temperature than the setpoint indicates. At 100 percent heat  
load, this droop can be significant.  
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.  
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A strip of four inserts is included with TRADELINE T874. Drop  
a strip into the recessed area behind the scaleplate so  
selected LEDs show. Make sure insert is completely seated in  
recessed area. Let scaleplate pop back; then set levers to  
desired position.  
CAUTION  
Due to calibration techniques used for T874  
Thermostats with outdoor reset, the C815A Thermistor  
must be wired into the system at all times. Failure to  
do so will result in serious degradation of performance.  
Equipment Damage Hazard.  
FILTER LED lights when the filter is clogged and needs  
replacement.  
CHECK LED lights when something needs to be checked  
or done to maintain efficient operation of system. See  
heating system instructions for CHECK LED meaning.  
EM. HT. LED lights when the emergency heat is operating.  
SUPL. HT. LED lights when the supplemental heat is  
operating.  
Service and Replacement of C815A Outdoor  
Thermistor  
To check and verify thermistor operation, perform the  
following steps:  
LOCKOUT LED lights when the system is shut down and  
needs maintenance.  
AUXILIARY HEAT LED lights when the auxiliary heat is  
1
2
.
.
Disconnect wire from T terminal on subbase.  
Measure resistance with ohmmeter across the T sub-  
base wire and X subbase terminal.  
Take outdoor temperature measurement at thermistor  
location and find correct thermistor resistance on the  
chart in Fig. 29.  
Compare resistance on the chart with measured resis-  
tance. Replace C815A if resistance varies more than 15  
percent. Contact installing dealer for packaged replace-  
ment outdoor thermistor.  
operating.  
SERVICE or MALFUNCTION LED can have several  
meanings. Consult heating system instructions.  
3
.
LEDs cannot be replaced or added in the field.  
4
.
LEDS  
EM.  
HEAT FILTER SERV. HEAT  
AUX  
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  
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.  
8
6
4
2
00  
00  
00  
00  
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  
In a standard residential heating-cooling circuit, changeover  
between heating and cooling can be done either automatically  
or manually.  
LED Indicators  
The light-emitting diodes (LED) indicators on the subbase  
light on command when something specific happens in the  
system. See Fig. 30.  
Up to four different LEDs are available. The thermostat has a  
clear lens window for viewing each LED. On TRADELINE  
models, a small insert is used so the LED function desired can  
be selected. This must be done during installation.  
Automatic changeover is done by an AUTO position on the  
subbase system switch. See Fig. 31. When the switch is in the  
AUTO position, the thermostat automatically changes between  
heat and cool modes, depending on the indoor temperature.  
A blank insert is factory-installed in some T874 models. To  
remove it, push both temperature setting levers to the far ends  
of the thermostat. Use index fingernail to gently pull out the  
scaleplate a fraction of an inch. Turn thermostat upside-down,  
and the blank insert falls out.  
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When enthalpy is low enough, the thermostat and a sensor  
tell the economizer damper to open to let in cool air from the  
outside. When enthalpy is high, the thermostat and sensor tell  
the damper to close. No more outside air comes in, so the air  
conditioner is used instead.  
OFF EM. HT. HEAT AUTO COOL  
Heat Pump Application  
M5828  
A heat pump system operates much like an air conditioner.  
However, the heat pump is capable of moving heat in two  
directions; from inside the home to outdoors for cooling, or  
from outdoors to inside for heating.  
Fig. 31. System switching for automatic changeover.  
Automatic changeover is frequently used in areas where there  
are large temperature differences in 24 hours. Some uses for  
the automatic changeover feature are in the southern states  
The heat pump can be controlled by separate relay/contactor  
components, which are energized or de-energized by the T874.  
(
cool in the morning and hot in the afternoon) and in  
commercial applications.  
Changeover between heating and cooling for heat pump  
circuits can be accomplished either manually or automatically.  
Manual changeover requires a system switch movement to  
change the mode. See Fig. 32. Most manual changeover  
switches consist of HEAT-OFF-COOL choices. For heating,  
switch to HEAT. For cooling, switch to COOL. It is not automatic.  
Manual changeover control requires a system switch  
movement to change the mode. The reversing valve is  
activated by moving the system switch to COOL for cooling or  
to HEAT for heating operation.  
Manual changeover is often used in areas where the  
temperature is relatively stable between day and night.  
Automatic changeover is accomplished by one of the mercury  
bulb switches in the thermostat. On a system with automatic  
changeover on cooling, the changeover valve is energized by  
a mercury switch that makes on a temperature rise. On  
heating, the changeover valve is energized by a mercury  
switch that makes on a temperature fall.  
A description of the methods used to control heat pumps  
follows.  
OFF EM. HT. HEAT COOL  
M5829  
Fig. 32. System switching for manual changeover.  
Space Temperature Sensing Low Voltage  
Controls  
Control systems for heat pumps usually utilize low voltage  
control circuits. The variety of control functions required and  
the relative complexity require a versatile and economical  
method of control. There are several interlocks and indication  
functions that would be more difficult and expensive to wire in  
line voltage circuits. Low voltage controls have the precision  
and flexibility needed.  
Commercial Rooftop Application  
The T874 can be used as part of a commercial rooftop  
application to run mechanical cooling.  
An economizer is often used as part of a rooftop application to  
save on air conditioning. See Fig. 33.  
The thermostat used is a low voltage device that has good  
temperature sensitivity and several manual and automatic  
switching capabilities.  
OUTDOOR  
AIR  
Two-Stage Thermostat  
In most installations, the heat pump itself requires auxiliary  
heat, usually in the form of electric strip heaters, to carry the  
building heating load at design conditions. Since the heat  
pump is usually more efficient, the control strategy is to run  
the heat pump whenever possible and the auxiliary heat only  
when necessaryduring 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.  
2
3
60-24858  
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T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES  
one controlling the auxiliary heat) makes only when the heat  
pump alone cannot handle the load. That is how the strategy  
of using the most economical heat first is implemented.  
This is particularly important on the 3-stage T874. The  
additional stage means that more droop is introduced when  
the system is in operation. That is the reason the TRADELINE  
T874W is sold in a package with the outdoor sensor and the  
correct subbase.  
Cycling Rate  
Normally, thermostat anticipation is set to cycle a fossil fuel  
furnace about five or six times an hour (at 50 percent load).  
This provides a reasonable balance between comfort, stability  
and economy.  
Outdoor ResetHow 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  
Outdoor Reset  
Outdoor reset, which a C815A Outdoor Thermistor makes  
possible, is sometimes applied in heat pump systems to help  
minimize the effect of the differential between stages. It takes  
Changeover between heating and cooing can be  
accomplished either automatically or manually, depending on  
the application. Changeover can occur in heating or cooling.  
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.