In this troubleshooting situation, the equipment is a package unit heat pump, and the customer has called for service because although they have the thermostat set at 74° F, the temperature in the building remains at 64° with the system “running constantly.” When you arrive, you confirm the thermostat settings and the uncomfortable indoor temperature. And, when you check the outdoor section of the equipment, you find the outdoor coil severely frosted. The compressor and the outdoor fan motor are running.

After reviewing the manufacturer’s wiring diagram shown in Figure One and the legend in Figure Two, you decide that your first step would be to check the condition of the Defrost Thermostat (DFT), and your voltmeter shows 0-volts at the switch connections.

Figure Two

Heat Pump Definitions

As your next step, you locate and attach a jumper across the pins that allow you to initiate a defrost mode. With this step accomplished, the reversing valve switches to re-direct refrigerant flow, allowing the outdoor coil to warm up. The outdoor fan motor continues to run. When you test at the Defrost Relay (DR) terminals wired in series with the OFM, your meter shows 0-VAC.

Your two-part troubleshooting question:

  1. Which component needs to be replaced to restore this equipment to normal operation?
  2. What condition did the failure of this component cause that led to the frost build-up?

Compare your answer with ours here.