How to Install an Outside Thermostat for a Heat Pump

The purpose of the outside thermostat for a heat pump is to determine if the auxiliary heat needs to turn on. There is a balance point, when the temperature outside is too cold for the heat pump to meet the heat demands of the house and needs the auxiliary heat, which normally consists of electric resistive heat elements. When the auxiliary heat comes on, the watt count goes up, but this is a necessary evil. If the outdoor thermostat fails, the contacts might be stuck, causing the auxiliary heat to stay on all the time. However, if the contacts will not close, the auxiliary heat will not come on and when the temperature outside dips below the balance point, and the unit will not supply sufficient heat.

Things You'll Need

• New outdoor thermostat
• Screwdriver
• Multi-meter

Instructions

1. Turn off the power to the unit. Remove the control panel with a screwdriver. Set the multi-meter on AC volts. The thermostat will be located in the control panel.

2. Remove the wires, marking them as you go. Using a screwdriver, remove the mounting screws. The outdoor thermostat will have a sensing element and be mounted with two screws.

3. Mount the new thermostat, reinstalling the two screws. Then, reattach the wires in the same order as before. Use caution when handling the sensing element. If it is a type that contains a charge of gas or if it is broken or cracked, it will loose its charge and not function.

4. Reinstall the control panel, using the screwdriver. Turn on the power, and test the unit.

References

• InspectAPedia: Heat Pump Backup Heat Outdoor Thermostat Diagnosis, Inspection, Repair Guide
• G&S Mechanical: heating service and heat pump repair