By Ron Walker

Winter will soon be upon us and it is time to brush up on our heat pump defrost controls. Here is a quick review of what you need to know. This is a general overview and not brand specific.

Iced-CondenserDefrost Control Board Operation

The first thing to remember is that the heat pump operation in defrost is the same as when the system is cooling with 2 differences. First, the condenser fan motor is turned off during defrost, and second, the defrost control board energizes the ‘W’ terminal turning on the electric strip heat. The electric strip heat is turned on to rewarm the air that is cooled by the evaporator coil during defrost.

The defrost control board will initiate the defrost cycle when certain conditions are met. The following is the defrost sequence:

1. The defrost sensor, which is usually mounted on a tube of the condenser, must be closed. The sensor closes when the coil temperature is cold enough to frost or freeze. Once closed, the control board begins saving accumulated run time of the compressor.

2. The accumulated compressor run time, as selected by the jumper on the control board, must be reached. This is the jumper on the board that usually has the 30, 60, or 90 next to it. Note – if the defrost sensor warms up and opens before the accumulated run time is met, accumulated run time is reset to zero.

3. When the defrost sensor is closed and the accumulated run time is met, the defrost control board will simultaneously:

  • Energize (apply 24 volts) to the ‘W’ terminal on the defrost control board. This energizes the electric strip heat.
  • Energize (apply 24 volts) to the reversing valve to switch it from the heating to the cooling mode.
  • De-energize the condenser fan relay, turning off the condenser fan.

4. The system is now in defrost. The condenser heats up quickly because the condenser fan motor is off, melting the frost. The electric strip heat tempers the air that is being cooled by the evaporator.

5. Defrost will continue until the defrost sensor opens. This tells the defrost control, that the coil is warm enough and that no frost is present. If, for some reason, the defrost sensor fails or defrost continues for an extended period of time, the defrost control board will terminate the defrost after specific amount of time (About 10 minutes depending on the manufacturer.)

6. Once the defrost cycle is terminated::

  • The ‘W’ terminal on the defrost control board is de-energized, turning off the electric strip heat.
  • The reversing valve is de-energized switching from cooling to heating.
  • The condenser fan is turned back on.

Checking the Defrost Control Board

Checking the defrost control board is quite easy once you know how it operates. Remember, the defrost sensor must be closed and the compressor accumulated run time must be greater than the 30, 60 ,90 pin settings on the board. Here are the steps to check the defrost control board.

  • Observe all safety precautions.
  • Find and remove the defrost sensor connection from the defrost control board and place a jumper across them. This simulates a closed defrost sensor. At this point the board begins accumulating compressor run time.
  • Look on the board, there will be two pins that are usually labeled “Speed Up’. Place a jumper or carefully short the two pins together. This will speed up the accumulated run time count to several seconds rather 30, 60 ,90 minutes.
  • Wait 30 seconds and the system should go into the defrost mode as outlined above.
  • Remove the short or jumpers from the “Speed Up” pins. The system should still be in defrost.
  • Check that there is 24V AC on the  ‘W’ terminal.
  • Check that the condenser fan is off.
  • Check that the condenser coil is heating up.
  • Remove the jumpers from the defrost connection pins on the defrost board.
  • The defrost cycle should terminate and the system should be in the heating mode.

Checking the Defrost Sensor

The defrost sensor is a normally open switch that closes on temperature fall. If you have a defrost sensor on your truck, you can easily test out the procedure below to see how it works.

  • Remove power from the condenser.
  • Find and remove the defrost sensor from the condenser and unplug it from the control board.
  • Place the sensor in a container of ice water and using your multimeter, ohm across the terminals, the sensor should read zero ohms (closed).
  • Remove the sensor and hold it in your hand to warm it up. Again, using your meter, ohm across the terminals. You should be reading infinite ohms (open).
  • Any readings other than indicated above means the sensor is faulty and must be replaced.

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Ron Walker
Ron Walker

After retiring from the U.S. Marines and achieving his B.S. degree, Ron Walker entered the HVAC field. He has been an HVAC technician, service manager, and business owner. Working as a service manager, he spent many years training HVAC technicians to be more technically competent and really understand their trade. His passion for teaching and helping others resulted in the creation of HVAC Training Solutions, LLC.