Most GHP units can be equipped (optionally) with a device called a desuperheater to partially heat domestic hot water (DHW). In the summer, this device uses some of the "waste" heat from the air conditioning to heat hot water. As a result, during the cooling season, this heat is free. In the winter, some of the capacity of the heat pump is diverted from space heating to heat domestic hot water. It is important to understand, however, that the heat pump only produces domestic hot water when it is running for either space heating or cooling purposes. As a result, only a portion of the annual domestic hot water heating needs are met by the desuperheater.
The percentage of annual DHW heating needs met depends upon the run time of the heat pump and DHW use patterns in the home. The largest savings occur in applications where the heat pump runs a large number of hours (particularly in the cooling mode) and where alternative water heating is by electric resistance.
For an average family size (3.5 persons), with a 3-ton heat pump, the annual savings on domestic hot water would be in the range of 25% (colder climates) to 35% (warmer climates), or about $100 - $150 per year at $0.08/kWh. Since desuperheater capacity is directly related to heat pump capacity, the savings from a 4- or 5-ton system would be greater than the 3-ton savings cited above.