Heat pump is a device that takes heat from one area and delivers it to another area at a higher temperature. In heating a building, a heat pump absorbs heat from outside the building and delivers it inside. The same heat pump can also cool a building by pumping heat from the inside to the outside.
The fluid that circulates through a heat pump is called a refrigerant. For heating, a cold refrigerant first flows through coils of pipe that are exposed to an outside heat source. The heat source may be outside air, well water, or the ground–all of which contain heat even at 0 °F (-18 °C). The refrigerant absorbs heat from these sources, boils, and becomes a vapor. The vapor then goes to a compressor that increases its temperature and pressure. The heated vapor then passes through an inside coil that resembles an automobile radiator. The vapor gives up some of its heat to room air which is circulating through the inside coil. When the vapor loses its heat, it condenses and turns back into a liquid. The liquid refrigerant then passes through a pressure-reducing restrictor, and the liquid becomes cold again. The cycle is repeated as the refrigerant circulates again through the coils of pipe and picks up heat from the heat source.
For cooling purposes, valves reverse the direction of the refrigerant flow. The refrigerant vapor flows from a compressor at high temperature and pressure through the outside coils. At this point, water, earth, or outside air absorbs heat from the hotter refrigerant. The refrigerant condenses and then passes through the restrictor, which lowers its pressure and thus decreases its temperature. The refrigerant absorbs heat from the room air in the inside coil. The warm refrigerant, now a vapor again, returns to the compressor and the cycle is repeated. Heat pumps are controlled by thermostats that sense the temperature of the room and turn the compressor on and off.
