A heat pump is a machine or device that moves heat from one location (the ‘source’) to another location (the ‘sink’ or ‘heat sink’), using work. Most heat pump technology moves heat from a low temperature heat source to a higher temperature heat sink. Common examples are food refrigerators and freezers and air conditioners and reversible-cycle heat pumps for providing thermal comfort.
Heat pumps can be thought of as an heat engine which is operating in reverse. One common type of heat pump works by exploiting the physical properties of an evaporating and condensing fluid known as a refrigerant. In heating, ventilation, and cooling (HVAC) applications, a heat pump normally refers to a vapor-compression refrigeration device that includes a reversing valve and optimized heat exchangers so that the direction of heat flow may be reversed. Most commonly, heat pumps draw heat from the air or from the ground. Air-source heat pumps do not work well when temperatures fall below around −5°C (23°F).