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APPLICATION
These thermostat/subbase combinations provide low voltage (24 Vac) control in multistage systems as follows:
Number of
Stages
Thermostat/
Subbase
Heat
T874A/Q674Q
1
T874D/Q674F
2
T874R/Q674B
2
T874D/Q674D
2
T874D/Q674B
2
OPERATION
On a two-stage heat thermostat, the two stages of heat
make sequentially as the temperature drops. Make refers
to the mercury switch initiating a call for heat or cool.
There are about 2°F (1°C) between stages so that the
second stage makes only when the first stage cannot
handle the load. This is called the interstage differential .
The LED indicators are light emitting diodes on the
subbase that light up when something specific happens
within the system. When an LED lights up, refer to this list
for the meaning:
AUX. HEAT: Auxiliary heat is operating, which means
the weather is so cold that the heat pump alone
cannot handle the load.
HEAT PUMP: System needs to be checked. See
heating system instructions for specific meaning.
LEDs are not field replaceable.
RECYCLING NOTICE
This control contains mercury in a sealed tube. Do
not place the 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.
®U.S. Registered Trademark
Copyright © 1996 Honeywell Inc. • All Rights Reserved
T874A,D,R Thermostats and
Cool
1
HEAT-OFF-COOL
2
AUX HT-HEAT PUMP-2 STG. HEAT-OFF-COOL
1
HEAT-OFF-COOL
2
2
HEAT-OFF-COOL
Q674B,D,F,Q Subbases
INSTALLATION INSTRUCTIONS
Switching Positions
System
None
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.
INSTALLATION
When Installing this Product...
1. Read these instructions carefully. Failure to follow
them could damage the product or cause a hazard-
ous condition.
2. Check the ratings given in the instructions and 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.
X-XX UL
Wiring
Fig.
Fan
LO-MED-HIGH-ON
AUTO-ON
AUTO-ON
6,7
None
AUTO-ON
69-0107-2
4
5
8
9
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Related Manuals for Honeywell T874A
Summary of Contents for Honeywell T874A
- Page 1 If this control is replacing a control that contains mercury in a sealed tube, do not place your old control in the trash. ®U.S. Registered Trademark 69-0107-2 Copyright © 1996 Honeywell Inc. • All Rights Reserved X-XX UL...
- Page 2 T874A,D,R THERMOSTATS AND Q674B,D,F,Q SUBBASES VERTICAL MOUNTING CAUTION OUTLET SCREWS (2) Disconnect power supply to prevent electrical COVER ADAPTER shock or equipment damage. PLATE RING Run wires as close as possible to the subbase. To prevent interference with the thermostat linkage, keep wire length to a minimum.
- Page 3 O RELAY COMPRESSOR CONTACTOR POWER SUPPLY. PROVIDE DISCONNECT MEANS AND OVERLOAD PROTECTION AS REQUIRED. 3-SPEED FAN SCHEMATIC SINGLE SETPOINT LEVER AND BIMETAL. M8005 Fig. 4. Internal schematic and typical hookup of T874A Thermostat with Q674Q Subbase with automatic changeover. 69-0107—2...
- Page 4 T874A,D,R THERMOSTATS AND Q674B,D,F,Q SUBBASES THERMOSTAT SUBBASE SYSTEM COMPONENTS FAN RELAY AUTO H1 ANTICIPATOR SWITCH (HOT) FALL SYSTEM SWITCH H2 ANTICIPATOR COMPRESSOR AUX. HT. RELAY HEAT PUMP LED (GREEN) HEAT PUMP AUX. HT. RELAY FALL 2 STG. HEAT RISE AUX. HT...
- Page 5 T874A,D,R THERMOSTATS AND Q674B,D,F,Q SUBBASES SUBBASE SYSTEM COMPONENTS THERMOSTAT FAN RELAY FAN SWITCH AUTO H1/C1 ANTICIPATOR FALL (HOT) HEAT CHANGEOVER VALVE SYSTEM SWITCH FALL HEAT AUX. HT. RELAY COOL COMPRESSOR CONTACTOR M5998 POWER SUPPLY. PROVIDE DISCONNECT MEANS AND OVERLOAD PROTECTION AS REQUIRED.
-
Page 6: Mounting The Thermostat
T874A,D,R THERMOSTATS AND Q674B,D,F,Q SUBBASES SUBBASE SYSTEM COMPONENTS THERMOSTAT (HOT) HEAT RELAY 1 SWITCH HEAT RELAY 2 FALL AUTO H1 ANTICIPATOR HEAT CHANGEOVER VALVE FALL H2 ANTICIPATOR SYSTEM SWITCH COOL CHANGEOVER VALVE HEAT RISE FAN RELAY COOL COMPRESSOR CONTACTOR 2... -
Page 7: Temperature Setting
T874A,D,R THERMOSTATS AND Q674B,D,F,Q SUBBASES HOLES FOR INSULATED After mounting the thermostat, set the adjustable LOCKING LEVER SCREWS heat anticipator(s) to match the respective COOL HOLE WITH reading(s) recorded in step . See Fig. 11. LEVER BRASS INSERT HEAT BRACKET... - Page 8 T874A,D,R THERMOSTATS AND Q674B,D,F,Q SUBBASES CALIBRATION Fan switch settings control fan operation as follows: ON: Fan operates continuously. AUTO: Fan operates with cooling equipment as Thermostat controlled by the thermostat or with the heating equipment as controlled by the plenum switch.