Heat Pump Operation

Heat pumps work like air conditioners during the summer and reverse to become air heaters during the winter. During the summer, refrigerant is piped through the indoor coils, absorbs heat from the room air, and vaporizes. The cooled room air is then re-circulated in the house by a blower. The vaporized refrigerant flows into the compressor, which pumps the refrigerant to the outdoor coil, where it condenses back into a liquid by releasing its heat to the outdoor air. Air is circulated through the outside unit by a fan. The cooled refrigerant then flows back to the indoor coil, where the heat transfer cycle is repeated.

In the heating mode, the refrigerant flow is reversed, bringing heat inside from outdoors, essentially working like a conventional air conditioner in reverse. Cold refrigerant is piped through the outdoor coils, absorbing heat from the outside air. The refrigerant vaporizes and flows into the compressor, which pumps it to the indoor coil, where it condenses back into a liquid by releasing its heat to the indoor air. The refrigerant then flows back to the outdoor coils, where the heat transfer cycle starts again.

Like refrigerators, most heat pumps have defrost cycles that minimize frost buildup on the evaporator during the winter heating cycle. Defrost occurs automatically at pre-set time intervals. Defrosting works against the efficiency of the unit when it switches into defrost mode unnecessarily, wasting heating and cooling capacity. Microprocessor controls in some units prevent this from happening. Some controls even determine whether the heat pump or back-up heat is more economical at a particular outdoor air temperature and switch to that heating system.