HVAC Training - Defrost Control

Understanding a Heat Pump Defrost Control Board – Part 2

by Ron Walker

So how exactly does a defrost control board work? If you break it down into the three different functions it performs it will be a little easier to understand. The three functions are cooling, heating, and defrost control. We’ll start with the simplest cooling.

HVAC Training - Defrost ControlCooling

There are four inputs from the air handler and thermostat. R and C are the 24 volts in from the air handler’s transformer. These inputs are always there in all three modes and power up the board.

O terminal is always energized when the thermostat is in the cooling mode. The control board sends 24 volts to the reversing valve to hold it in the cooling mode. Some manufacturers like Rheem, energize the reversing valve in the heating mode – make sure you check.

Y terminal has 24 volts applied when the thermostat calls for cooling. The control board sends 24 volts to the contactor to power up the condenser fan and contactor.

That’s it for the cooling mode (other than the optional high and low pressure cutout switches, which we’ll discuss later). All of the other inputs and outputs are not used in the cooling mode.

Heating-

There are only three inputs from the air handler and thermostat when a heat pump is in the heating mode. Once again, R and Care the 24 volts in from the air handler’s transformer.

Y terminal-has 24 volts applied when the thermostat calls for cooling. The control board sends 24 volts to the contactor to power up the condenser fan and compressor.

O terminal is not energized. This puts the reversing valve in the heating position.

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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.