In areas where winters and summers are fairly mild, heat pumps offer an affordable alternative to other heating and cooling systems. As temperatures become more extreme, however, their advantages wane.
Heat pumps extract heat from the air outdoors, so they operate more efficiently when temperatures are moderate. If the heat pump is unable to provide sufficient heat, it may resort to emergency heating mode, which relies on inefficient electrical resistance to generate warm air. Boilers and furnaces often offer better efficiency during long, cold winters.
However, heat pumps offer another advantage: almost all provide cooling as well and can operate more efficiently than air conditioners. As a result, homeowners can purchase a single unit that supplies both heating and cooling.