Honeywell T874H Installation Manual

Honeywell T874H 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.

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 theinterstage differential.

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

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.

Honeywell T874H 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

Ha

Q

W

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

Switch Positions

Model

System

Fan

B

Heat-Off-Cool

Auto-On

C

Off-Auto

Auto-On

G

Off-Auto

None

L

Em Ht-Heat-Off-Cool

Auto-On

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.)

  1. 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.
  2. Move the system switch to AUTO (if applicable).
  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 Ⓢ.

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:

  1. OFF: Both the heating and cooling systems are off. If the fan switch is in the AUTO position, the cooling fan is also off.
  2. AUTO: Thermostat automatically changes between the heat and cool modes, depending on the indoor temperature.
  3. HEAT: Heating system is controlled by the thermostat. Cooling system is off.
  4. 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.

The proper thermistor operation must be verified to assure the correct operation of the thermostat. Check the thermistor operation as follows:

  1. Disconnect the T wire on the subbase.
  2. Use an ohmmeter to measure resistance between the T wire and the transformer common.
  3. Take the outdoor temperature at thermistor location and find the correct thermistor resistance on the chart in Fig. 10.
  4. 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.

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 equip- ment 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.

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Title

: Honeywell T874H Installation Manual

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Pages

: 24 Pages

Language

: English

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