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T874A,D,R THERMOSTATS AND Q674B,D,F,Q SUBBASES
THERMOSTAT
H1
FALL
H1 ANTICIPATOR
H2
FALL
H2 ANTICIPATOR
C1
RISE
C1 ANTICIPATOR
C2 ANTICIPATOR
C2
RISE
1
2
Fig. 9. Internal schematic and typical hookup of T874D Thermostat with Q674B Subbase in heat pump system
FOR STRAIGHT
INSERTION–
STRIP 5/16 IN. (8 MM)
SUBBASE TERMINAL SCREW
Fig. 10. Wiring connections.
Mounting the Thermostat
Remove the thermostat cover by pulling the bottom
edge of the cover upward until it snaps free from the
cover clip.
NOTE: The cover is hinged at the top and can be
removed by pulling up at the bottom.
Carefully remove and discard the polystyrene
packing insert that protects the mercury switches
during shipment.
NOTE: If the thermostat is being used with a Q674
Subbase with LEDs and the thermostat
setpoint scale is not labeled (such as EM.
HT., CHECK), install the desired LED label
insert (packed with the subbase):
- Push both thermostat setpoint levers to
the far ends of the thermostat.
- Gently pull the setpoint scale out about
1/4 in. (6 mm).
- Insert the desired LED label.
- Reposition the setpoint levers.
69-0107—2
1
4
6
8
9
10
11
POWER SUPPLY. PROVIDE DISCONNECT MEANS AND OVERLOAD
PROTECTION AS REQUIRED.
JUMPER RH, RC FOR SINGLE TRANSFORMER.
with changeover in heating and cooling.
FOR WRAPAROUND–
STRIP 7/16 IN. (11 MM)
BARRIER
M928
SUBBASE
FAN
SWITCH
AUTO
ON
SYSTEM
SWITCH
HEAT
OFF
COOL
3
OPTIONAL LED ON SOME MODELS.
Turn over the thermostat base and note the spring
fingers that engage the subbase contacts. Make
sure the spring fingers are not bent flat, which
prevents proper electrical contact with the subbase.
Set the heat anticipator indicators at the respective
current setting for each stage. See Setting the Heat
Anticipator section.
Mount the thermostat base on the subbase noting
the tabs along the top inside edge of the thermostat
base. The tabs fit into the subbase notches.
Tighten the captive mounting screws. See Fig. 1.
Place the upper edge of the thermostat cover on the
thermostat base and swing the cover downward until
it engages with the cover clip on the base.
SETTINGS
Setting the Heat Anticipator
Set the heat anticipator to match the current draw of the
primary control. If the primary control nameplate is not
labeled with a rating, or if further adjustment is necessary,
use the following procedure to determine the current draw
for each stage. Measure the current draw of each heating
stage with the thermostat removed from the subbase and
the power on:
Connect an ac ammeter of appropriate range
between the heating terminals of the subbase:
Stage 1: between W1 and R.
Stage 2: between W2 and R.
Move the system switch to HEAT or AUTO.
After one minute, read the ammeter and record the
reading.
6
SYSTEM COMPONENTS
RH
HEAT RELAY 1
3
W1
HEAT RELAY 2
W2
HEAT CHANGEOVER
VALVE
B
X1
O
COOL CHANGEOVER
VALVE
G
FAN RELAY
2
Y2
COMPRESSOR
CONTACTOR 2
Y1
COMPRESSOR
CONTACTOR 1
RC
L1
(HOT)
L2
1
L2
L1
(HOT)
1
M933C
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