Honeywell THM5421C1008 Benutzerhandbuch

68-0287—1

22

The thermostat uses a red LED indicator that lights when the 

thermostat is in the Emergency Heat mode. The LED is located 

in the upper right corner of the thermostat. It is visible only 

when on.

In this operation, there is no external fossil fuel kit (dual fuel kit) 

installed; the thermostat controls this function.

1. Choose correct heat pump application in Installer Setup 

Number 0172.

2. Choose Fossil Fuel Option as the backup heat source 

in Installer Setup Number 0200.

3. Choose No External Fossil Fuel Kit Option is control-

ling back up heat in Installer Setup Number 0210.

4. Choose Outdoor Temperature Sensor for Heat Pump 

Temperature Lockouts in Installer Setup Number 0342.

5. Choose appropriate Balance Point Temperature in 

Installer Setup Number 0350.

NOTE: System must have an outdoor temperature sensor.

When the outdoor temperature is above the selected balance 

point temperature, only the compressor operates and the fan 

(G terminal) energizes when the thermostat calls for heat. See 

Fig. 30. When the outdoor temperature is below the selected 

balance point temperature, only the fossil fuel (auxiliary heat) 

operates and the fan (G terminal) does not energize when the 

thermostat calls for heat.

The fossil fuel will turn on when the outdoor temperature is 

above the balance point if the room temperature droops 

approximately 2 degrees below the temperature setting, and 

the compressor will be disabled.

When the outdoor temperature is above the Auxiliary Heat 

Lockout Temperature only the compressor operates; below the 

balance point temperature (setup function 0360) only the fossil 

fuel operates. Between the balance point temperature and 

auxiliary heat lockout temperature the compressor will operate. 

However, the fossil fuel will turn on if the room temperature 

droops approximately 2°F below the temperature setting, 

compressor will be disabled.

AUXILIARY LOCKOUT FOR ELECTRIC HEAT BACKUP
Heat Pump systems with electric heat backup can select a 

compressor lockout temperature (function 0350) and/or 

auxiliary heat lockout temperature (function 0360). When the 

outdoor temperature is below the compressor lockout 

temperature, only the auxiliary heat operates. When the 

outdoor temperature is between the compressor and auxiliary 

lockout temperatures, both the compressor and auxiliary heat 

can operate.

NOTE: System must have an outdoor temperature sensor.

1. Choose correct heat pump application in Installer Setup 

Number 0172.

2. Choose Electric as Auxiliary (Backup) Heat Source in 

Installer Setup Number 0200.

3. Choose Outdoor Temperature Sensor in Installer 

Setup Number 0342.

4. Choose Compressor Lockout Temperature in Installer 

Setup Number 0350.

5. Choose Auxiliary Lockout Temperature in Installer 

Setup Number 0360.

NOTE: There is a 5°F deadband between the Compressor 

and Auxiliary Heat Lockout Temperatures.

When the outdoor temperature is below the Compressor 

Lockout Temperature, only the Auxiliary Heat operates.

When the outdoor temperature is above the Auxiliary Lockout 

Temperature, only the Compressor operates. See Fig. 32.

30

M22441

BALANCE

POINT

COMPRESSOR ONLY

FOSSIL FUEL ONLY

OUTDOOR TEMPERATURE

50

35

COMPRESSOR ONLY

COMPRESSOR+2 °F

DROOP UPSTAGE TO

FURNACE 

FURNACE ONLY

M23497