Page 1
T874A-F Multistage Thermostats
M3375
with Restricted Set Point
Q674A-E,G Subbases
Installation Instructions for the Trained Service Technician.
Application
The T874A-F Thermostats provide 24 to 30 Vac control
for heating and/or cooling systems. See Tables 1 and 2.
TABLE 1—HEATING AND/OR COOLING STAGES.
T874 A B C D E F
Heating Stages 1122— 2
Cooling Stages 1212 2 —
The Q674A-E,G Subbases provide wiring terminals,
system and fan switching, and mounting bases for T874
Multistage Thermostats.
TABLE 2—SWITCHING POSITIONS.
Q674 Switch Positions
Model System Fan
A Heat-Auto-Cool Auto-On
B Heat-Off-Cool Auto-On
C Off-Auto Auto-On
D None None
E Off-Heat-Auto-Cool Auto-On
G Off-Auto None
Operation
On a 2-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 can not
handle the load. This is the interstage differential.
The LED indicators are light emitting diodes on the
subbase that light when something specific happens within the system.
Up to four of the LEDs listed below may be included on
your subbase. When an LED lights, refer to the following
list for the meaning:
FILTER: Clogged filter needs replacement.
EM. HT.: Emergency heat is operating. The compres-
sor has failed, and the heat pump is not operating.
LED lights when system switch is placed in the EM.
HT. position.
AUX. HT.: Auxiliary heat is operating, which means the
weather is so cold that the heat pump can not handle
the load alone.
LOCKOUT: System is shut down and needs maintenance.
CHECK: System needs to be checked. See heating sys-
tem instructions for specific meaning.
SERVICE: 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 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.
Installation
WHEN INSTALLING THIS PRODUCT…
1. Read these instructions carefully. Failure to fol-
low them could damage the product or cause a hazardous
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 the product
operation as provided in these instructions.
1 60-1227—1S. M. • Rev. 6-93 • • ©Honeywell Inc. 1993 • Form Number 60-1227—1
M3375
Page 2
!
CAUTION
1. Disconnect the power supply to prevent electrical shock and equipment damage.
2. Run wires as close as possible to the subbase.
To prevent interference with the thermostat
linkage, keep wire length to a minimum. Push
excess wire back into the hole, and plug the
hole to prevent drafts from affecting thermostat operation.
3. Do not tighten the thermostat captive mounting screws to the point of damaging the subbase threads.
4. Do not short across coil terminals on the relay.
This may burn out the heat anticipator.
IMPORTANT: Thermostats are calibrated at the factory by
using subbases mounted at true level. Inaccurate subbase leveling will cause thermostat control deviation.
LOCATION
Install the thermostat about 5 ft [1.5 m] above the floor
in an area with good air circulation at average temperature.
Do not install thermostat where it can be affected by:
—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, such as an outside wall,
behind the thermostat.
MOUNT THE SUBBASE
The thermostat subbase can be mounted on a vertical
outlet box, horizontal outlet box or directly on the wall.
1. To mount on a vertical outlet box, order Honeywell
193121A Adapter Assembly. See Fig. 1. 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. The horizontal outlet box requires
placing the subbase on the box and tightening two mounting screws.
For a wall installation, hold the subbase in position and
mark the holes for the anchors. See Fig. 2. Obtain wall
anchors locally. Be careful that the wires do not fall back
into the wall opening. Set subbase aside. Drill two 3/16 in.
[5 mm] holes and gently tap anchors into the holes until
flush with the wall.
2. Pull electrical wires through the cover plate (if used)
and subbase cable opening. See Fig. 3. See Wire the Subbase section before pulling any wires.
IMPORTANT: Use 18 gauge, color-coded thermostat
cable for proper wiring.
3. Secure the cover plate (if used) and subbase with the
screws provided. Do not fully tighten the subbase screws.
4. Level the subbase using a spirit level, see Fig. 3, and
firmly tighten the subbase mounting screws. The subbase
mounting holes allow for minor out-of-level adjustments.
IMPORTANT: An incorrectly leveled subbase will cause
the temperature control to deviate from set point.
Fig. 1—Installation of subbase on outlet box.
VERTICAL
OUTLET
1
BOX
ADAPTER
RING
MOUNTING
SCREWS (2)
SUBBASE
HORIZONTAL
OUTLET
1
BOX
NOT INCLUDED WITH UNIT.
1
ACCESSORY PARTS AVAILABLE (193121A).
2
SUBBASE
2
COVER
PLATE
MOUNTING
SCREWS (2)
MOUNTING
SCREWS (2)
2
M925
Fig. 2—Installation of subbase on wall.
WALL
WIRES THROUGH
WALL OPENING
WALL
ANCHORS
(2)
M926
Fig. 3—Subbase components and leveling
procedure.
SPIRIT LEVEL
MOUNTING
HOLES (2)
TOP
MOUNTING
HOLES (2)
SUBBASE
WIRING
TERMINAL
POST (2) FOR
MOUNTING
THERMOSTAT
MOUNTING
HOLES
MOUNTING
SCREWS (2)
THERMOSTAT
CABLE OPENING
TO SPRING FINGER
CONTACTS ON THE
THERMOSTAT
(UP TO 12)
M927
2
Page 3
WIRE THE SUBBASE
Disconnect power supply before beginning installation
to prevent electrical shock or equipment damage. All wiring
must comply with local electrical codes and ordinances.
Follow equipment manufacturer’s wiring instructions when
available. To wire the subbase, proceed as follows:
1. Connect the system wires to the subbase. See Figs. 5
through 11. A letter code is located near each terminal for
identification. The terminal barrier permits straight or conventional wraparound wiring connections. See Fig. 4.
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 the hole with nonflammable insulation to prevent drafts from affecting the thermostat.
Fig. 5—T874D/Q674A in standard heating/cooling system.
Fig. 4—Wiring connections.
FOR STRAIGHT
INSERTION–
STRIP 5/16 in. [8 mm]
SUBBASE TERMINAL SCREW
BARRIER
FOR WRAPAROUND–
STRIP 7/16 in. [11 mm]
M928
FALL
H1
H1 ANTICIPATOR
H2 ANTICIPATOR
FALL
H2
RISE
C1
C1
ANTICIPATOR
C2 ANTICIPATOR
RISE
C2
POWER SUPPLY. PROVIDE DISCONNECT MEANS AND OVERLOAD PROTECTION AS REQUIRED.
1
JUMPER RC, RH FOR SINGLE TRANSFORMER.
2
OPTIONAL CHECK-LITE ON SOME MODELS.
3
FAN
SWITCH
AUTO
ON
SYSTEM
SWITCH
HEAT
AUTO
COOL
CHECK
3
RH
HEAT RELAY 1
W1
W2
X1
2
G
Y2
Y1
RC
HEAT RELAY 2
FAN RELAY
COMPRESSOR
CONTACTOR 2
COMPRESSOR
CONTACTOR 1
L1
(HOT)
L2
L1
(HOT)
1
L2
1
M932A
3 60-1227—1
Page 4
Fig. 6—T874D/Q674B in standard heating/cooling system.
FALL
H1
H1 ANTICIPATOR
H2 ANTICIPATOR
FALL
H2
RISE
C1
C1
ANTICIPATOR
C2 ANTICIPATOR
RISE
C2
1
POWER SUPPLY. PROVIDE DISCONNECT MEANS AND OVERLOAD PROTECTION AS REQUIRED.
2
JUMPER RC AND RH FOR SINGLE TRANSFORMER SYSTEMS.
OPTIONAL LIGHT ON SOME MODELS.
3
FAN
SWITCH
AUTO
ON
SYSTEM
SWITCH
HEAT
OFF
COOL
Fig. 7—T874D/Q674C in standard heating/cooling system.
FALL
H1
H1 ANTICIPATOR
H2 ANTICIPATOR
FALL
H2
RISE
C1
C1
ANTICIPATOR
C2 ANTICIPATOR
RISE
C2
POWER SUPPLY. PROVIDE DISCONNECT MEANS AND OVERLOAD PROTECTION AS REQUIRED.
1
JUMPER TERMINALS RC-RH FOR SINGLE SYSTEM TRANSFORMER ONLY.
2
B,O TERMINALS ON SOME MODELS.
3
FAN
SWITCH
AUTO
ON
SYSTEM
SWITCH
OFF
AUTO
3
RH
HEAT RELAY 1
W1
HEAT RELAY 2
W2
HEAT DAMPER OR
CHANGEOVER RELAY
B
2
X1
O
COOL DAMPER OR
CHANGEOVER RELAY
G
Y2
COMPRESSOR
CONTACTOR 2
Y1
COMPRESSOR
CONTACTOR 1
RC
RH
2
HEAT RELAY 1
W1
HEAT RELAY 2
W2
HEAT DAMPER
B
3
O
COOL DAMPER
G
Y2
COMPRESSOR
CONTACTOR 2
Y1
COMPRESSOR
CONTACTOR 1
2
RC
FAN RELAY
FAN RELAY
L1 (HOT)
L2
L1
(HOT)
L2
M933A
L1
(HOT)
L2
L2
L1
(HOT)
1
1
1
1
M5122
4
Page 5
Fig. 8—T874D/Q674D in standard heating/cooling system.
FALL
H1
H1 ANTICIPATOR
H2 ANTICIPATOR
H2
FALL
C1
RISE
C1 ANTICIPATOR
C2 ANTICIPATOR
C2
RISE
POWER SUPPLY. PROVIDE DISCONNECT MEANS AND OVERLOAD PROTECTION AS REQUIRED.
1
2
JUMPER RC, RH FOR SINGLE TRANSFORMER.
3
B TERMINAL OPTIONAL.
NO
FAN
SWITCH
NO
SYSTEM
SWITCH
Fig. 9—T874D/Q674E in standard heating/cooling system.
FALL
H1
H1 ANTICIPATOR
H2 ANTICIPATOR
FALL
H2
RISE
C1
C1
ANTICIPATOR
C2 ANTICIPATOR
RISE
C2
12POWER SUPPLY. PROVIDE DISCONNECT MEANS AND OVERLOAD PROTECTION AS REQUIRED.
JUMPER RC, RH FOR SINGLE TRANSFORMER.
FAN
SWITCH
AUTO
ON
SYSTEM
SWITCH
OFF
HEAT
AUTO
COOL
3
RH
B
HEAT RELAY 1
W1
HEAT RELAY 2
W2
2
Y2
COMPRESSOR
CONTACTOR 2
Y1
COMPRESSOR
CONTACTOR 1
RC
2
RH
HEAT RELAY 1
W1
HEAT RELAY 2
W2
O
B
G
FAN RELAY
Y2
COMPRESSOR
CONTACTOR 2
Y1
COMPRESSOR
CONTACTOR 1
RC
L1
(HOT)
L2
1
L2
L1
(HOT)
M3667
L1
(HOT)
1
L2
L1
(HOT)
1
L2
M934
5 60-1227—1
Page 6
Fig. 10—T874D/Q674G in standard heating/cooling system.
FALL
H1
H1 ANTICIPATOR
H2
ANTICIPATOR
FALL
H2
RISE
C1
C1
ANTICIPATOR
C2
ANTICIPATOR
RISE
1
C2
2
RH
NO FAN
SWITCH
W1
HEAT RELAY 1
2
W2
HEAT RELAY 2
B
SYSTEM SWITCH
OFF
AUTO
O
Y2
COMPRESSOR
CONTACTOR 2
Y1
COMPRESSOR
CONTACTOR 1
RC
POWER SUPPLY. PROVIDE DISCONNECT MEANS AND OVERLOAD PROTECTION AS REQUIRED.
JUMPER RC, RH FOR SINGLE TRANSFORMER.
L1
(HOT)
L2
1
1
L2
L1
(HOT)
M3726
Fig. 11—Typical Q674 terminal designations. The thermostat used determines the number of connections.
STAGE 1
HEATING
RELAY
STAGE 2
HEATING
RELAY
STAGE 3
HEATING
RELAY
CLOGGED
FILTER
SWITCH
SYSTEM
MONITOR
OUTDOOR
THERMISTOR
HEATING DAMPER
MOTOR (OR
CHANGEOVER
VALVE)
EMERGENCY
HEAT
RELAY
RC
OW3
1
RH
W2
E
L
Y1 Y2
W1
X
B
X1
G
T
Q674
SUBBASE
COOLING DAMPER
MOTOR (OR
CHANGEOVER
VALVE)
STAGE 1
COOLING
RELAY
STAGE 2
COOLING
RELAY
COOLING
FAN RELAY
COIL
M954
LED
1 ADD JUMPER (RH-RC) FOR HOOKUP USING COMMON TRANSFORMER.
2
POWER
SUPPLY
2 PROVIDE DISCONNECT MEANS AND OVERLOAD PROTECTION AS REQUIRED.
POWER
SUPPLY
2
6
Page 7
MOUNTING THE THERMOSTAT
1. Remove the thermostat cover by pulling the bottom
edge of the cover upward until it snaps free of the cover clip.
NOTE: The cover is hinged at the top. Remove it by pull-
ing up at the bottom.
2. Carefully remove and discard the polystyrene packing
insert, which protects the mercury switches during shipment.
3. Turn the thermostat base over 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.
4. Set the heat anticipator indicators to the respective
current setting for each stage. See Setting the Heat Anticipator section.
5. 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.
6. Place the upper edge of thermostat cover on the
thermostat base and swing cover downward until it engages
with the cover clip on the base.
Setting
SETTING THE HEAT ANTICIPATOR
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. The
current draw of each heating stage must be measured with
the thermostat removed and the power on.
1. Connect an ac ammeter of appropriate range between
the heating terminals of the subbase:
Stage 1: between W1 and RH or R.
Stage 2: between W2 and RH or R.
2. Move the system switch to HEAT or AUTO.
3. After one minute, read the ammeter and record the
reading.
4. After mounting the thermostat, set the adjustable heat
anticipator(s) to match the respective reading(s) measured
in step 3.
TEMPERATURE SETTING
Move the heating and cooling levers to the desired
positions. The minimum differential between the heating
and cooling set points is 3° F [2° C], which means the
setting levers cannot be set closer together than 3° F [2° C].
The T874 is equipped with a restricted set point feature
for fuel efficiency.
Factory-installed stops are set so the set point levers
cannot be set above 68° F [20° C] or 72° F [22° C] on
heating (depending on the model) or below 78° F [26° C] on
cooling (Fig. 12).
Do not attempt to change the location of these stops,
because they are nonadjustable.
Fig. 12—Restricted set point.
SUBBASE SETTING
System switching positions control thermostat opera-
tion 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 (Q674B,C,E or G only).
HEAT: Heating system is controlled by the thermostat.
Cooling system is off (Q674A,B or E only).
COOL: Thermostat controls the cooling system. Heat-
ing system is off (Q674A, B or E only).
AUTO: The thermostat automatically switches between
heat and cool modes, depending on the indoor temperature (Q674A,C,E or G only).
Fan switching positions control fan operation as fol-
lows (Q674A,B,C or E):
ON: Fan operates continuously.
AUTO: Fan operates with cooling equipment as con-
trolled by the thermostat or with the heating equipment as controlled by the plenum switch.
To switch positions, use thumb and index finger to slide
lever to the desired position. The switch lever must stop
over the desired function indicator mark for proper circuit
operation.
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
should start and the fan should run after a short delay.
Move the heating lever about 10° F [6° C] below room
temperature. The heating equipment should shut off, and
the fan should run for a short time and then shut off.
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 should start. Move the cooling lever
about 10° F [6° C] above room temperature. The cooling
equipment and fan should stop.
FAN
Move the subbase system switch to OFF or AUTO, and
the fan switch to ON. The fan should run continuously.
When the fan switch is in the AUTO position, the fan
operation is controlled by the heating or cooling system.
7 60-1227—1
Page 8
Calibration
THERMOSTAT
T874 Thermostats are accurately calibrated at the fac-
tory. They do not have provision for field calibration.
THERMOMETER
1. Remove thermostat cover by pulling up from the
bottom of cover until it clears the locking springs.
2. Set the cover on a table near an accurate thermometer.
3. After allowing five or ten 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 thermometer; see step 4.
4. Insert a small screwdriver in the thermometer shaft
(Fig. 13) and turn it until the thermometers read the same.
When the thermometer is calibrated, replace the cover and
place the system into operation.
NOTE: Hand heat will offset the thermometer reading.
After making each adjustment, wait five or ten minutes
for the thermometer to stabilize before comparing.
Fig. 13—Thermometer calibration.
M5070
Home and Building Control Home and Building Control Helping You Control Your World
Honeywell Inc. Honeywell Limited—Honeywell Limitée
1985 Douglas Drive North 740 Ellesmere Road
Golden Valley, MN 55422 Scarborough, Ontario
Printed in U.S.A. www.honeywell.com/yourhome
M1P 2V9
QUALITY IS KEY
8
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