Heat Pump Diagnostics: Reversing Valve, Defrost, and Mode Confusions

The short field answer:
The reversing valve changes refrigerant flow so the indoor and outdoor coils switch jobs. In many common OEM heat pump designs, the reversing valve is energized in cooling and also during defrost. Defrost logic usually watches outdoor coil temperature, ambient conditions, and compressor runtime, then ends defrost at a termination temperature or a maximum timer.

Lennox documents that the reversing valve coil is energized during cooling demand and during defrost, and its newer heat pump controls show defrost initiation based on compressor run time plus outdoor coil temperature. Some Lennox controls also initiate a defrost after consecutive low-pressure trips in heating under certain weather conditions.

Big exam confusion:
Do not confuse a reversing valve problem with a defrost event. If a heat pump temporarily changes behavior during winter operation, ask whether the board intentionally forced a defrost cycle before you condemn the valve. On the other hand, if pressures never move the right way and the unit stays in the wrong mode, the valve or its control circuit moves up your list fast. (Copeland Webapps)