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Operation

Heat pumps work like air conditioners during the summer and reverse to become air heaters during the winter.

 During the summer it operates in the cooling mode. 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 the 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 "backup" heat is more economical at a particular outdoor air temperature and switch to that heating system. An example of this might be a heat pump with natural gas backup heat. This type system is known as a dual-fuel or hybrid heat pump. When it gets very cold, and the heat pump's efficiency drops off, the control would turn the electric heat pump off and the natural gas system on. This is because even thought the gas system may only be 70% or 80% efficient, it still may be more economical to operate than the electric heat pump because of the difference in cost between gas and electricity.