Honeywell Q674G, Q674B, Q674C User Manual

T874H,Q,W Multistage Thermostats and

Q674B,C,G,L Subbases

INSTALLATION INSTRUCTIONS

APPLICATION

The T874H,Q,W Multistage Thermostats provide 24 to 30
Vac control for heating and cooling systems as listed in
Table 1.

Table 1. Heating and Cooling Stages.

T874 Model

Stage H

Heating Stages 1 1 3
Cooling Stages 1 — 2

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.

Table 2. Switching Positions.

Q674

Model

B Heat-Off-Cool Auto-On
C Off-Auto Auto-On

G Off-Auto None

L Em Ht-Heat-Off-Cool Auto-On

System Fan

a

Switch Positions

QW

OPERATION

On a two-stage or three-stage heat thermostat, the stages
of heat

make

refers to the mercury switch initiating a call for heat or cool.

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

Select models include LEDs (light-emitting diodes). The
LED lights to indicate a problem with the system; refer to
the system manufacturer instructions.

sequentially as the temperature drops.

.

Make

When an LED lights, refer to this list for the meaning:

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.

LEDs are not field addable or 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
control in the trash.

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.

INSTALLATION

When Installing this Product…

 Read these instructions carefully. Failure to follow

them could damage the product or cause a hazard­ous condition.

 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.

not

place your old

Copyright © 1996 Honeywell Inc. • • All Rights Reserved

X-XX UL

69-0087-6

T874H,Q,W MULTISTAGE THERMOSTATS AND Q674B,C,G,L SUBBASES

CAUTION

1. Disconnect the power supply to prevent
electrical shock or equipment damage.

2. 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.

3. Do not tighten the thermostat captive mount­ing screws to the point of damaging the
subbase threads.

4. Do not short across the coil terminals on the
relay. This can burn out the heat anticipator.

5. Never install more than one wire per terminal
unless a factory-supplied jumper with a spade
terminal is used.

IMPORTANT

These thermostats are calibrated at the factory
by using subbases mounted at true level.
Inaccurate subbase leveling causes thermostat
control deviation.

Location

Install the thermostat about 5 ft (1.5m) above the floor in
an area with good air circulation at average temperature.

Do not install the 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 behind the thermostat

such as an outside wall.

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:

 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.

IMPORTANT

Use 18 gauge, color-coded thermostat cable for
proper wiring.

 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.

VERTICAL
OUTLET
BOX

1
HORIZONTAL
OUTLET
BOX

SUBBASE

SUBBASE
MOUNTING SCREWS (2)

CAPTIVE
MOUNTING SCREWS (2)

1 NOT INCLUDED WITH UNIT.

2 ACCESSORY PARTS AVAILABLE (193121A).

MOUNTING
SCREWS (2)

1

2

6

0

5

8

0

7

0

L

O

O

C

0

0

6

0

5

COVER
PLATE

THERMOSTAT

THERMOSTAT
COVER

0

8

0

7

ADAPTER
RING

0

8

0

7

0

6

0

5

T

A

E

H

Fig. 1. Installation on vertical or horizontal outlet box.

SPIRIT LEVEL

MOUNTING
HOLES (2)

TOP
MOUNTING
HOLES (2)

WIRING
TERMINAL

POST (2) FOR
MOUNTING
THERMOSTAT

THERMOSTAT
CABLE OPENING

TO SPRING FINGER
CONTACTS ON THE
THERMOSTAT
(UP TO 12)

Fig. 2. Leveling subbase.

 Connect the system wires to the subbase. See Fig.

4 through 9. A letter code for identification is
located near each terminal. The terminal barrier
permits straight or conventional wraparound wiring
connections. See Fig. 3. Either method is accept­able. 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.

IMPORTANT

An incorrectly leveled subbase causes the
temperature control to deviate from the setpoint.

2

M6009

M927

69-0087—6

2

T874H,Q,W MULTISTAGE THERMOSTATS AND Q674B,C,G,L SUBBASES

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.

THERMOSTAT

H1
ANTICIPATOR

FALL

H1

RISE

C1
ANTICIPATOR

CO

RISE

C1

POWER SUPPLY. PROVIDE DISCONNECT MEANS

1

AND OVERLOAD PROTECTION AS REQUIRED.

2

SERVICE LED ON SELECT MODELS.

2

3

4

8

9

10

FAN SWITCH

AUTO

ON

SYSTEM
SWITCH

OFF

AUTO

SUBBASE

SERVICE LED
(YELLOW)

Fig. 4. T874H/Q674C in heating-cooling system.

FOR STRAIGHT
INSERTION–
STRIP 5/16 in. (8 mm)

SUBBASE TERMINAL SCREW

Y1

G

R

Y2

B

O

2

X

COMPRESSOR
FAULT

C

FOR WRAPAROUND–
STRIP 7/16 in. (11 mm)

BARRIER

Fig. 3. Wiring connections.

SYSTEM COMPONENTS

COOL CHANGEOVER
VALVE

FAN RELAY

COMPRESSOR
CONTACTOR

1

L2

L1
(HOT)

M3680A

M928

THERMOSTAT

H1 ANTICIPATOR

H1

FALL

RISE

CO

C1 ANTICIPATOR

RISE

C1

1

POWER SUPPLY. PROVIDE DISCONNECT MEANS
AND OVERLOAD PROTECTION AS REQUIRED.

2

3

4

8

9

10

Fig. 5. T874H/Q674C in heating-cooling system with SERVICE LED and FILTER LED.

FAN SWITCH

AUTO

ON

SYSTEM
SWITCH

OFF

AUTO

SUBBASE

SYSTEM COMPONENTS

Y1

G

R

Y2

B

O

X

SERVICE
LED (YELLOW)

X1

C

FILTER LED (RED)

X2

3

COOL CHANGEOVER
VALVE

FAN RELAY

COMPRESSOR
CONTACTOR

24 VAC

1

L2

L1
(HOT)

M5902

69-0087—6

T874H,Q,W MULTISTAGE THERMOSTATS AND Q674B,C,G,L SUBBASES

THERMOSTAT SUBBASE SYSTEM COMPONENTS

3

H1

H2

RISE

H1 ANTICIPATOR

FALL

H2 ANTICIPATOR

FALL

C1

C1 ANTICIPATOR

1

4

6

8

9

10

NO FAN
SWITCH

SYSTEM
SWITCH

AUTO

OFF

1

POWER SUPPLY. PROVIDE DISCONNECT MEANS AND
OVERLOAD PROTECTION AS REQUIRED.

Fig. 6. T874Q/Q674G for heating only system.

W1

B

W2

G

Y1

R

CHANGEOVER
RELAY

FAN RELAY

COMPRESSOR
CONTACTOR

1

L2

L1
(HOT)

M6008A

THERMOSTAT

H1 ANTICIPATOR

FALL

H1

H2 ANTICIPATOR

FALL

H3

H3 ANTICIPATOR

FALL

H2

C1

C2

RESET HEATER

C1 ANTICIPATOR

C2 ANTICIPATOR

RISE

RISE

POWER SUPPLY. PROVIDE DISCONNECT

1

MEANS AND OVERLOAD PROTECTION
AS REQUIRED.

Fig. 7. T874W/Q674B with outdoor reset in heating and cooling system.

SUBBASE

1

2

FAN SWITCH

3

AUTO

4

ON

5

SYSTEM

6

8

10

11

SWITCH

HEAT

OFF

COOL

SYSTEM COMPONENTS

R

C815A THERMISTOR

T

W1

W2

HEAT RELAY 2

W3

B

HEAT CHANGEOVER VALVE

G

O

COOL CHANGEOVER VALVE

Y2

COMPRESSOR CONTACTOR 2

Y1

COMPRESSOR CONTACTOR 1

1

HEAT RELAY 1

HEAT RELAY 3

FAN RELAY

L1 (HOT)

L2

M3674

69-0087—6

4

T874H,Q,W MULTISTAGE THERMOSTATS AND Q674B,C,G,L SUBBASES

THERMOSTAT

RESET

1

2

3

FAN SWITCH

4

5

6

8

9

10

11

12

SYSTEM

SWITCH

EM.HEAT

FALL

H1 ANTICIPATOR

H1

H2

FALL

FALL

1

2

H2 ANTICIPATOR

H3 ANTICIPATOR

H3

RISE

C1

C1 ANTICIPATOR

C2 ANTICIPATOR

RISE

C2

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.

AUTO

HEAT

OFF

COOL

SUBBASE

T

W3

AUXILIARY
HEAT LED
(GREEN)

ON

EMERGENCY
HEAT LED
(RED)

CHECK LED
(RED)

U

G

R

E

L

X

B

2

O

Y1

Y2

SYSTEM COMPONENTS

C815A OD THERMISTOR

COMPRESSOR
MONITOR

CHANGEOVER

RELAY (HEAT)

COMPRESSOR
CONTACTOR 1

1

AUX. HEAT RELAY

EM. HEAT RELAY

L1
(HOT)

FAN RELAY

CHANGEOVER
RELAY (COOL)

COMPRESSOR
CONTACTOR 2

L2

M156A

Fig. 8. T874W/Q674L for three-stage heat and two-stage cool with outdoor reset and compressor monitor.

5

69-0087—6

T874H,Q,W MULTISTAGE THERMOSTATS AND Q674B,C,G,L SUBBASES

THERMOSTAT

RESET

1

2

3

FAN SWITCH

4

5

6

8

9

10

11

12

SYSTEM

SWITCH

EM.HT.

FALL

H1 ANTICIPATOR

H1

H2

FALL

FALL

RISE

1

2

H2 ANTICIPATOR

H3 ANTICIPATOR

H3

RISE

C1

C1 ANTICIPATOR

C2 ANTICIPATOR

C2

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.

AUTO

HEAT

OFF

COOL

SUBBASE

AUX. HEAT
LED (GREEN)

ON

EM. HEAT
LED (RED)

CHECK LED
(YELLOW)

SYSTEM COMPONENTS

T

C815A OUTDOOR
THERMISTOR

W3

G

R

E

X1

C

CHANGEOVER
RELAY (HEAT)

B

O

X2

COMPRESSOR
CONTACTOR 1

Y1

Y2

AUX. HT. RELAY

FAN RELAY

EM. HT. RELAY

2

CHANGEOVER
RELAY (COOL)

COMPRESSOR
CONTACTOR 2

L1
(HOT)

L2

M6999

1

Fig. 9. T874W/Q674L for three-stage heat and two-stage cool with outdoor reset.

Mounting the Thermostat

 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 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
proper electrical contact with the subbase.

69-0087—6

not

bent flat, preventing

 T874Q only: Set the heat anticipator indicators to

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.

 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.

6

T874H,Q,W MULTISTAGE THERMOSTATS AND Q674B,C,G,L SUBBASES

SETTING

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.)

 Connect an ac ammeter of appropriate range

between the heating terminals of the subbase as
follows:

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.

 After mounting the thermostat, set the adjustable

heat anticipator(s) to match the respective
reading(s) measured in step .

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
(1.7°C) (5° F [2.8°C] for T874W).

Subbase Setting

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.

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.

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.

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.

4600
4400
4200
4000
3800
3600
3400
3200
3000
2800
2600
2400
2200
2000
1800
1600
1400

THERMISTOR RESISTANCE

1200
1000

800
600
400
200

0

-20 0 20 40 60 80 100 120 140

C815A THERMISTOR RESISTANCE
R = 400 ohms ± 10% AT 77°F (25°C)

TEMPERATURE OF THERMISTOR (°F)

M1590A

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:

 Disconnect the
 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.

 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.

T

wire on the subbase.

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.

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
10°F (6°C) above room temperature. The cooling equip­ment and fan stop.

Emergency Heat

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

69-0087—6

T874H,Q,W MULTISTAGE THERMOSTATS AND Q674B,C,G,L SUBBASES

Fan (Select Models)

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 thermom­eter; see step 

 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.

NOTE: Hand heat offsets the thermometer

reading. After making each adjustment,
wait 10 or 15 minutes for the thermometer
to stabilize before comparing.

Fig. 11. Thermometer calibration.

M5070

Automation and Control Solutions

Honeywell Internationl, Inc.
1985 Douglas Drive North
Golden Valley, MN 55422

69-0087—6

69-0087—6 C.H. Rev. 9-96 www.honeywell.com

Honeywell Limited-Honeywell Limitée
35 Dynamic Drive
Scarborough, Ontario
M1V 4Z9

Printed in U.S.A. on recycled
paper containing at least 10%
post-consumer paper fibers.

8