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Understanding a Heat Pump Defrost Control Board – Part 3

by Ron Walker

Finally we get the defrost part of the defrost control board. A heat pump’s outdoor unit will begin to frost and ice up somewhere around 40F or below outside temperature, depending on the humidity. For the heat pump to heat properly the ice must be removed.

Note – Frost on the outdoor unit is normal when in the heating mode.

Online HVAC TrainingFirst, there are several different ways a heat pump can be put in the defrost mode. But the most common is time-temperature initiated and time-temperature terminated.

Time-temperature initiation

  • Every 30, 60, or 90 minutes of accumulated run time (these times can vary depending on the manufacturer and can be set on some control boards) the defrost board sends a signal to the defrost thermostat to test for icing.
  • Both conditions, time and temperature must be met to initiate a defrost. If the defrost thermostat is open and the board initiates a defrost signal the system will not go into defrost.
  • Conversely, if the defrost thermostat is closed but the defrost board has not initiated a defrost, the system will not go into defrost. Remember, both conditions must be met time and temperature.
  • If the defrost thermostat is closed and the defrost board has initiated a defrost cycle, the system will go into the defrost mode. Here’s what happens:
  • The reversing valve solenoid de-energizes, directing hot gas from the compressor to the outdoor coil to defrost.
  • The outdoor fan stops. If it didn’t, cold air from the fan prevents the melting effect of the hot refrigerant.
  • The defrost control energizes the electric heat elements to warm the indoor air. (Because we are basically in the cooling mode during defrost.)

Time-temperature termination

  • Either condition, time or temperature independent of each other, will terminate the defrost cycle.
  • If the defrost thermostat warms up to a temperature where it is obvious that there is no more ice on the coil, it breaks the defrost circuit terminating the defrost cycle.
  • Otherwise, if its too cold for the defrost thermostat contacts to open, after a predetermined amount of time the defrost control board will terminate the defrost cycle. Normally the maximum amount of time for defrost is 10 minutes.

After the defrost terminates here’s what happens:

  • The reversing valve shifts back to the heating position and directs hot refrigerant gas to the indoor coil.
  • The outdoor fan operates.
  • The electric elements cycle off.
  • The unit is now in the normal heating mode.

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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.