Heat pump water heaters move heat from one location to another, rather than generating heat directly for providing hot water. They can be two to three times more energy efficient than conventional electric resistance water heaters.
To move the heat, heat pump systems work like a refrigerator in reverse: while a refrigerator pulls heat from inside a box and dumps it into the surrounding room, a stand-alone air-source heat pump water heater pulls heat from the surrounding air and dumps it—at a higher temperature—into a tank to heat water. Stand-alone heat pump water heating systems can be bought as an integrated unit with a built-in water storage tank and back-up resistance heating elements, but you can also retrofit a heat pump to work with an existing conventional storage water heater.
Homeowners primarily install pumps for their geothermal heating and cooling, which draw heat from the ground during the winter and from the indoor air during the summer. For water heating, a desuperheater is added to a geothermal heat pump system and uses superheated gases from the heat pump's compressor to heat water. This hot water then circulates through a pipe to the home's storage water heater tank.
In the summer, the desuperheater uses the excess heat that would otherwise be expelled to the ground. Therefore, when the geothermal heat pump runs frequently during the summer, it can heat all of your water. During the remaining seasons - when the desuperheater isn't producing as much excess heat - you'll need to rely more on your storage or demand water heater to heat the water.