In this troubleshooting situation, the equipment is an air source heat pump and the customer’s complaint is that even though the thermostat is set at 70° F, the temperature in the conditioned space remains near 65°.

When you arrive, you confirm that the thermostat is properly set and that the equipment is not operating in order to reach the set-point. When you go to the outdoor unit, you note that the outdoor coil is severely frosted and that the outdoor fan motor (OFM) is running.

Accessing the electrical components and using the diagram shown in Figure One, you decide to first check across the terminals of the DFT (defrost thermostat). Your meter shows no voltage difference, indicating that the switch is closed and calling for defrost.

After checking the manufacturer’s electrical information sheet, you confirm that you can put the system manually into defrost by using a jumper on the Speed Up pins located on the control board. When you take this step, you note the following:


  1.  The OFM continues to run.
  2. The reversing valve switches.
  3. A temperature change of the refrigeration system tubing indicates a change from the heat mode to the cooling mode.


Accomplishing your next voltmeter check at the OF1 and OF terminal connections on the DR (defrost relay), you note that there is no voltage difference shown on your meter display.

Your troubleshooting question:

What is the next step you need to take in servicing this equipment?

Compare your answer with ours HERE.

Publication date: 3/4/2019

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