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 mechanical 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, air conditioners, and
reversible-cycle heat pumps for providing
thermal comfort.
Heat pumps can be thought of as a 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 air conditioning (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. Some air-source heat pumps do not work as well when temperatures fall below around −5 °C (23 °F).
