How Heat Pumps Work
Heat pumps are exactly as they are named, a pump that happens to pump heat. The heart of the heat pump is the compressor. The compressor is, more or less, a high pressure pump. In summer, this pump constantly pumps or re-circulates refrigerant in a closed loop from the compressor to the outside coil of your home and then to the inside coil. The refrigerant acts as a transfer medium or vehicle, if you will, for the heat to travel in.
In the summer, the refrigerant, in a hot gaseous state, leaves the compressor and travels to and through the outside coil, also known as the condenser, where the condenser fan cools the coil thereby cooling the gas. As it cools, the gas condenses, turning into a liquid. The liquid then travels to the inside coil where, just before going through the coil, it is forced, under pressure, though a small orifice. This orifice creates a drop in pressure, which now lowers the refrigerant's boiling point. As the warm household air blows across the the coil, also known as the evaporator, the heat is transferred to the the refrigerant, causing it to boil or evaporate. Just as our sweat cools us on a hot day by evaporating, the evaporating refrigerant absorbs heat from the coil, which in turn absorbs heat from the home's air. The now gaseous refrigerant travels back to the compressor, where the process is repeated over and over.
In winter, a valve, known as a reversing valve, reverses the direction of the refrigerant flow so that the heat is picked up outside and then brought into the home. In a nutshell, the heat pump manipulates the boiling point of the refrigerant at a certain point, thereby controlling the point or area that the refrigerant is ready to evaporate and absorb heat. There is one more stage we must mention - the defrost cycle.
In winter, the outside coil has a propensity to freeze up due to the fact that the outside coil is absorbing heat from air that is already cold,thus making that air even colder. Although there is usually little moisture in winter air, there is some. This moisture now freezes to the outside coil and must be periodically removed by the defrost cycle. When the heat pump, via electronic sensors, realizes the outside coil is in need of de-icing, the defrost cycle is initiated. At this time, the unit reverts back to the summer or cooling cycle. The heat is picked up from the inside coil and transferred to the outside coil. A couple of additional things take place, however, to make this work. The outside condensor fan stops, which allows the outside coil to heat up quicker. Secondly, the indoor supplemental heating elements, usually electric or gas, are turned on. This is to compensate for the now cold indoor coil, by adding additional heat back into the indoor air stream. Usually after only a few minutes, the defrost cycle is terminated and back to heating the home again, using only the heat being transferred from the outside to the inside via the refrigerant.
Also see: Wikipedia - Air Source Heat Pumps