Honeywell T874H Multistage Thermostat Instruction 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  

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.  

The T874H,Q,W Multistage Thermostats provide 24 to 30  

Vac control for heating and cooling systems as listed in  

Table 1.  

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.  

Switch Positions  

System  

Q674  

Model  

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…  

³

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.  

X-XX UL  

•

All Rights Reserved  

^{Copyright © 1996 Honeywell Inc. •} firealarmresources.com  

69-0087-6  

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General	Details
Name	Honeywell T874H Multistage Thermostat Instruction Manual
Make	Honeywell
Language	English
Filetype	PDF (Download)
File size	0.19 MB

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• ALL THERMOSTAT INSTRUCTIONS & USER MANUALS
• Honeywell

Honeywell T874H Multistage Thermostat Instruction Manual Overview

Summary of Contents

• Page 1: T874H,Q,W multistage thermostats provide 24 to 30 Vac control for heating and cooling systems. When an LED lights, refer to this list for the meaning. Emergency heat is operating when the compressor has failed, and the heat pump is not operating. Auxiliary heat is operating because the weather is so cold that the heat pump alone cannot handle the load. The Q674B,C,G,L subbases provide wiring terminals, system and fan switching, and mounting bases for T874 multistage thermostats. This control contains mercury in a sealed tube; do not place it in the trash at the end of its useful life. On a two-stage or three-stage heat thermostat, the stages of heat make sequentially as the temperature drops. 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 ensure suitability for your application. Installer must be a trained, experienced service technician.
• Page 2: T874H,Q,W multistage thermostats and Q674B,C,G,L subbases Caution: 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. Do not tighten the thermostat captive mounting screws to the point of damaging the subbase threads. Never install more than one wire per terminal unless a factory-supplied jumper with a spade terminal is used. These thermostats are calibrated at the factory using subbases mounted at true level. Install the thermostat about 5 ft (1.5m) above the floor in an area with good air circulation. Do not install the thermostat where it can be affected by drafts, dead spots, hot or cold air from ducts, radiant heat, concealed pipes, or unheated areas. Mount the subbase on a wall or horizontal outlet box. All wiring must comply with local electrical codes and ordinances. An incorrectly leveled subbase causes the temperature control to deviate from the setpoint.
• Page 3: T874H, Q, W multistage thermostats and Q674B, C, G, L subbases For straight insertion, strip 5/16 in. (8 mm) For wraparound, strip 7/16 in. (11 mm) 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. Provide disconnect means and overload protection as required. Service LED on select models. Power supply.
• Page 4: T874H, Q, W multistage thermostats and Q674B, C, G, L subbases Thermostat Subbase System components H1 anticipator W1 no fan switch Changeover relay Power supply. Provide disconnect means and overload protection as required. C1 anticipator Heat changeover valve
• Page 5: T874H,Q,W multistage thermostats and Q674B,C,G,L subbases System components 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. Fig. 8. T874W/Q674L for three-stage heat and two-stage cool with outdoor reset and compressor monitor.
• Page 6: T874H,Q,W multistage thermostats and Q674B,C,G,L subbases System components 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 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. 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. Carefully remove and discard the polystyrene packing insert that protects the mercury switches during shipment. Make sure the spring fingers are not bent flat, preventing proper electrical contact with the subbase.
• Page 7: T874H,Q,W multistage thermostats and Q674B,C,G,L subbases Setting the heat anticipator (T874Q only) involves matching each heat anticipator to its primary control current draw. The proper thermistor operation must be verified to assure correct thermostat operation. The minimum differential between the heating and cooling setpoints is 3°F (1.7°C). System switching positions control thermostat operation: OFF, AUTO, HEAT, EM. HT. Heating checkout requires moving the system switch to HEAT or AUTO and adjusting the heating lever. Fan switching positions control fan operation: ON and AUTO. Cooling checkout involves moving the system switch to COOL or AUTO and adjusting the cooling lever. The T874W must use a C815 thermistor for proper operation. The EM. HT. LED indicates when the emergency heat mode is active. Adjustments must be made carefully to ensure proper circuit operation.
• Page 8: T874H,Q,W multistage thermostats and Q674B,C,G,L subbases Fan operation can be set to ON for continuous running or AUTO for system control. Thermostats are accurately calibrated at the factory and do not allow for field calibration. To calibrate the thermometer, remove the thermostat cover and compare readings after stabilization. If readings differ, use a small screwdriver to adjust the thermometer shaft until they match. Replace the cover and put the system into operation after calibration.