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Understanding a Heat Pump Defrost Control Board – Part 1

by Ron Walker

Heat Pump RepairMany new air conditioning technicians are intimidated by the number of wires going to and from a heat pump defrost control board and exactly what they do. We’ll start out with a general overview of the control wiring (most defrost control boards have the same basic connections).

  • R terminal – 24 volt power coming in from the air handler’ s transformer (this provides power to the board) C terminal the common side of the 24 volt transformer
  • Y terminal – when the thermostat calls for heating or cooling it sends 24 volts to the Y terminal. It is routed through the defrost board and applied to the 24 volt terminals of the contactor. This energizes the contactor and turns on the compressor. The 24 volts on the Y terminal also routes through the defrost board, activates the condenser fan relay and turns on the condenser fan.
  • O terminal – if the thermostat is in the cooling mode, the thermostat sends 24 volts to the O terminal. Once again it is routed through the board. It is then sent out to energize the reversing valve which puts it into the cooling mode. (Note this hold true in most cases, however there may be some older heat pumps that are in the heating mode when the reversing valve is energized.)
  • C terminal – 24 volt neutral.

All of the terminals above are INPUTS from the air handler or thermostat.

W terminal is an OUTPUT terminal, it sends 24 volts back to the W terminal of the air handler when the condenser goes into defrost. The 24 volts on the W terminal on the air handler energizes the electric strip heat for the duration of the defrost cycle.

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About The Author

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.