Heat naturally moves from warmer places to cooler places. Heat pumps use electricity to move heat in the opposite direction, from cooler places to warmer places, making the cool space cooler and the warm space warmer. Your refrigerator is a heat pump, moving heat from inside the refrigerated cabinet (cooler place) into your kitchen (warmer place). An air conditioner is a heat pump, moving heat from inside your home (cooler place) to the outside summer air (warmer place).
Heat pumps are also used to heat homes, taking advantage of the fact that in the spring, fall, and yes, even in the winter, there is some heat in the outdoor air. An air-source heat pump collects heat from the outdoor air, concentrates it via an outdoor compressor, and distributes it inside through an indoor room unit or a home’s ductwork. During the summer, a heat pump can operate in reverse, as an air conditioner, by moving heat from indoors to the outside air. Heat pumps require electricity to run, but can deliver more energy than they use.
Air-source heat pumps have been used for many years in nearly all parts of the United States, but until recently they have not been used in areas that experienced extended periods of subfreezing temperatures. However, in recent years, air-source heat pump technology has advanced so that it now offers a legitimate space heating alternative in colder regions. Over 300,000 heat pumps were sold in New England, New York, New Jersey, Pennsylvania, Delaware, Maryland, and Washington DC in 2015 alone.