Honeywell Q674C Thermostat User Manual

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T874H,Q,W Multistage Thermostats and  
Q674B,C,G,L Subbases  
INSTALLATION INSTRUCTIONS  
When an LED lights, refer to this list for the meaning:  
APPLICATION  
The T874H,Q,W Multistage Thermostats provide 24 to 30  
Vac control for heating and cooling systems as listed in  
Table 1.  
FILTER: clogged filter needs replacement.  
EM. HT.: emergency heat is operating. The compressor  
has failed, and the heat pump is not operating. The  
LED lights when the homeowner places the system  
switch in the EM. HT. position.  
AUX. HT.: auxiliary heat is operating because the  
weather is so cold that the heat pump alone cannot  
handle the load.  
SERVICE or CHECK: system needs to be checked.  
See heating system instructions for specific  
meaning.  
Table 1. Heating and Cooling Stages.  
T874 Model  
a
Stage  
H
Q
1
W
3
Heating Stages  
Cooling Stages  
1
1
2
LEDs are not field addable or replaceable.  
a
Plus a changeover stage that operates with cooling.  
The Q674B,C,G,L Subbases provide wiring terminals,  
system and fan switching, and mounting bases for T874  
Multistage Thermostats as listed in Table 2.  
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.  
Table 2. Switching Positions.  
Q674  
Model  
Switch Positions  
System  
If this control is replacing a control that contains  
mercury in a sealed tube, do not place your old  
control in the trash.  
Fan  
B
C
G
L
Heat-Off-Cool  
Off-Auto  
Auto-On  
Auto-On  
None  
Contact your local waste management authority for  
instructions regarding the recycling and the proper  
disposal of this control, or of an old control  
containing mercury in a sealed tube.  
Off-Auto  
Em Ht-Heat-Off-Cool  
Auto-On  
OPERATION  
On a two-stage or three-stage heat thermostat, the stages  
of heat make sequentially as the temperature drops. Make  
refers to the mercury switch initiating a call for heat or cool.  
INSTALLATION  
When Installing this Product…  
Read these instructions carefully. Failure to follow  
them could damage the product or cause a hazard-  
ous condition.  
There are about 2°F (1°C) between the stages so that the  
second stage makes only when the first stage can not  
handle the load. About one degree later, the third stage  
makes (T874W only). This degree difference is called the  
interstage differential.  
Check the ratings given in the instructions and on  
the product to make sure the product is suitable for  
your application.  
Installer must be a trained, experienced service  
technician.  
After installation is complete, check out product  
operation as provided in these instructions.  
Select models include LEDs (light-emitting diodes). The  
LED lights to indicate a problem with the system; refer to  
the system manufacturer instructions.  
Copyright © 1996 Honeywell Inc. •  
All Rights Reserved  
69-0087-6  
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T874H,Q,W MULTISTAGE THERMOSTATS AND Q674B,C,G,L SUBBASES  
VERTICAL  
MOUNTING  
SCREWS (2)  
OUTLET  
BOX  
CAUTION  
1
1
.
.
Disconnect the power supply to prevent  
electrical shock or equipment damage.  
Run the wires as close as possible to the  
subbase. To prevent interference with the  
thermostat linkage, keep the wire length to a  
minimum. Push the excess wire back into the  
hole, and plug the hole to prevent drafts from  
affecting the thermostat operation.  
COVER  
PLATE  
ADAPTER  
2
RING  
2
2
1
3.  
Do not tighten the thermostat captive mount-  
ing screws to the point of damaging the  
subbase threads.  
HORIZONTAL  
OUTLET  
BOX  
4
.
.
Do not short across the coil terminals on the  
relay. This can burn out the heat anticipator.  
Never install more than one wire per terminal  
unless a factory-supplied jumper with a spade  
terminal is used.  
5
THERMOSTAT  
IMPORTANT  
These thermostats are calibrated at the factory  
COOL  
THERMOSTAT  
COVER  
8
0
7
0
6
0
5
0
by using subbases mounted at true level.  
Inaccurate subbase leveling causes thermostat  
control deviation.  
SUBBASE  
80  
7
0
6
0
EAT  
50  
H
SUBBASE  
MOUNTING SCREWS (2)  
7
0
8
0
6
0
Location  
50  
CAPTIVE  
MOUNTING SCREWS (2)  
Install the thermostat about 5 ft (1.5m) above the floor in  
an area with good air circulation at average temperature.  
1
2
NOT INCLUDED WITH UNIT.  
Do not install the thermostat where it can be affected by:  
ACCESSORY PARTS AVAILABLE (193121A).  
M6009  
drafts, or dead spots behind doors and in corners.  
hot or cold air from ducts.  
radiant heat from sun or appliances.  
concealed pipes and chimneys.  
unheated (uncooled) areas behind the thermostat  
such as an outside wall.  
Fig. 1. Installation on vertical or horizontal outlet box.  
TOP  
WIRING  
THERMOSTAT  
MOUNTING  
HOLES (2)  
TERMINAL  
CABLE OPENING  
SPIRIT LEVEL  
Mounting and Wiring the Subbase  
Mount the subbase on a wall or horizontal outlet box. If  
you mount it on a vertical outlet box, order Honeywell part  
no. 193121A Adapter Assembly, which includes an  
adapter ring with two screws, and a cover plate to cover  
the marks on the wall.  
All wiring must comply with local electrical codes and  
ordinances. Follow the equipment manufacturer wiring  
instructions, when available.  
To install the subbase:  
MOUNTING  
HOLES (2)  
POST (2) FOR  
MOUNTING  
THERMOSTAT  
TO SPRING FINGER  
CONTACTS ON THE  
THERMOSTAT  
Prepare a hole for the thermostat wires at the  
chosen location. Run wires to the location.  
Pull about 6 in. (152 mm) of wire through the hole.  
(UP TO 12)  
M927  
Fig. 2. Leveling subbase.  
IMPORTANT  
Use 18 gauge, color-coded thermostat cable for  
Connect the system wires to the subbase. See Fig.  
through 9. A letter code for identification is  
proper wiring.  
4
located near each terminal. The terminal barrier  
permits straight or conventional wraparound wiring  
connections. See Fig. 3. Either method is accept-  
able. Run the wires as close as possible to the  
subbase, keeping the wire length to a minimum.  
Push the excess wire back into the hole. Plug the  
hole to prevent drafts.  
When mounting the subbase on a vertical outlet box,  
install an adapter ring with the two screws provided  
with the adapter assembly. See Fig. 1.  
Pull the wires through the cover plate (if used) and  
the subbase wire opening. Secure the cover plate  
and subbase with the two screws provided, but do  
not fully tighten.  
The subbase mounting slots provide for minor out-  
of-level adjustments. Level the subbase by using a  
spirit level and tightening the subbase mounting  
screws. See Fig. 2.  
IMPORTANT  
An incorrectly leveled subbase causes the  
temperature control to deviate from the setpoint.  
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T874H,Q,W MULTISTAGE THERMOSTATS AND Q674B,C,G,L SUBBASES  
FOR STRAIGHT  
Mounting C815A Thermistor (T874W only)  
The T874W is used with the C815A Thermistor mounted in  
the outdoor section. Locate it in an area where it can  
measure the true outdoor ambient temperature. Avoid  
mounting the C815A in an area where it is exposed to high  
temperatures or direct sunlight. Refer to heat pump  
equipment instructions for proper mounting.  
INSERTION–  
STRIP 5/16 in. (8 mm)  
FOR WRAPAROUND–  
STRIP 7/16 in. (11 mm)  
BARRIER  
SUBBASE TERMINAL SCREW  
M928  
Fig. 3. Wiring connections.  
THERMOSTAT  
H1  
SUBBASE  
SYSTEM COMPONENTS  
2
3
4
Y1  
ANTICIPATOR  
COOL CHANGEOVER  
VALVE  
FALL  
H1  
FAN SWITCH  
AUTO  
FAN RELAY  
G
R
ON  
Y2  
SYSTEM  
SWITCH  
8
9
COMPRESSOR  
CONTACTOR  
OFF  
RISE  
B
AUTO  
C1  
ANTICIPATOR  
CO  
1
10  
O
X
L1  
L2  
HOT)  
(
2
RISE  
C1  
SERVICE LED  
YELLOW)  
COMPRESSOR  
FAULT  
(
1
2
POWER SUPPLY. PROVIDE DISCONNECT MEANS  
AND OVERLOAD PROTECTION AS REQUIRED.  
C
SERVICE LED ON SELECT MODELS.  
M3680A  
Fig. 4. T874H/Q674C in heating-cooling system.  
THERMOSTAT  
SUBBASE  
SYSTEM COMPONENTS  
2
Y1  
H1 ANTICIPATOR  
3
COOL CHANGEOVER  
VALVE  
H1  
FALL  
FAN SWITCH  
1
AUTO  
ON  
G
R
L2  
L1  
FAN RELAY  
4
(HOT)  
COMPRESSOR  
CONTACTOR  
Y2  
B
RISE  
CO  
8
O
9
SYSTEM  
SWITCH  
X
SERVICE  
LED (YELLOW)  
C1 ANTICIPATOR  
OFF  
10  
AUTO  
X1  
C1  
RISE  
C
24 VAC  
FILTER LED (RED)  
X2  
1
POWER SUPPLY. PROVIDE DISCONNECT MEANS  
AND OVERLOAD PROTECTION AS REQUIRED.  
M5902  
Fig. 5. T874H/Q674C in heating-cooling system with SERVICE LED and FILTER LED.  
3
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T874H,Q,W MULTISTAGE THERMOSTATS AND Q674B,C,G,L SUBBASES  
THERMOSTAT  
SUBBASE  
SYSTEM COMPONENTS  
H1 ANTICIPATOR  
1
W1  
NO FAN  
SWITCH  
3
CHANGEOVER  
RELAY  
FALL  
B
H1  
4
6
W2  
H2 ANTICIPATOR  
FAN RELAY  
G
FALL  
H2  
SYSTEM  
SWITCH  
OFF  
Y1  
8
9
COMPRESSOR  
CONTACTOR  
1
L2  
AUTO  
C1  
R
L1  
(
HOT)  
C1 ANTICIPATOR  
1
POWER SUPPLY. PROVIDE DISCONNECT MEANS AND  
OVERLOAD PROTECTION AS REQUIRED.  
RISE  
10  
M6008A  
Fig. 6. T874Q/Q674G for heating only system.  
THERMOSTAT  
SUBBASE  
SYSTEM COMPONENTS  
1
1
L1 (HOT)  
H1 ANTICIPATOR  
H2 ANTICIPATOR  
R
FALL  
H1  
C815A THERMISTOR  
L2  
2
T
FAN SWITCH  
AUTO  
FALL  
3
4
H3  
W1  
W2  
ON  
HEAT RELAY 1  
HEAT RELAY 3  
H3 ANTICIPATOR  
HEAT RELAY 2  
5
6
W3  
B
FALL  
H2  
SYSTEM  
SWITCH  
RESET HEATER  
HEAT CHANGEOVER VALVE  
FAN RELAY  
HEAT  
OFF  
RISE  
8
G
O
C1  
C1 ANTICIPATOR  
C2 ANTICIPATOR  
COOL  
10  
COOL CHANGEOVER VALVE  
COMPRESSOR CONTACTOR 2  
RISE  
11  
Y2  
Y1  
C2  
1
POWER SUPPLY. PROVIDE DISCONNECT  
MEANS AND OVERLOAD PROTECTION  
AS REQUIRED.  
COMPRESSOR CONTACTOR 1  
M3674  
Fig. 7. T874W/Q674B with outdoor reset in heating and cooling system.  
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T874H,Q,W MULTISTAGE THERMOSTATS AND Q674B,C,G,L SUBBASES  
THERMOSTAT  
FALL  
SUBBASE  
SYSTEM COMPONENTS  
1
L1  
HOT)  
(
1
L2  
H1 ANTICIPATOR  
2
T
W3  
U
H1  
C815A OD THERMISTOR  
H2  
H2 ANTICIPATOR  
3
AUX. HEAT RELAY  
FALL  
FALL  
FAN SWITCH  
AUXILIARY  
HEAT LED  
(GREEN)  
H3 ANTICIPATOR  
4
AUTO  
ON  
G
FAN RELAY  
H3  
EMERGENCY  
HEAT LED  
R
(
RED)  
5
E
L
6
RESET  
COMPRESSOR  
MONITOR  
EM. HEAT RELAY  
CHECK LED  
RED)  
(
RISE  
X
B
8
SYSTEM  
SWITCH  
C1  
EM.HEAT  
HEAT  
CHANGEOVER  
RELAY (HEAT)  
9
2
CHANGEOVER  
RELAY (COOL)  
C1 ANTICIPATOR  
OFF  
1
0
1
O
COMPRESSOR  
CONTACTOR 1  
COOL  
C2 ANTICIPATOR  
RISE  
Y1  
1
COMPRESSOR  
CONTACTOR 2  
C2  
12  
Y2  
1
2
POWER SUPPLY. PROVIDE DISCONNECT MEANS AND OVERLOAD PROTECTION AS REQUIRED.  
CONNECT CHANGEOVER RELAY TO 'O' TERMINAL FOR CHANGEOVER IN COOLING OR TO 'B' TERMINAL FOR CHANGEOVER IN HEATING.  
M156A  
Fig. 8. T874W/Q674L for three-stage heat and two-stage cool with outdoor reset and compressor monitor.  
5
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T874H,Q,W MULTISTAGE THERMOSTATS AND Q674B,C,G,L SUBBASES  
THERMOSTAT  
FALL  
SUBBASE  
SYSTEM COMPONENTS  
1
H1 ANTICIPATOR  
2
T
H1  
C815A OUTDOOR  
THERMISTOR  
H2  
H2 ANTICIPATOR  
3
W3  
FALL  
FALL  
AUX. HT. RELAY  
FAN SWITCH  
AUTO  
AUX. HEAT  
LED (GREEN)  
H3 ANTICIPATOR  
4
G
R
ON  
FAN RELAY  
H3  
L1  
(HOT)  
5
L2  
1
EM. HEAT  
LED (RED)  
E
6
EM. HT. RELAY  
RESET  
X1  
CHECK LED  
(
YELLOW)  
C
B
RISE  
CHANGEOVER  
RELAY (HEAT)  
2
8
SYSTEM  
SWITCH  
C1  
CHANGEOVER  
RELAY (COOL)  
EM.HT.  
HEAT  
9
O
C1 ANTICIPATOR  
OFF  
1
0
1
X2  
Y1  
COMPRESSOR  
CONTACTOR 1  
COOL  
C2 ANTICIPATOR  
RISE  
1
COMPRESSOR  
CONTACTOR 2  
C2  
12  
Y2  
1
2
POWER SUPPLY. PROVIDE DISCONNECT MEANS AND OVERLOAD PROTECTION AS REQUIRED.  
CONNECT CHANGEOVER RELAY TO 'O' TERMINAL FOR CHANGEOVER IN COOLING OR TO 'B' TERMINAL FOR CHANGEOVER IN HEATING.  
M6999  
Fig. 9. T874W/Q674L for three-stage heat and two-stage cool with outdoor reset.  
T874Q only: Set the heat anticipator indicators to  
Mounting the Thermostat  
the respective current setting for each stage. See  
Setting the Heat Anticipator section. T874H and  
T874W have fixed anticipation; the settings are  
preselected and cannot be adjusted.  
Remove the thermostat cover by pulling the bottom  
edge of the cover upward until it snaps free of the  
cover clip.  
Note the tabs along the top inside edge of the  
thermostat base. The tabs fit into the subbase  
notches. Mount the thermostat base on the subbase  
and 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.  
NOTE: The cover is hinged at the top and must be  
removed by pulling up at the bottom.  
Carefully remove and discard the polystyrene  
packing insert that protects the mercury switches  
during shipment.  
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.  
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T874H,Q,W MULTISTAGE THERMOSTATS AND Q674B,C,G,L SUBBASES  
4600  
SETTING  
4400  
4200  
4000  
3800  
3600  
C815A THERMISTOR RESISTANCE  
R = 400 ohms ± 10% AT 77°F (25°C)  
Setting the Heat Anticipator (T874Q Only)  
Set each heat anticipator to match its primary control  
current draw. If the primary control nameplate has no  
rating or if further adjustment is necessary, use the  
following procedure to determine the current draw of each  
stage. (Measure the current draw of each heating stage  
with the thermostat removed and the power on.)  
3400  
3
3
200  
000  
2800  
2
2
600  
400  
2200  
2
1
000  
800  
Connect an ac ammeter of appropriate range  
between the heating terminals of the subbase as  
follows:  
1600  
1400  
1200  
1000  
800  
Stage 1: between W1 and RH or R.  
Stage 2: between W2 and R.  
Move the system switch to AUTO (if applicable).  
After one minute, read the ammeter and record the  
reading.  
6
4
00  
00  
200  
0
-20  
0
20  
40  
60  
80  
100  
120 140  
M1590A  
TEMPERATURE OF THERMISTOR (°F)  
After mounting the thermostat, set the adjustable  
heat anticipator(s) to match the respective  
reading(s) measured in step .  
Fig. 10 Thermistor resistance chart.  
The proper thermistor operation must be verified to assure  
the correct operation of the thermostat. Check the  
thermistor operation as follows:  
Temperature Setting  
Move the heating and cooling levers to the desired  
positions. The minimum differential between the heating  
and cooling setpoints is 3°F (1.7°C), which means the  
setting levers cannot be set closer together than 3°F  
Disconnect the T wire on the subbase.  
Use an ohmmeter to measure resistance between  
the T wire and the transformer common.  
Take the outdoor temperature at thermistor location  
and find the correct thermistor resistance on the  
chart in Fig. 10.  
(1.7°C) (5° F [2.8°C] for T874W).  
Subbase Setting  
If the resistance measured in step  and the  
calculated resistance in step vary by more than  
15 percent, the thermistor requires replacement.  
Contact the installing dealer for the packaged  
replacement outdoor thermistor.  
System switching positions control thermostat operation as  
follows:  
OFF: Both the heating and cooling systems are off. If  
the fan switch is in the AUTO position, the cooling  
fan is also off.  
AUTO: Thermostat automatically changes between the  
heat and cool modes, depending on the indoor  
temperature.  
HEAT: Heating system is controlled by the thermostat.  
Cooling system is off.  
EM. HT.: Emergency heat relay is energized. The  
cooling system is off. When the compressor has  
failed and the heat pump is not operating, switch to  
the EM. HT. position.  
CHECKOUT  
Heating  
Move the system switch on the Q674 Subbase to HEAT or  
AUTO. Move the heating lever on the T874 about 10°F  
(6°C) above room temperature. Both stages of heating  
start and the fan runs after a short delay. Move the heating  
lever about 10°F (6°C) below room temperature. The  
heating equipment shuts off, and the fan runs for a short  
time, and then shuts off.  
Fan switching positions control fan operation as follows  
Q674B,C, or L):  
ON: Fan operates continuously.  
(
AUTO: Fan operates with cooling equipment as  
controlled by the thermostat or with the heating  
equipment as controlled by the plenum switch.  
Cooling  
Move the system switch on the Q674 Subbase to COOL  
or AUTO. Move the cooling lever on the T874 Thermostat  
about 10°F (6°C) below room temperature. The cooling  
equipment and fan start. Move the cooling lever about  
To switch positions, use thumb and index finger to slide  
the lever to the desired position. The switch lever must  
stop in detent over the desired function indicator mark for  
proper circuit operation.  
10°F (6°C) above room temperature. The cooling equip-  
ment and fan stop.  
Emergency Heat  
C815 Outdoor Reset Thermistor (T874W only)  
The T874W is used with a C815 Thermistor mounted in  
the outdoor section. The thermistor must be used; if not,  
thermostat performance will deviate radically from proper  
operation.  
Move the system switch to EM. HT. The EM. HT. LED  
comes on. Move the heating lever about 10°F (6°C) above  
room temperature. The electric strip heater(s) comes on.  
Move the heating lever about 10°F (6°C) below room  
temperature. The electric strip heater de-energizes. The  
EM. HT. LED remains on until the system switch is moved  
to another postion.  
7
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T874H,Q,W MULTISTAGE THERMOSTATS AND Q674B,C,G,L SUBBASES  
Fan (Select Models)  
NOTE: Hand heat offsets the thermometer  
reading. After making each adjustment,  
wait 10 or 15 minutes for the thermometer  
to stabilize before comparing.  
Move the subbase system switch to OFF, and the fan  
switch to ON. The fan runs continuously. When the fan  
switch is in the AUTO position, the fan operation is  
controlled by the heating or cooling system.  
CALIBRATION  
Thermostat  
T874 Thermostats are accurately calibrated at the factory.  
They do not have provision for field calibration.  
Thermometer  
Remove the thermostat cover by pulling up from the  
bottom of the cover until it clears the locking springs.  
Set the cover on a table near an accurate thermom-  
eter.  
After allowing 10 or 15 minutes for stabilization,  
compare the readings. If they are the same, replace  
the cover and put the system into operation. If they  
are different, recalibrate the thermostat thermom-  
eter; see step ꢃ  
M5070  
Insert a small screwdriver into the thermometer shaft  
and turn it until the thermometers read the same.  
See Fig. 11. When the thermometer is calibrated,  
replace the cover and place the system into  
operation.  
Fig. 11. Thermometer calibration.  
Automation and Control Solutions  
Honeywell Internationl, Inc.  
Honeywell Limited-Honeywell Limitée  
35 Dynamic Drive  
1
985 Douglas Drive North  
Golden Valley, MN 55422  
Scarborough, Ontario  
M1V 4Z9  
Printed in U.S.A. on recycled  
paper containing at least 10%  
post-consumer paper fibers.  
69-00876 C.H. Rev. 9-96  
www.honeywell.com  

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