A heat pump is an electrical device that extracts heat fromone place and transfers it to another. The heat pump is nota new technology; it has been used in Canada and aroundthe world for decades. Refrigerators and air conditionersare both common examples of this technology.
Heat pumps transfer heat by circulating a substance called arefrigerant through a cycle of evaporation and condensation(see Figure 1). A compressor pumps the refrigerant betweentwo heat exchanger coils. In one coil, the refrigerant is evaporatedat low pressure and absorbs heat from its surroundings.The refrigerant is then compressed en route to the other coil,where it condenses at high pressure. At this point, it releasesthe heat it absorbed earlier in the cycle.
Refrigerators and air conditioners are both examples ofheat pumps operating only in the cooling mode. A refrigeratoris essentially an insulated box with a heat pump systemconnected to it. The evaporator coil is located inside thebox, usually in the freezer compartment. Heat is absorbedfrom this location and transferred outside, usually behindor underneath the unit where the condenser coil is located.Similarly, an air conditioner transfers heat from inside ahouse to the outdoors.
The heat pump cycle is fully reversible, and heat pumpscan provide year-round climate control for your home –heating in winter and cooling and dehumidifying in summer.Since the ground and air outside always contain someheat, a heat pump can supply heat to a house even on cold winter days. In fact, air at –18°C contains about 85 percentof the heat it contained at 21°C.
An air-source heat pump absorbs heat from the outdoor air in winter and rejects heat into outdoor air in summer. It isthe most common type of heat pump found in homes at this time. However, ground-source (also calledearth-energy, geothermal, geoexchange) heat pumps, whichdraw heat from the ground or ground water, are becoming more widely used.