Heat pumps provide very efficient heat in all by the coldest days of the year.

Reversible-cycle heat pumps are often seen in providing building-space heating in high latitude climates that are much warmer than comfortable in one season, but colder in another season. In heating, ventilation, and air conditioning applications, the term heat pump normally refers to a vapor-compression refrigeration device that includes a reversing valve and optimized heat exchangers so that the direction of thermal energy flow may be changed without loss of efficiency. Most commonly, when used in heating, heat pumps draw heat from the air or from the ground.