User's Manual

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S. M. • Rev. 5-93 • • ©Honeywell Inc. 1993 • Form Number 60-1147—4

T874/Q674

Thermostat/Subbase Combinations

Installation Instructions for the Trained Service Technician.

Application

The T874/Q674 Thermostat/Subbase combinations provide 24 to 30 Vac control for heat pump systems. See Table 1.

TABLE 1—T874/Q674 THERMOSTAT/SUBBASE COMBINATIONS.

Thermostat/ Number Changeover

Subbase

of Stages Stage Type of Switching

Model Heating Cooling Heating Cooling System Fan See Fig.

T874N/Q674C 2 1 1 — OFF-AUTO AUTO-ON 5

T874N/Q674F 2 1 — 1 OFF-EM. HT.- AUTO-ON 6

HEAT-AUTO-COOL

T874R/Q674L 2 1 — 2

EM. HT.-HEAT- AUTO-ON 7

OFF-COOL

Q674 Subbase provides the wiring terminals, system switch and fan switch for system operation.

Provides manual changeover with the subbase system switch in COOL position.

Installation

WHEN INSTALLING THIS PRODUCT…

1. Read these instructions carefully. Failure to follow

them could 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 applica-

tion.

3. Installer must be a trained, experienced service tech-

nician.

4. After installation is complete, check out product opera-

tion as provided in these instructions.

CAUTION

1. Disconnect power supply to prevent electrical

shock and equipment damage.

2. To prevent interference with the thermostat link-

age, keep wire length to a minimum and run

wires as close as possible to the subbase. Push

excess wire back into the hole and plug hole to

prevent drafts from affecting thermostat opera-

tion.

3. Do not overtighten thermostat captive mounting

screws because damage to subbase threads may

result.

4. Do not short across coil terminals on the relay.

This may burn out the thermostat heat

anticipator.

IMPORTANT: Thermostats are calibrated at the factory

using subbases mounted at true level. An inaccurately

leveled subbase will cause thermostat control devia-

tion.

Operation

On a two-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 the interstage differential.

One or two LEDs (light emitting diodes) are included on

your subbase. Refer to the list below for specific meaning.

The CHECK LED lights when something needs to be

checked or done to maintain efficient operation of the system.

See your heating system instructions for the specific mean-

ing.

The EM. HT. LED lights when the system switch is placed

in the EM. HT. position. Emergency heat is operating; in

most systems, the compressor has failed and the heat pump

is not operating.

The AUX. HT. LED lights when the auxiliary heat stage

is operating. The weather is cold enough that the first stage

cannot handle the load alone.

LEDs are not field replaceable or addable.

M3375

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 in-

structions regarding recycling and the proper disposal of this

control, or of an old control containing mercury in a sealed

tube.

If you have questions, call Honeywell at 1-800-468-1502.

Summary of content (8 pages)

PAGE 1
T874/Q674 Thermostat/Subbase Combinations Installation Instructions for the Trained Service Technician. Application The T874/Q674 Thermostat/Subbase combinations provide 24 to 30 Vac control for heat pump systems. See Table 1. TABLE 1—T874/Q674 THERMOSTAT/SUBBASE COMBINATIONS. Thermostat/ Subbasea Model T874N/Q674C T874N/Q674F Number of Stages Heating Cooling 2 1 2 1 T874R/Q674L 2 1 Changeover Stage Heating Cooling 1 — — 1 — 2b Type of Switching System Fan OFF-AUTO AUTO-ON OFF-EM. HT.
PAGE 2
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 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 such as an outside wall behind the thermostat. Fig. 1—Installation of Q674 Subbase on outlet box.
PAGE 3
2. Carefully remove and discard the polystyrene packing insert that protects the mercury switches during shipment. 3. If LED indication (EM.HT., CHECK, etc.) is to be used with the Q674 Subbase, install the preprinted insert under the thermostat set point scale (Fig. 8). To install, push both thermostat set point levers to the far ends of the thermostat. Use index finger to gently pull out the plastic set point scale about 1/4 in. [6 mm].
PAGE 4
Fig. 6—Internal schematic and typical wiring diagram for T874N and Q674F. W1 DEFROST RELAY H1 ANTICIPATOR OUTDOOR THERMOSTAT REVERSING VALVE HEAT RELAY 2 EM.HT. RELAY CONTACT EM.HT. RELAY CONTACT HEAT RELAY 3 H1 W3 FALL FAN SWITCH FAN RELAY H2 ANTICIPATOR AUTO G ON 1 L1 HOT L2 R H2 FALL W2 SYSTEM SWITCH OFF EM. HEAT LED (RED) CHECK LED (GREEN) HEAT OPTIONAL COMPRESSOR FAILURE DEVICE L RISE AUTO C1 ANTICIPATOR C1 HIGH PRESSURE SWITCH COOL C0 60-1147-4 EM. HEAT RELAY X EM. HT.
PAGE 5
Fig. 7—Internal schematic and typical wiring diagram for T874R and Q674L. L1 HOT L2 R EM.HT. RELAY CONTACT 1 EM.HT. RELAY CONTACT HEAT CONTACTOR 3 W3 H1/C1 ANTICIPATOR DEFROST RELAY OUTDOOR THERMOSTAT HEAT CONTACTOR 2 HEAT RELAY 1 HIGH LIMIT SWITCH DEFROST RELAY REVERSING VALVE W2 H1 C1 FALL B FAN SWITCH L AUTO H2 AUX. HEAT LED (GREEN) ON OPTIONAL COMP FAILURE DEVICE X2 H2 ANTICIPATOR FALL X1 SYSTEM SWITCH EM. HEAT RELAY EM. HEAT LED (RED) E EM. HT.
PAGE 6
TEMPERATURE SETTING Fig. 9—Mounting thermostat on subbase. Fig. 11—Location of external components (T874N shown).
PAGE 7
If the set point lever and the thermometer reading do not agree: 1. Remove the thermostat cover by pulling up from the bottom of cover until it clears the mounting slots. 2. Set the thermostat cover on a table near an accurate thermometer. 3. Allow 10 or 15 minutes for cover thermometer to sense area temperature; compare the readings. Be careful not to touch the thermometer or breathe on it. 4. If the readings are the same, replace the cover and put the system into operation. 5.
PAGE 8
Automation and Control Solutions Honeywell International Inc. 1985 Douglas Drive North Golden Valley, MN 55422 60-1147 –4 S.M. Rev. 5-93 60-1147-4 Honeywell Limited-Honeywell Limitée 35 Dynamic Drive Scarborough, Ontario M1V 4Z9 8 www.honeywell.