Heat sources and sinks for heat pumps can be the ground surrounding a building, water in lakes, rivers or wells, and outdoor air. Although still largely in the research stage, considerable interest has also been shown in the use of solar energy as a heat source for heat pumps. The units were sized to cope with the cooling load and in most applications provided sufficient heating capacity. When heat pumps were marketed in more northerly regions, however, the heating capacity had to be supplemented with electrical resistance heaters and other special features needed for operation in colder climates. The heat pump employs the same basic principle as the common household refrigerator, extracting heat from a space at low temperature and discharging it to another space at higher temperature.
Electrical power is needed to run the compressor in the system.
By reversing the direction of the refrigerant flow with a valve, the system can be used, as required, for either heating or cooling. The performance of a heat pump may be obtained by dividing the total useful heat output by the electrical energy needed to operate the system. Both the heating capacity and COP of the heat pump are reduced as the outdoor temperature drops. Heat pump capacity and building heat loss for a home in the Ottawa area. Supplementary heaters are staged to come on in such a manner that only the portion actually required to make up the difference between the heat pump capacity and the space heating requirement is activated.