T874 Multistage Thermostats
and Q674 Subbases
PRODUCT DATA
These thermostats and subbases provide low voltage control of multistage heating and cooling systems, including heat pump systems.
•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.
•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 |
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Copyright © 2001 Honeywell • All Rights Reserved |
60- 2485-8 |
T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES
IMPORTANT
The specifications given in this publication do not include normal manufacturing tolerances. Therefore, this unit may not exactly match the listed specifications. This product is tested and calibrated under closely controlled conditions, and some minor differences in performance can be expected if those conditions are changed.
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 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.
Super Tradeline Models
T874 THERMOSTAT
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.
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. Available in beige or Premier White® color.
SUPER TRADELINE FEATURES:
•SUPER TRADELINE package with cross reference label and special instruction sheet.
•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 Models
T874 THERMOSTAT
•T874 TRADELINE models provide staged heat and/or cool operation. See Table 1.
Table 1. Heating and Cooling Stages.
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Models |
A |
B |
C |
D |
E |
F |
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Heating Stages |
1 |
1 |
2 |
2 |
— |
2 |
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Cooling Stages |
1 |
2 |
1 |
2 |
2 |
— |
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• T874A-F are standard models.
TRADELINE FEATURES:
•TRADELINE package with cross reference label and special instruction sheet.
•T874A,C model available with factory stops for DoD applications.
•T874A,C model available with adjustable temperature locking stops.
Q674 SUBBASE
Q674 switching subbases provide system and fan switching. See Table 2.
Table 2. System and Fan Switching.
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Q674 |
System |
Fan |
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A |
Heat-Auto-Cool |
Auto-On |
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B |
Heat-Off-Cool |
Auto-On |
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C |
Off-Auto |
Auto-On |
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D |
None |
None |
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E |
Off-Heat-Auto-Cool |
Auto-On |
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F |
Em. Ht.-Off-Heat-Auto-Cool |
Auto-On |
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G |
Off-Auto |
None |
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J |
Em. Ht.-Auto-Off |
Auto-On |
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L |
Em. Ht.-Heat-Off-Cool |
Auto-On |
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TRADELINE FEATURE:
•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.Your local Home and Building Control Sales Office (check white pages of your phone directory).
2.Home and Building Control Customer Relations Honeywell, 1885 Douglas Drive North
Minneapolis, Minnesota 55422-4386 (800) 328-5111
In Canada—Honeywell Limited/Honeywell Limitée, 35 Dynamic Drive, Scarborough, Ontario M1V 4Z9.
International Sales and Service Offices in all principal cities of the world. Manufacturing in Australia, Canada, Finland, France, Germany, Japan, Mexico, Netherlands, Spain, Taiwan, United Kingdom, U.S.A.
60-2485—8 |
2 |
T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES
Standard Models
T874 THERMOSTATS
Models: See Table 3.
Electrical Rating: 24 to 30 Vac.
Switching: Coiled bimetal elements operate mercury switches.
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.
Dimensions: See Fig. 1.
Temperature: Scale Range: 42° to 88°F (6° to 31°C) standard; optional ranges available.
Thermometer Range: 42° to 88°F (6 to 31°C)
Changeover Differential: 4°F (2°C) minimum between heating and cooling (5°F [3°C] on T874W). Levers can be set apart for greater separation.
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.
Special Models: See Table 3.
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 193121A Adapter Plate Assembly.
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THERMOSTAT MOUNTED |
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ON SUBBASE |
SUBBASE |
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FRONT |
SIDE |
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3/8 (10) |
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5-5/8 (143) |
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3-9/32 (83) |
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HEAT |
50 60 70 80 |
50 60 70 80 |
COOL |
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1-1/2 |
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(38) |
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50 |
60 |
70 80 |
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3-7/16 |
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3-1/2 |
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(87) |
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(89) |
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FAN |
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OFF EM. HT. HEAT AUTO COOL |
FAN |
OFF EM. HT. HEAT AUTO COOL |
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AUTO ON |
AUTO ON |
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1-7/8 (48) |
5-1/8 (130) |
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2-3/16 (56) |
M5849 |
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Fig. 1. T874 Thermostat and Q674 Subbase dimensions in in. (mm).
Optional Specifications (T874 Only):
Temperature scale ranges are 40° to 75°F (4° to 24°C) heating and 75° to 90°F (24° to 32°C) cooling with stop; 44° to 68°F (7° to 20°C) heating, 80° to 86°F (27° to 30°C) cooling; 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 Accessories).
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.
Thermostat Accessories:
Locking Cover and Locking Lever Assembly: Part no. 194559R with thermometer; 194559S without thermometer. 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.
Universal Versaguard™ Thermostat Guard: Includes wallplate, ring base, guard cover, tumbler lock, two keys and optional Honeywell logo insert. Double-wall construction provides extra measure of tamper-resistance. Tamperresistant lock; key cannot be removed without being in locked position. Vents in guard base allow airflow for optimum thermostat performance. See form 68-0104 for more information.
—TG511A1000: Clear cover.
—TG511B1008: Opaque cover.
—TG511D1004: Painted steel (off-white) cover. See Fig. 2.
3 |
60-2485—8 |
T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES
Key Lock Cover: Part no. TG504A replaces existing T874 |
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TG504A1025: Blank face, internal thermometer. |
cover. Mounts on T874 base and covers thermostat set- |
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TG504A1033: External thermometer. See Fig. 2. |
tling levers and subbase switches. Includes LED window |
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and two keys. Should not be used with 193121A Adapter |
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Plate. |
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194559R Locking Cover |
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TG504A1033 Key Lock |
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TG511D1004 Includes |
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with Thermometer |
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Cover with External |
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Painted Steel Cover, |
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Allen Wrench |
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Thermometer |
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Opaque Base and Wallplate |
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Fig. 2. T874 Thermostat accessories. |
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Table 3. T874 Thermostat Specifications. |
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Anticipation |
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System Stages |
Heating (Adj) |
Cooling (Fixed) |
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Models and Options |
Replaces |
Applications |
Heat |
Cool |
Other |
Stage 1 |
Stage 2 |
Stage 1 |
Stage 2 |
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T874A—Standard and TRADELINE®. |
T872A |
Standard |
1 |
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1 |
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0.1-1.2A |
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0-1.5A |
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— 72°F/78°F (22°C/26°C)setpoint |
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stops with locking cover. |
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— Adjustable anticipator set 0.4A. |
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— Adjustable locking temperature |
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stops (TRADELINE®). |
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— 72°F/78°F (22°C/26°C) setpoint |
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stops with locking cover, no |
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thermometer (for DoDa). |
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T874B—Standard and TRADELINE. |
T872B |
Standard |
1 |
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2 |
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0.1-1.2A |
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0-1.2A |
0-1.0A |
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T874C—Standard and TRADELINE. |
T872C |
Standard |
2 |
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1 |
— |
0.1-1.2A |
0.1-1.2A |
0-1.5A |
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— 72°F/78°F (22°C/26°C)set stops |
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with locking cover, no thermometer |
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(for DoDa). |
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— 12°F (7°C) differential between |
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H1 and H2 stages (T874C1125). |
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— Fast cycling. |
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0.12- |
0.12- |
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0.6A |
0.6A |
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T874D—Standard and SUPER |
T872D |
Standard |
2 |
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2 |
— |
0.1-1.2A |
0.1-1.2A |
0-1.2A |
0-1.0A |
TRADELINE®. |
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— Adjustable locking temperature |
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stops (SUPER TRADELINE). |
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T874E—Standard and TRADELINE. |
T872E |
2-Stage Cool |
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2 |
— |
— |
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0-1.2A |
0-1.0A |
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T874F—Standard and TRADELINE. |
T872F |
2-Stage Heat |
2 |
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0.1-1.2A |
0.1-1.2A |
— |
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— Locking cover. |
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aDepartment of Defense.
bChangeover stage operates with heating.
cFixed voltage type anticipation.
dChangeover stage operates with heating; a secondary changeover is provided in cooling switch.
eProvides night setback used with standard T874 and timer-operated remote switching.
fManual changeover stage—use Q674B,L subbase.
gChangeover stage operates with cooling.Q674 Subbases
60-2485—8 |
4 |
T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES
— Fast cycling. |
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Electric Heat |
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0.12- |
0.12- |
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0.6A |
0.6A |
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T874G—Heat pump, cool |
T872G |
Heat Pump or |
2 |
1 |
1b |
0-1.0Ac |
0.1-1.2A |
0-1.0A |
— |
changeover, with fast cycling. |
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— Fixed anticipator for H2. |
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0.1-1.5A |
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— Fast cycling. |
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0.12- |
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0.6A |
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T874H—Use with Q674C. |
T872H |
Heat Pump or |
1 |
1 |
1b |
0-1.0Ac |
— |
0-1.0A |
— |
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Standard |
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T874J—Heat pump. |
None |
Heat Pump |
2 |
1 |
2d |
0-1.0Ae |
0-1.5Ac |
0-1.0Ac |
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T874K—Heat pump. |
None |
Heat Pump |
2 |
1 |
1b |
0.1-1.2A |
0-1.5Ac |
0-1.5A |
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T874L—Heat pump. |
None |
Heat Pump |
2 |
1 |
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0.1-1.2A |
0.1-1.2A |
0-1.5A |
— |
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T874N—Heat pump, heat |
T872N |
Heat Pump |
2 |
1 |
1b |
0.1-1.2A |
0.1-1.2A |
0-1.0A |
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changeover. |
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T874P—Heat pump. |
None |
Heat Pump |
2 |
1 |
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0.1-1.2A |
0.1-1.2A |
0-1.5A |
— |
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T874Q—Night setback heating. |
T872Q |
Standard |
1e |
— |
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0.1-1.2A |
— |
— |
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T874R—Heat pump. |
T872R |
Heat Pumpf |
2 |
1 |
— |
0-1.5Ac |
0.1-1.2A |
0-1.5A |
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T874S—Two-speed compressor heat |
None |
Heat Pump |
2 |
2 |
1g |
0-1.2Ac |
0-1.2Ac |
0-1.2A |
0-1.0A |
pump. |
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T874V—Standard. |
None |
Standard |
— |
1 |
— |
— |
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0-1.5A |
— |
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T874W—Heat pump and standard. |
T872W |
Heat Pump or |
3 |
2 |
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0-1.2Ab |
0-1.2Ac,d |
0-1.2A |
0-1.0A |
— Night setback heating. |
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aDepartment of Defense.
bChangeover stage operates with heating.
cFixed voltage type anticipation.
dChangeover stage operates with heating; a secondary changeover is provided in cooling switch.
eProvides night setback used with standard T874 and timer-operated remote switching.
fManual changeover stage—use Q674B,L subbase.
gChangeover stage operates with cooling.Q674 Subbases
Models:
See Table 4.
Electrical Ratings:
Switch Contacts: 2.5A at 30 Vac (7.5A inrush).
LED Lights (Optional): 30 Vac.
Switches: Two slide switches (one switch on Q674G and K; no switches on Q674D) operated by levers. Switch position is shown on scaleplate.
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).
Finish: Dark brown or gray.
Dimensions in in. (mm): 3-1/2 (89)height; 5-5/8 (143) width; 5/16 (8) depth. See Fig. 1.
Table 4. Q674 Subbase Specifications.
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Switch Positions |
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Models and Options |
Replaces |
Application |
System |
Fan |
Use With |
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Q674A—Standard and TRADELINE®. |
Q672A |
Standard |
HEAT-AUTO-COOL |
AUTO-ON |
T874A-D |
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— Indicator LEDs. |
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Q674B—Standard and TRADELINE. |
Q672B |
Standard |
HEAT-OFF-COOL |
AUTO-ON |
T874A-D |
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— Provision for fan relay operation from |
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Heat Pump |
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T874G,H,L,R |
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external fan switch (isolate G terminal). |
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OVERRIDE-HEAT- |
AUTO-ON |
T874P |
— Indicator LEDs. |
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OFF-COOL |
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Q674C—Standard and TRADELINE. |
Q672C |
Standard |
OFF-AUTO |
AUTO-ON |
T874A-H,W |
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— Indicator LEDs. |
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Heat Pump |
OVERRIDE-AUTO- |
AUTO-ON |
T874K,L,N |
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OFF |
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T874P |
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Q674D—Standard and TRADELINE. |
Q672D |
Standard |
None |
None |
T874A-F |
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— For use when subbase switching is not |
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required. |
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5 |
60-2485—8 |
T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES
Q674E—Standard and TRADELINE. |
Q672E |
Standard |
OFF-HEAT-AUTO- |
AUTO-ON |
T874A-D |
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COOL |
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Q674F—Two LED models. |
Q672F |
Heat Pump |
OFF-EM.HT.-HEAT- |
AUTO-ON |
T874C,D,G,N,S |
— EM.HT. light. |
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AUTO-COOL |
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— Provision for AUTO fan operation in |
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OFF-COOL-AUTO- |
AUTO-ON |
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EM.HT. |
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HEAT-EM.HT. |
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Q674G—O and B terminals. |
Q672G |
Standard |
OFF-AUTO |
None |
T874A-F |
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Q674J—Provision for AUTO fan operation |
Q672J |
Heat Pump |
EM.HT.-AUTO-OFF |
AUTO-ON |
T874A,D,G,J |
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in EM.HT. |
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EM.HT.-ON-OFF |
AUTO-ON |
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— Provision for outdoor thermistor. |
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SUPL.HT.-ON-OFF |
AUTO-ON |
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Q674K—Standard. |
Q672K |
Standard |
OFF-HEAT-AUTO- |
None |
T874F |
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COOL |
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OFF-WOOD-WOOD/ |
None |
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OIL-OIL |
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Q674L—Provision for AUTO fan operation in |
Q672L |
Heat Pump |
EM.HT.-HEAT-OFF- |
AUTO-ON |
T874R,W |
EM.HT. |
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COOL |
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— Indicator LEDs. |
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SUPL.HT.-HEAT- |
AUTO-ON |
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OFF-COOL |
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Q674N—Standard. |
Q672N |
Evaporative |
EVAP-COOL-OFF- |
AUTO-ON |
T874C |
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Cooler |
HEAT |
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Q674P—Standard. |
Q672P |
Heat Pump |
SUPL.HT.-HEAT- |
AUTO-ON |
T874G |
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COOL |
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Q674Q—Standard. |
None |
Fan Coil |
HEAT-OFF-COOL |
LO-MED-HI- |
T874A |
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ON |
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Q674R—Standard. |
None |
Fan Coil |
OFF-COOL |
LO-HI- |
T874V |
— International symbols. |
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CONT. |
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Q674S—Indicator LED. |
None |
Standard |
HEAT-COOL |
None |
T874C |
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Optional Specifications (Q674 Only):
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 systems 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.
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.
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 |
T874 Thermostat with |
Q674 Subbase |
One Setpoint Lever |
Separate |
with Four LEDs |
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Heating and Cooling Levers |
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Fig. 3. Heating, cooling levers and system LED indicators. |
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60-2485—8 |
6 |
T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES
MERCURY NOTICE
This control contains mercury in a sealed tube. Do not place control in the trash at the end of its useful life.
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 1-800-468-1502.
1.Read these instructions carefully. Failure to follow them could damage the product or cause a hazardous condition.
2.Check the ratings given on the product to make sure the product is suitable for your application.
3.Installer must be a trained, experienced service technician.
4.After installation is complete, check out product operation as provided in these instructions.
CAUTION
Hazardous Voltage.
Can damage heating/cooling system.
1.Disconnect power supply before beginning installation to prevent electrical shock or equipment damage.
2.Do not short across coil terminals on relay. This can burn out thermostat heat anticipator.
3.To prevent interference with the thermostat linkage, keep wire length to a minimum and run wires as close as possible to the subbase.
4.Do not overtighten thermostat captive mounting screws because damage to subbase threads can result.
IMPORTANT
An incorrectly leveled thermostat will cause the temperature control to deviate from setpoint. It is not a calibration problem.
Install the thermostat about 5 ft (1.5m) above the floor in an area with good air circulation at average temperature.
Do not mount the thermostat where it can be affected by:
—drafts or dead spots behind doors, in corners or under cabinets.
—hot or cold air from ducts.
—radiant heat from the sun, fireplace, or appliances.
—concealed pipes and chimneys.
—unheated (uncooled) areas such as an outside wall behind the thermostat.
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.
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
3/16 in. (4.8 mm) holes and gently tap anchors into the holes until flush with the wall.
2.Pull wires through the cover plate (if used) and subbase cable opening. See Fig. 6.
3.Secure the cover plate (if used) and subbase with the screws provided. Do not fully tighten the subbase
screws.
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.
VERTICAL MOUNTING
OUTLET SCREWS (2)
BOX 1
COVER
ADAPTER PLATE 2
RING 2
1 HORIZONTAL OUTLET BOX
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THERMOSTAT |
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COO |
L |
THERMOSTAT |
7 |
80 |
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COVER |
0 |
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SUBBASE |
70 |
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80 |
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HE |
SUBBASE
MOUNTING SCREWS (2)
CAPTIVE
MOUNTING SCREWS (2)
1 NOT INCLUDED WITH UNIT.
2 ACCESSORY PART AVAILABLE (193121A). |
M6009 |
Fig. 4. Installing Q674 Subbase on outlet box.
7 |
60-2485—8 |
T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES
WALL
WIRES THROUGH
WALL OPENING
WALL |
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ANCHORS |
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MOUNTING |
(2) |
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HOLES |
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MOUNTING |
M926 |
SUBBASE |
SCREWS (2) |
Fig. 5. Installing Q674 Subbase on wall.
WIRING |
THERMOSTAT |
TERMINAL |
CABLE OPENING |
SPIRIT LEVEL
POST FOR |
MOUNTING |
TO SPRING FINGER |
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MOUNTING |
HOLES (4) |
CONTACTS ON |
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THERMOSTAT (2) |
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THE THERMOSTAT |
M927 |
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(UP TO 12) |
Fig. 6. Subbase components and leveling procedure.
Disconnect power supply before beginning installation to prevent electrical shock or equipment damage.
All wiring must comply with local electrical codes and ordinances.
IMPORTANT
Use 18 gauge, solid-conductor wire whenever possible. If using 18 gauge stranded wire, no more than 10 wires can be used. Do not use larger than
18 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 designation 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.
FOR STRAIGHT |
FOR WRAPAROUND– |
INSERTION– |
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STRIP 5/16 IN. (8 MM) |
STRIP 7/16 IN. (11 MM) |
BARRIER |
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SUBBASE TERMINAL SCREW |
M928 |
Fig. 7. Barrier configuration.
CAUTION
Equipment Damage Hazard.
Never install more than one wire per terminal unless using factory-supplied jumper with spade terminal.
2.Firmly tighten each terminal screw.
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.
60-2485—8 |
8 |
T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES
JUMPER WIRE
(SUPPLIED WITH SOME MODELS)
1
1
1TWO 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
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Standard Terminal |
Alternate Designations or |
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Designation |
Customer Specials |
Typical Connection |
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B |
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Heating damper motor; changeover valve |
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E |
K |
Emergency heat relay |
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G |
F |
Fan relay coil |
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L |
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System monitor |
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O |
R |
Cooling damper motor; changeover valve |
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R |
V |
Power connection to transformer (internally connected for heating |
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and cooling) |
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RC |
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Power connection to cooling transformer |
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RH |
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Power connection to heating transformer |
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W1 |
H1, R3 |
Stage 1 heating control |
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W2 |
H2, Y, R4 |
Stage 2 heating control |
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W3 |
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Stage 3 heating controlb |
Y1 |
C1, M |
Stage 1 cooling control |
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Y2 |
C2 |
Stage 2 cooling control |
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Y3 |
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Stage 3 cooling control |
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X |
X1,X2,C |
Clogged filter switch or common connection |
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T |
A |
Outdoor thermistor |
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L, C, H |
HSII control panel |
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P |
Defrost |
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O |
Momentary circuit, changeover |
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A, A1, A2, Z, C, L |
LEDs |
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aOther terminal designations can be used that are not listed on this table. Refer to the hookup drawing and internal schematic for exact connections.
bW3 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 |
60-2485—8 |
T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES
Table 5. Terminal Designationsa
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Standard Terminal |
Alternate Designations or |
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Designation |
Customer Specials |
Typical Connection |
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T |
External temperature readout, T relay |
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R1, R2 |
LO and HI speed fan relays |
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RS |
Cooling contactor |
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Y |
M |
Compressor contactor |
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aOther terminal designations can be used that are not listed on this table. Refer to the hookup drawing and internal schematic for exact connections.
bW3 controls the auxiliary heat like W2, and allows adding additional stages of auxiliary heat with outdoor thermostats while maintaining the proper second stage anticipation.
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.
The stop brackets should be installed only if there is a need to restrict the adjustable range of the heating and cooling temperature setpoint levers. If adjustable lever stops are desired, order 4074ECK Envelope Assembly, which contains two adjustable lever stop brackets, one brass insert, one
HOLES FOR INSULATED
LOCKING LEVER SCREWS
COOL |
HOLE WITH |
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LEVER |
BRASS INSERT |
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HEAT |
BRACKET |
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LEVER |
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TABS |
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ADJUSTABLE |
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MOUNTING |
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LEVER STOP |
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BRACKET |
SCREW |
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BRACKETS |
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SLOTS |
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ADJUSTABLE STOPS
mounting screw and two locking screws with insulated heads. When installed, the stop brackets limit the movement of the T874 HEAT and COOL levers.
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.
3.Push the brass insert into the hole with finger.
4.Turn to the front of the T874 Thermostat.
5.Place the two stop brackets in position with the tabs in the slot between the HEAT and COOL levers. See Fig. 9.
ADJUSTABLE LOCKING LEVERS
NONADJUSTABLE STOPS
WITH LOCKING LEVER SCREWS 75°F (24°C) MAX. HEAT
75°F (24°C) MIN. COOL
NONADJUSTABLE D.O.D. STOPS
72°F (22°C) MAX. HEAT M7626
78°F (26°C) MIN. COOL
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.
60-2485—8 |
10 |
T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES
8.Move the HEAT and COOL levers to the maximum temperature desired.
9.Slide the stop brackets until one rests against the HEAT lever and the other against the COOL lever.
10.Firmly tighten the mounting screw.
11.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.
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.
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.
NOTE: The cover is hinged at the top and must be removed by pulling up at the bottom.
2.Carefully remove and discard the polystyrene packing insert that protects the mercury switches during shipment.
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.
4.Turn over the thermostat base and note the spring fingers that engage the subbase contacts. Make sure the spring fingers are not bent flat, preventing proper electrical contact with the subbase.
5.Set the heat anticipator indicator(s) to the respective current setting of each stage. See Set The Heat Anticipator section.
6.If the thermostat provides optional locking cover assembly, start the Allen locking screws in the cover with the wrench provided. See Fig. 10.
7.Note the two tabs along the top inside edge of the thermostat base. The tabs fit into corresponding slots on top of the subbase. Mount the thermostat on the subbase.
8.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 subbase posts can result.
ALLEN
RETAINING
SCREWS (2)
M956
Fig. 10. Installation of locking cover assembly.
CAUTION
Equipment Damage Hazard.
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).
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.
The current draw of each heating stage must be measured with the thermostat removed and power on to the heating system.
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.
2.Move the system switch to HEAT or AUTO.
3.After one minute, read the ammeter and record the reading:
a.Stage 1—__________A;
b.Stage 2—__________A;
c.Stage 3—__________A.
NOTE: If equipment cycles too fast, set the indicator to a higher current rating, but not more than one-half division at a time, and recheck the cycle rate. Most conventional 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
11 |
60-2485—8 |
T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES
anticipator at 140 percent of the actual primary control current draw to reduce the cycling rate. See Fig. 11.
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.
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STAGE ONE |
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12 |
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ANTICIPATOR |
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HEATING |
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15 |
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CONTROL |
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. 2 |
. 8 |
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1 |
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. 1 |
0 |
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.12 |
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.15 |
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1.2.8.6 |
.4 |
.3 |
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STAGE TWO |
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MOVE INDICATOR TO |
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ANTICIPATOR |
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MATCH CURRENT RATING |
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HEATING CONTROL |
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OF PRIMARY CONTROL |
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M5069 |
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Fig. 11. Adjustable heat anticipator scales.
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 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.
STAGE 1 |
HEATING |
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HEATING |
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LEVER |
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STAGE 3 |
COOLING |
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LEVER |
STAGE 1 |
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HEATING |
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COOLING |
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STAGE 2 |
STAGE 2 |
CAPTIVE |
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HEATING |
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MOUNTING |
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COOLING |
M7625 |
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SCREWS (2) |
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Fig. 12. Internal view of T874W (three stages of heating, two stages of cooling).
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.
HEAT—heating system is automatically controlled by the thermostat. Cooling system is off.
AUTO—thermostat automatically changes between heating and cooling system operation, depending on the indoor temperature.
COOL—cooling system is automatically controlled by the thermostat. Heating system is off.
EM.HT.—emergency heat relay is automatically controlled by the thermostat. Cooling system is off. Compressor is de-energized.
SUPL.HT.—supplemental heat relay is energized. Cooling system is off. Compressor is de-energized.
WOOD—heating system is operating with only the woodburning 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.
ON—heating system is controlled by the thermostat. EM. HT. or SUPL. HT. relay is not energized.
FAN SWITCH positions control fan operation as follows: ON or CONT.—fan operates continuously.
AUTO—fan operates as controlled by the thermostat in heat pump systems or conventional cooling mode; fan operates as controlled by the plenum switch in conventional heating mode.
LO—fan operates constantly at low speed. MED—fan operates constantly at medium speed. HI—fan operates constantly at high speed.
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.
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.
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.
60-2485—8 |
12 |
T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES
Each mark on the scale represents 1°F (0.6°C) The differential is factory set at 2°F (1°C) the differential can be set as high as 12°F (7°C) To set the adjustable interstage differential, loosen the tension screw. See Fig. 13. Slide the adjustable scale to align with the number of degrees desired between stages. Use the lower edge of the tension screw bracket as a guide for alignment. In heating, slide the lever wider apart for a larger differential, or closer together for a smaller differential. In cooling, slide the lever closer together for a larger differential, or wider apart for a smaller differential. While supporting the scale with hand, tighten the tension screw.
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 |
HEAT |
SCREW |
50 60 70 80 |
FIRST
STAGE
SWITCH
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1 |
.5 |
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.2 |
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1.2 |
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.8 |
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SCALE |
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SECOND STAGE |
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(ADJUSTABLE) |
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SWITCH |
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TENSION
SCREW
ALIGN LOWER |
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EDGE WITH |
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SCALE |
SLIDE LEVER |
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NOTCH |
WIDER APART |
10°F |
FOR LARGER |
SCALE |
DIFFERENTIAL |
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1 |
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TENSION
SCREW
ALIGN LOWER |
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EDGE WITH |
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SCALE |
SLIDE LEVER |
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NOTCH |
CLOSER TOGETHER |
4°F |
FOR LARGER |
SCALE |
DIFFERENTIAL |
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1 |
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1 EACH MARK ON THE SCALE REPRESENTS 1°F (0.6°C).
M937
Fig. 13. Set adjustable interstage differential.
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.
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.
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 temperatures is the interstage differential, which should match the number set on the scale with the tension screw.
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.
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.
Cooling
CAUTION
Equipment Damage Hazard.
Do not operate cooling if outdoor temperature is below 50°F (10°C). Refer to manufacturer recommendations.
13 |
60-2485—8 |
T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES
Move the system switch on the Q674 Subbase to COOL or AUTO. Move the cool setting lever on the T874 Multistage Thermostat about 10°F (6°C) below room temperature. See Fig. 12. The cooling equipment and fan should start. If the system has two stages of cooling, both stages should start. Move the cool lever about 10°F (6°C) above room temperature. The cooling equipment and fan should stop.
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.
If the system is supplied with a thermistor, it must be used; if not used, thermostat performance deviates radically from proper operation.
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.
4.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 thermistor, part no. C815A1005.
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4600 |
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4400 |
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(ohms) |
4200 |
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3800 |
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R = 400 ohms ± 10% AT 77°F (25°C) |
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4000 |
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C815A THERMISTOR RESISTANCE |
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RESISTANCE |
3600 |
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2600 |
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3400 |
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3200 |
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3000 |
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2800 |
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THERMISTOR |
2400 |
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1400 |
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2200 |
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2000 |
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1800 |
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1600 |
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1000 |
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800 |
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600 |
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400 |
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200 |
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0 |
20 |
40 |
60 |
80 |
100 |
120 |
140 |
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-20 |
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TEMPERATURE OF THERMISTOR (°F) |
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M1590A |
Fig. 14. Thermistor resistance chart.
T874 Thermostats are accurately calibrated at the factory.
They do not have provision for field calibration.
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:
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.
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.
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 operation.
M5070
Fig. 15. Thermometer calibration.
To understand wiring diagrams, it is important to know what all the symbols mean and how to trace the path of the circuits from the transformer. See Fig. 16 through 25.
Circuit descriptions and terminology are defined as follows: For standard heating-cooling circuits:
Auto changeover—refers to the presence of an AUTO position in the system switching (EX: Q674E with OFF- HEAT-AUTO-COOL switching). The thermostat automatically changes between heat and cool modes as indoor temperature changes.
Manual changeover—requires a system switch movement to change mode (EX: Q674B with HEAT-OFF-COOL switching). T874D Multistage Thermostats with 2 heat or 2 cool switches are shown on most standard circuits. Most standard or TRADELINE® subbases (Q674A-E,G) can be used with T874A-F standard or TRADELINE
60-2485—8 |
14 |
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).
For heat pump circuits:
Cool changeover valve—operates on cooling. The reversing valve or relay is activated either by moving the system 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 reversing valve or relay is activated either by moving the system 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 compressor and indicates the malfunction by activating the EMERGENCY HEAT LED on the Q674 Switching Subbase. The system monitor relay is usually wired into the L terminal on the Q674.
Each mercury switch is identified by function: H1—Stage 1 Heating.
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).
Each anticipator is identified and each switch affected is named (EX: H1 anticipator, C1 anticipator).
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.
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.
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.
2.Completely trace only one circuit at a time (for example: heat or cool).
3.Connections are indicated by small dots at the point of intersection. If there is no dot, there is no connection.
4.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-
perature rise. Fixed anticipation is represented by a zigzag 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 anticipator 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 circles represent the switch positions available on this particular Q674, with the solid circle representing where it is actually switched on the diagram.
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.
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.
KEY TO HOOKUP SYMBOLS
TRANSFORMER
(24 VAC SECONDARY)
RELAY OR CONTACTOR COIL
MERCURY SWITCH
FIXED ANTICIPATOR HIGH RESISTANCE (TYPICALLY 5 KILOHMS)
ADJUSTABLE ANTICIPATOR LOW RESISTANCE (TYPICALLY 0 TO 5 OHMS)
RELAY/CONTACTOR CONTACTS
B
TERMINAL
LED
ODT
OUTDOOR THERMOSTAT
EHR
EMERGENCY HEAT RELAY
RTD
TIME DELAY RELAY
RD
DEFROST RELAY
CHP
PRESSURE SWITCH
LACO
LOW AMBIENT CUTOFF
M5848
Fig. 16. Key to hookup symbols.
15 |
60-2485—8 |
T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES
THERMOSTAT |
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SUBBASE |
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SYSTEM COMPONENTS |
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(HOT) L1 |
L2 |
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1 |
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H1/C1 |
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ANTICIPATOR |
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FALL |
2 |
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R |
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ODT 1 |
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H1 |
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RTD 1 |
EHR 1 |
RTD 2 |
C1 |
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W3 |
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3 |
FAN SWITCH |
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ODT 2 |
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4 |
AUTO |
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H2 |
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ON |
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EHR 2 |
RTD 3 |
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ANTICIPATOR |
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W2 |
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5 |
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AUX. HT. |
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RTD 1 |
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LED |
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H2 |
6 |
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(GREEN) |
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2 |
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FALL |
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X |
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SYSTEM |
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EM. HT. |
SYSTEM |
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LED (RED) |
MONITOR |
RD |
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SWITCH |
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L |
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FAN RELAY |
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EM. HT. |
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G |
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HEAT |
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O |
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EM. HT. |
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COOL CHANGEOVER |
RELAY |
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OFF |
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E |
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VALVE |
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COOL |
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LACO |
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CHP |
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Y |
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COMPRESSOR B CONTACTOR
1 POWER SUPPLY. PROVIDE DISCONNECT MEANS AND OVERLOAD PROTECTION AS REQUIRED.
M5072A
2 AUXILIARY HEAT LED AVAILABLE ON SOME MODELS.
Fig. 17. Internal schematic and typical hookup of a T874R Thermostat and Q674L Subbase in a heat pump application. The thermostat provides two-stage heating and one-stage cooling manual changeover operates on cooling.
60-2485—8 |
16 |
T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES
THERMOSTAT |
H1/C1 |
ANTICIPATOR |
FALL |
2 |
H1 |
C1 |
3 |
4 |
H2 |
ANTICIPATOR |
5 |
6 |
H2 |
FALL |
1POWER SUPPLY. PROVIDE DISCONNECT MEANS AND OVERLOAD PROTECTION AS REQUIRED.
THERMOSTAT |
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H1/ C1 |
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ANTICIPATOR |
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FALL |
2 |
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H1 |
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C1 |
3 |
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4 |
H2 |
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ANTICIPATOR |
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5 |
H2 |
6 |
FALL |
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1POWER SUPPLY. PROVIDE DISCONNECT MEANS AND OVERLOAD PROTECTION AS REQUIRED.
SUBBASE |
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SYSTEM COMPONENTS |
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L1 |
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R |
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(HOT) |
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ODT 1 |
L2 |
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1 |
FAN SWITCH |
RTD 1 |
EHR 1 |
RTD 2 |
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W3 |
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AUTO |
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ODT 2 |
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ON |
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W2 |
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EHR 2 |
RTD 3 |
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AUX. HT. |
RTD 1 |
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LED |
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(GREEN) |
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X |
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SYSTEM |
EM. HT. |
SYSTEM |
RD |
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LED (RED) |
MONITOR |
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SWITCH |
L |
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FAN RELAY |
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EM. HT. |
G |
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HEAT |
O |
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EM. HT. |
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CHANGEOVER |
RELAY |
OFF |
E |
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VALVE |
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COOL |
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LACO |
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CHP |
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Y |
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B |
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COMPRESSOR |
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CONTACTOR |
M5840 |
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Fig. 18. Tracing the changeover relay circuit.
SUBBASE |
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SYSTEM COMPONENTS |
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L1 |
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R |
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(HOT) |
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ODT 1 |
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L2 |
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1 |
FAN SWITCH |
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RTD 1 |
EHR 1 |
RTD 2 |
W3 |
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AUTO |
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ODT 2 |
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ON |
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W2 |
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EHR 2 |
RTD 3 |
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AUX. HT. |
RTD 1 |
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LED |
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(GREEN) |
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X |
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SYSTEM |
EM. HT. |
SYSTEM |
RD |
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LED (RED) |
MONITOR |
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SWITCH |
L |
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FAN RELAY |
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EM. HT. |
G |
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HEAT |
O |
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EM. HT. |
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CHANGEOVER |
RELAY |
OFF |
E |
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VALVE |
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COOL |
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LACO |
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CHP |
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Y |
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B |
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COMPRESSOR |
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CONTACTOR |
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M5841 |
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Fig. 19. Tracing the heat 1 and anticipation circuit.
17 |
60-2485—8 |
T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES
|
THERMOSTAT |
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|
H1/ C1 |
|
|
ANTICIPATOR |
|
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FALL |
2 |
H1 |
C1 |
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3 |
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4 |
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H2 |
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ANTICIPATOR |
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5 |
H2 |
6 |
FALL
1POWER SUPPLY. PROVIDE DISCONNECT MEANS AND OVERLOAD PROTECTION AS REQUIRED.
SUBBASE |
|
|
SYSTEM COMPONENTS |
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L1 |
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R |
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(HOT) |
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ODT 1 |
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L2 |
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1 |
FAN SWITCH |
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RTD 1 |
EHR 1 |
RTD 2 |
W3 |
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AUTO |
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ODT 2 |
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ON |
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W2 |
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EHR 2 |
RTD 3 |
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AUX. HT. |
RTD 1 |
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LED |
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(GREEN) |
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X |
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SYSTEM |
EM. HT. |
SYSTEM |
RD |
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LED (RED) |
MONITOR |
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SWITCH |
L |
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FAN RELAY |
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EM. HT. |
G |
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HEAT |
O |
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EM. HT. |
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CHANGEOVER |
||
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RELAY |
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OFF |
E |
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VALVE |
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COOL |
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CHP |
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LACO |
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Y |
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B |
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COMPRESSOR |
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CONTACTOR |
M5842 |
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||
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|
Fig. 20. Tracing the heat 1 anticipation circuit.
THERMOSTAT |
|
|
H1/C1 |
|
ANTICIPATOR |
FALL |
2 |
H1 |
C1 |
|
|
|
3 |
|
4 |
|
H2 |
|
ANTICIPATOR |
|
5 |
H2 |
6 |
FALL |
|
1POWER SUPPLY. PROVIDE DISCONNECT MEANS AND OVERLOAD PROTECTION AS REQUIRED.
SUBBASE |
|
|
SYSTEM COMPONENTS |
|
|
|
|
|
L1 |
|
R |
|
|
(HOT) |
|
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ODT 1 |
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L2 |
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1 |
FAN SWITCH |
|
RTD 1 |
EHR 1 |
RTD 2 |
W3 |
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AUTO |
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ODT 2 |
|
ON |
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W2 |
|
EHR 2 |
RTD 3 |
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AUX. HT. |
RTD 1 |
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LED |
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(GREEN) |
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X |
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SYSTEM |
EM. HT. |
SYSTEM |
RD |
|
LED (RED) |
MONITOR |
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||
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|||
SWITCH |
L |
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FAN RELAY |
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EM. HT. |
G |
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HEAT |
O |
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EM. HT. |
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||
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CHANGEOVER |
RELAY |
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OFF |
E |
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VALVE |
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COOL |
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LACO |
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CHP |
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Y |
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COMPRESSOR |
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B |
|
CONTACTOR |
M5843 |
Fig. 21. Tracing the heat 2, anticipation and AUX. HT. LED circuit.
60-2485—8 |
18 |
T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES
|
THERMOSTAT |
|
|
H1/C1 |
|
|
ANTICIPATOR |
|
|
FALL |
2 |
H1 |
C1 |
|
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3 |
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4 |
|
H2 |
|
|
ANTICIPATOR |
|
|
|
5 |
|
H2 |
6 |
FALL |
|
|
1POWER SUPPLY. PROVIDE DISCONNECT MEANS AND OVERLOAD PROTECTION AS REQUIRED.
SUBBASE |
|
|
SYSTEM COMPONENTS |
|
|
|
|
|
L1 |
|
R |
|
|
(HOT) |
|
|
ODT 1 |
|
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L2 |
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1 |
FAN SWITCH |
|
RTD 1 |
EHR 1 |
RTD 2 |
W3 |
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AUTO |
|
|
ODT 2 |
|
ON |
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W2 |
|
EHR 2 |
RTD 3 |
|
AUX. HT. |
RTD 1 |
|
|
|
LED |
|
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|
|
(GREEN) |
|
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|
|
X |
|
|
|
SYSTEM |
EM. HT. |
SYSTEM |
RD |
|
LED (RED) |
MONITOR |
|
||
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|||
SWITCH |
L |
|
|
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FAN RELAY |
|
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EM. HT. |
G |
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O |
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EM. HT. |
|
HEAT |
|
|
CHANGEOVER |
|
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|
RELAY |
|
OFF |
E |
|
VALVE |
|
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||
|
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COOL |
|
LACO |
|
CHP |
|
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||
|
Y |
|
|
|
|
|
|
COMPRESSOR |
|
|
B |
|
CONTACTOR |
M5844 |
Fig. 22. Tracing the emergency heat and EM. HT. LED circuit.
THERMOSTAT
H1/C1
ANTICIPATOR
FALL |
2 |
H1 |
C1 |
|
3 |
4
H2
ANTICIPATOR
5
H2 |
6 |
FALL
1POWER SUPPLY. PROVIDE DISCONNECT MEANS AND OVERLOAD PROTECTION AS REQUIRED.
SUBBASE |
|
|
SYSTEM COMPONENTS |
|
|
|
|
|
L1 |
|
R |
|
|
(HOT) |
|
|
|
ODT 1 |
L2 |
|
|
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|
|
|
|
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|
1 |
|
|
RTD 1 |
EHR 1 |
RTD 2 |
FAN SWITCH |
W3 |
|
|
|
|
|
|
|
|
AUTO |
|
|
ODT 2 |
|
ON |
|
|
|
|
|
W2 |
|
EHR 2 |
RTD 3 |
|
AUX. HT. |
RTD 1 |
|
|
|
LED |
|
|
|
|
(GREEN) |
|
|
|
|
X |
|
|
|
SYSTEM |
EM. HT. |
SYSTEM |
RD |
|
LED (RED) |
MONITOR |
|
||
|
|
|||
SWITCH |
L |
|
|
|
|
|
|
FAN RELAY |
|
|
|
|
|
|
EM. HT. |
G |
|
|
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|
|
|
HEAT |
O |
|
|
EM. HT. |
|
|
|
||
|
|
CHANGEOVER |
RELAY |
|
|
|
|
||
OFF |
E |
|
VALVE |
|
|
|
|
||
|
|
|
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COOL |
|
LACO |
|
CHP |
|
|
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||
|
Y |
|
|
|
|
|
|
COMPRESSOR |
|
|
B |
|
CONTACTOR |
M5845 |
|
|
|
|
Fig. 23. Tracing the cooling circuit.
19 |
60-2485—8 |
T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES
|
THERMOSTAT |
|
|
H1/C1 |
|
|
ANTICIPATOR |
|
|
FALL |
2 |
H1 |
C1 |
3 |
|
||
|
|
|
|
|
4 |
|
H2 |
|
|
ANTICIPATOR |
|
|
|
5 |
|
H2 |
6 |
FALL |
|
|
1POWER SUPPLY. PROVIDE DISCONNECT MEANS AND OVERLOAD PROTECTION AS REQUIRED.
SUBBASE |
SYSTEM COMPONENTS |
|
|
|
L1 |
R |
ODT 1 |
(HOT) |
|
L2 |
|
|
|
|
|
|
1 |
RTD 1 |
EHR 1 |
RTD 2 |
FAN SWITCH |
|
|
W3 |
|
|
AUTO |
ODT 2 |
|
ON |
|
|
|
|
|
W2 |
|
EHR 2 |
RTD 3 |
|
AUX. HT. |
RTD 1 |
|
|
|
LED |
|
|
|
|
(GREEN) |
|
|
|
|
X |
|
|
|
SYSTEM |
EM. HT. |
SYSTEM |
RD |
|
LED (RED) |
MONITOR |
|
||
|
|
|||
SWITCH |
L |
|
|
|
|
|
|
FAN RELAY |
|
|
|
|
|
|
EM. HT. |
G |
|
|
|
|
|
|
|
|
HEAT |
O |
|
|
EM. HT. |
|
|
CHANGEOVER |
||
|
|
|
RELAY |
|
OFF |
E |
|
VALVE |
|
|
|
|
||
|
|
|
|
|
COOL |
|
LACO |
|
CHP |
|
|
|
||
|
Y |
|
|
|
|
|
|
COMPRESSOR |
|
|
B |
|
CONTACTOR |
M5846 |
THERMOSTAT |
|
|
H1/C1 |
|
ANTICIPATOR |
FALL |
2 |
H1 |
C1 |
|
|
|
3 |
|
4 |
|
H2 |
|
ANTICIPATOR |
|
5 |
H2 |
6 |
FALL |
|
1POWER SUPPLY. PROVIDE DISCONNECT MEANS AND OVERLOAD PROTECTION AS REQUIRED.
Fig. 24. Tracing the auto fan circuit.
SUBBASE |
SYSTEM COMPONENTS |
|
|
|
L1 |
R |
ODT 1 |
(HOT) |
|
L2 |
|
|
|
|
|
|
1 |
RTD 1 |
EHR 1 |
RTD 2 |
FAN SWITCH |
|
|
W3 |
|
|
AUTO |
ODT 2 |
|
ON |
|
|
|
|
|
W2 |
|
EHR 2 |
RTD 3 |
|
|
|
|
|
|
AUX. HT. |
RTD 1 |
|
|
|
LED |
|
|
|
|
(GREEN) |
|
|
|
|
X |
|
|
|
SYSTEM |
EM. HT. |
SYSTEM |
RD |
|
LED (RED) |
MONITOR |
|
||
|
|
|||
SWITCH |
L |
|
|
|
|
|
|
FAN RELAY |
|
|
|
|
|
|
EM. HT. |
G |
|
|
|
O |
|
|
|
|
|
|
|
EM. HT. |
|
HEAT |
|
|
|
|
|
|
CHANGEOVER |
RELAY |
|
|
|
|
||
OFF |
E |
|
VALVE |
|
|
|
|
||
COOL |
|
LACO |
|
CHP |
|
|
|
||
|
Y |
|
|
|
|
|
|
COMPRESSOR |
|
|
B |
|
CONTACTOR |
M5847 |
Fig. 25. Tracing the fan on circuit.
60-2485—8 |
20 |
T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES
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.
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.
There are two types of heat anticipation, adjustable and fixed.
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.
L1 L2 (HOT)
|
H1 ANTICIPATOR |
|
H1 |
FALL |
STAGE 1 HEAT RELAY |
|
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.
L1 L2 (HOT)
STAGE 1 |
HEAT RELAY |
FALL |
H1 ANTICIPATOR |
H1 |
M5824
Fig. 27. Fixed anticipation heater in parallel with load.
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.
L1 L2 (HOT)
RISE
STAGE 1
COOL RELAY
C1
C1 ANTICIPATOR
M5825
Fig. 28. Internal cooling anticipation schematic.
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.
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.
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.
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.
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.
Some T874/Q674 models are intended for use with the C815A Outdoor Thermistor. The outdoor thermistor provides significantly improved thermostat performance when compared to conventional multistage thermostats. The C815A is usually located in the heat pump condensing unit outdoors.
21 |
60-2485—8 |
T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES
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.
To check and verify thermistor operation, perform the following steps:
1.Disconnect wire from T terminal on subbase.
2.Measure resistance with ohmmeter across the T subbase wire and X subbase terminal.
3.Take outdoor temperature measurement at thermistor location and find correct thermistor resistance on the chart in Fig. 29.
4.Compare resistance on the chart with measured resistance. Replace C815A if resistance varies more than 15 percent. Contact installing dealer for packaged replacement outdoor thermistor.
|
4600 |
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4400 |
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(ohms) |
4200 |
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3800 |
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R = 400 ohms ± 10% AT 77°F (25°C) |
|
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|||||||||||
|
4000 |
|
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C815A THERMISTOR RESISTANCE |
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||||||||||
RESISTANCE |
3600 |
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2600 |
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3400 |
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3200 |
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3000 |
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2800 |
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THERMISTOR |
2400 |
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1400 |
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2200 |
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2000 |
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1800 |
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1600 |
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1200 |
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1000 |
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800 |
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600 |
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400 |
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Fig. 29. Thermistor resistance chart.
Two of the features of the T874/Q674 include LED indicators and restricted setpoint.
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.
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.
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.
•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.
•LOCKOUT LED lights when the system is shut down and needs maintenance.
•AUXILIARY HEAT LED lights when the auxiliary heat is operating.
•SERVICE or MALFUNCTION LED can have several meanings. Consult heating system instructions.
LEDs cannot be replaced or added in the field.
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Fig. 30. T874/Q674 LED location.
The Department of Defense (DoD) models are equipped with a restricted setpoint feature for fuel efficiency.
Fixed stops are factory-set so setpoint levers cannot be set above 72°F (22°C) on heating, or below 78°F (26°C) on cooling.
The T874/Q674 can be applied to standard residential systems for automatic or manual changeover, to commercial rooftop applications, or to heat pump applications.
In a standard residential heating-cooling circuit, changeover between heating and cooling can be done either automatically or manually.
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.
60-2485—8 |
22 |
T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES
OFF EM. HT. HEAT AUTO COOL
M5828
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 (cool in the morning and hot in the afternoon) and in commercial applications.
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 is often used in areas where the temperature is relatively stable between day and night.
OFF EM. HT. HEAT COOL
M5829
Fig. 32. System switching for manual changeover.
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.
OUTDOOR
AIR
H205 OR H705 |
RETURN |
DISCHARGE AIR SENSOR |
ENTHALPY |
AIR |
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Fig. 33. Typical commercial rooftop economizer application.
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.
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.
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.
The heat pump can be controlled by separate relay/contactor components, which are energized or de-energized by the T874.
Changeover between heating and cooling for heat pump circuits can be accomplished either manually or automatically.
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.
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.
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 thermostat used is a low voltage device that has good temperature sensitivity and several manual and automatic switching capabilities.
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).
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
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60-2485—8 |
T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES
one controlling the auxiliary heat) makes only when the heat pump alone cannot handle the load. That is how the strategy of using the most economical heat first is implemented.
This is particularly important on the 3-stage T874. The additional stage means that more droop is introduced when the system is in operation. That is the reason the TRADELINE T874W is sold in a package with the outdoor sensor and the correct subbase.
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.
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.
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.
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.
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.
L1 L2 (HOT)
Although not a specific requirement for heat pumps, several models of the T874 have voltage heat anticipation instead of current anticipation.
With voltage anticipation, the amount of heat added to the |
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Fig. 34. Location of thermistor. |
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independent of later thermostat adjustments. |
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 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 (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.
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.
Changeover between heating and cooing can be accomplished either automatically or manually, depending on the application. Changeover can occur in heating or cooling.
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.
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.
It is more common with new equipment to find that changeover occurs with cooling; that is, the changeover valve is energized to produce cooling.
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.
60-2485—8 |
24 |
T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES
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.
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.
NOTE: To determine the COP of a heat pump, use the following formula:
COP = Btu Out/Btu we pay for or Btuh Capacity
Unit Wattage x 3.413 Btu/Watt
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.
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.
Additional capacity control methods used on other refrigeration systems (unloading, multiple compressors) are not as likely to be seen on residential heat pumps.
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.
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.
THERMOSTAT
HEAT 2 HEAT 1
COMP.
STAGE 1
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.
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 way to control a two stage heat pump is with a threestage thermostat. The T874W is designed for this application as well as others. See Fig. 36.
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
AUXILIARY
HEAT
M5837
Fig. 36. Two-stage heat pump with three-stage thermostat.
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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60-2485—8 |
T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES
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 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.
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.
Defrost Control Functions
Besides changing over to the cooling mode, defrosting requires several more control initiated actions that follow.
OUTDOOR FAN
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.
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CONTROLLED BY OUTDOOR
DEFROST RELAY FAN MOTOR
M5839
Fig. 37. Outdoor fan with defrost control.
AUXILIARY HEAT DURING DEFROST
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.
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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.
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.
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.
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.
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.
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.
60-2485—8 |
26 |
T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES
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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.
There are two different ways to control auxiliary heat, staged or time modulated control.
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.
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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.
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.
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).
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 (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
178.8 kW) of heat pump auxiliary electric heat. The T874 can provide the same high quality 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.
The method any particular manufacturer selects depends on which arguments it finds persuasive and the opinions of its distributors and dealers.
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.
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 (100 percent) heat pump operation is required before auxiliary heat is brought on.
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 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.
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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60-2485—8 |
T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES
the thermostat. A set of contacts on the emergency heat relay is used to bypass each outdoor thermostat so that all electric heat is under immediate thermostat stage-two control. So the emergency heat relay simply bypasses the outdoor thermostats. See Fig. 41.
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.
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.
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.
THERMOSTAT
FUNCTION HEAT 2 HEAT 1 SWITCH
OUTDOOR
THERMOSTAT
EMERGENCY HEAT
RELAY CONTACT
COMPRESSOR
AUXILIARY
HEAT RELAY 1
AUXILIARY
HEAT RELAY 2
EMERGENCY
HEAT RELAY
M5833
Fig. 41. Emergency heat relay that bypasses the outdoor thermostats.
24 V
THERMOSTAT
FUNCTION HEAT 2 HEAT 1
SWITCH
EMERGENCY
HEAT RELAY
CONTACT
COMPRESSOR
AUXILIARY
HEAT RELAY
EMERGENCY
HEAT RELAY
M5832
Fig. 42. Emergency heat relay that energizes all of the auxiliary heat relays on a call for 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 compressor 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 changeover valve and dispersed in the piping and coils.
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.
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.
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.
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.
COMPRESSOR MOTOR |
FAULT RELAY |
||
L1 |
RUN |
R |
|
|
|
||
C |
CAP. |
|
|
L3 |
|
|
|
|
START |
S |
|
Y |
R |
L |
X |
L2 |
|
|
|
|
L1 |
|
|
|
|
(HOT) |
RISE |
|
|
LED |
|
|
|
|
|
CONTACTOR
THERMOSTAT-
M5831
SUBBASE
Fig. 43. Compressor fault relay schematic.
60-2485—8 |
28 |
T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES
The following section lists the current T874 Trade replacements for T872 and T874 Multistage Thermostats. It also lists the Q674 Trade replacements for Q672 and Q674 Subbases.
All thermostats listed carry the Honeywell logo unless otherwise noted.
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.
3.The Description column identifies certain characteristics of the existing device.
4.The Trade Replacement column identifies the correct replacement device.
5.The Remarks column lists any characteristics or adjustments to be made on the Trade replacement model.
6.The Fig. No. column lists the wiring hookup used for the specific model number. The wiring diagrams follow the Cross Reference section.
7.Refer to the circuit illustration that corresponds with the appropriate number. This figure illustrates the internal electric circuit for the desired model number.
Table 6. T872-T874 Thermostat Cross Reference.
|
|
|
|
|
|
T874 Trade |
Remarks |
Model Number |
Description |
Replacementa |
|
T872A |
1-stage heat, 1-stage cool |
|
|
|
|
|
|
A1006 |
Standard OEM |
T874A1036 |
|
|
|
|
|
A1014 |
72-78°F (22-26°C) stop; locking cover; no |
T874A1010 |
Use T874A1176 for Dept. of |
|
thermometer |
|
Defense (DoD) application. |
|
|
|
|
A1022 |
Heat anticipator set at 0.45A |
T874A1036 |
|
|
|
|
|
A1030 |
0.4A heat anticipator setting |
T874A1036 |
|
|
|
|
|
A1048 |
TRADELINE |
T874A1036 |
|
|
|
|
|
A1055 |
AAF logo; locking cover |
T874A1036 |
|
|
|
|
|
A1063 |
Climatrol logo |
T874A1036 |
|
|
|
|
|
A1071 |
Lennox logo; 0.5A heat anticipator setting |
T874A1036 |
|
|
|
|
|
A1089 |
Melco logo |
T874A1036 |
|
|
|
|
|
A1097 |
Westinghouse logo |
T874A1036 |
|
|
|
|
|
A1105 |
Amana logo; 1.2A heat anticipator setting |
T874A1036 |
|
|
|
|
|
A1113 |
Trane logo |
T874A1036 |
|
|
|
|
|
A1121 |
Airtemp (Chrysler) logo |
T874A1036 |
|
|
|
|
|
A1139 |
Singer logo; no. 02005384 |
T874A1036 |
|
|
|
|
|
A1147 |
American Standard logo |
T874A1036 |
|
|
|
|
|
A1154 |
Carrier logo and color; no. HH07AT074 |
T874A1036 |
|
|
|
|
|
A1162 |
General Electric logo; no. AY28X097 |
T874A1036 |
|
|
|
|
|
A1170 |
Sears logo; no. 4291441 |
T874A1036 |
|
|
|
|
|
A1188 |
Command-Aire logo |
T874A1036 |
|
|
|
|
|
A1196 |
Singer logo; Celsius scale; no. 02005521 |
T874A1036 |
|
|
|
|
|
A1204 |
Carteret logo |
T874A1036 |
|
|
|
|
|
A1212 |
General Electric; no. AY28X114; 72-78°F |
T874A1010 |
Use T874A1176 for Dept. of |
|
(22-26°C) stop |
|
Defense (DoD) application. |
|
|
|
|
A1220 |
Singer logo; no. 050240-01 |
T874A1036 |
|
|
|
|
|
A1238 |
Celsius scale |
T874A1036 |
|
|
|
|
|
A1246 |
TRADELINE; locking cover and adjustable locking |
T874A1150 |
|
|
lever stops |
|
|
|
|
|
|
A1253 |
Trane Comfort Corp |
T874A1036 |
|
|
|
|
|
A1261 |
TRADELINE; 72-78°F (22-26°C) stop; locking |
T874A1176 |
|
|
cover; no thermometer; meets DoD specs |
|
|
|
|
|
|
a When replacing a T872 Thermostat and Q672 Subbase, always replace both the thermostat and the subbase.
29 |
60-2485—8 |
T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES
Table 6. T872-T874 Thermostat Cross Reference.
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|
|
|
|
|
T874 Trade |
Remarks |
Model Number |
Description |
Replacementa |
|
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 |
T874A1036/ |
Set thermostat stage 1 heat |
|
shunt for auto fan in heating; heat anticipator set at |
Q674B1018 |
anticipator for total of fan and stage |
|
0.4A. |
|
1 primary control current draw. |
|
|
|
|
A1295 |
Lennox; Celsius scale |
T874A1036 |
|
|
|
|
|
A1303 |
Lennox logo; no. 64A0601; meets DoD specs |
T874A1176 |
|
|
|
|
|
A1311 |
Envirotron special; Celsius scale |
T874A1010 |
|
|
|
|
|
A1329 |
Carrier logo; Celsius scale; no. HH07AT074C |
T874A1036 |
|
|
|
|
|
A1337 |
Square D; no. HCD-3; heat anticipator set at 0.2A |
T874A1036 |
|
|
|
|
|
A1345 |
TRADELINE; Celsius scale |
T874A1010 |
|
|
|
|
|
A1352 |
Heat anticipator set 1.2A |
T874A1036 |
|
|
|
|
|
A1360 |
Friedrich logo; heat anticipator set at 0.2A |
T874A1036 |
|
|
|
|
|
A1378 |
Wesco logo; heat anticipator set at 0.8A |
T874A1036 |
|
|
|
|
|
T872B |
1-stage heat, 2-stage cool |
|
|
|
|
|
|
B1005 |
Standard OEM |
T874B1019 |
|
|
|
|
|
B1013 |
0.4A heat anticipator setting |
T874B1019 |
|
|
|
|
|
B1021 |
TRADELINE |
T874B1019 |
|
|
|
|
|
B1039 |
Westinghouse logo |
T874B1019 |
|
|
|
|
|
B1047 |
American Standard logo |
T874B1019 |
|
|
|
|
|
B1054 |
Climatrol logo |
T874B1019 |
|
|
|
|
|
B1062 |
Carrier logo |
T874B1019 |
|
|
|
|
|
B1070 |
Trane logo |
T874B1019 |
|
|
|
|
|
B1088 |
Fedders logo |
T874B1019 |
|
|
|
|
|
B1096 |
Climatrol logo |
T874B1019 |
|
|
|
|
|
B1104 |
Carteret logo |
T874B1019 |
|
|
|
|
|
B1112 |
Singer logo; no. 050240-03 |
T874B1019 |
|
|
|
|
|
B1120 |
Celsius scale |
T874B1019 |
|
|
|
|
|
B1138 |
Singer logo; no. 02005711; locking cover and |
T874B1019 |
|
|
lever. |
|
|
|
|
|
|
B1146 |
Lennox logo; no. P-8-8895 |
T874B1019 |
|
|
|
|
|
B1153 |
Lennox logo; Celsius scale |
T874B1019 |
|
|
|
|
|
T872C |
2-stage heat, 1-stage cool |
|
|
|
|
|
|
C1004 |
Standard OEM |
T874C1018 |
|
|
|
|
|
C1012 |
Heat anticipator set at 0.4A |
T874C1018 |
|
|
|
|
|
C1020 |
72-78°F (22-26°C) stop; locking cover; no |
T874C1141 |
Use T874C1141 for Dept. of |
|
thermometer; 42-88°F (6-31°C) setpoint range. |
|
Defense (DoD) application. |
|
|
|
|
C1038 |
TRADELINE |
T874C1018 |
|
|
|
|
|
C1046 |
American Standard logo |
T874C1018 |
|
|
|
|
|
C1053 |
Friedrich logo |
T874C1018 |
|
|
|
|
|
C1061 |
Lennox logo; no. P-8-8896 |
T874C1018 |
|
|
|
|
|
C1079 |
Westinghouse logo; stage 1 heat anticipator set at |
T874C1018 |
|
|
0.75A; stage 2 anticipator set at 1.2A |
|
|
|
|
|
|
C1087 |
Fast cycle performance on both heat stages; |
T874C1018 |
Adjust anticipator faster for correct |
|
anticipator range 0.12-0.6A |
|
performance. |
|
|
|
|
C1095 |
Payne logo; no. 0.1-0.175 |
T874C1018 |
|
|
|
|
|
a When replacing a T872 Thermostat and Q672 Subbase, always replace both the thermostat and the subbase.
60-2485—8 |
30 |