A desuperheater is a secondary heat exchanger that transfers heat from the earth in the winter, and from your home in the summer, into your domestic hot water tank. The desuperheater is part of the geothermal heat pump's domestic hot water generating system (HWG).
Hot water generators only heat domestic water when your geothermal heat pump is heating or cooling your home. The hot water generator's water circulating pump moves the cold water from the bottom of your hot water tank through a water pipe to the desuperheater itself, where the water is heated by the heat that has been transferred from the earth when you are heating your home, and from the inside of your home when you are cooling your home. The heated water is then circulated back into your hot water tank.
Heating water with a geothermal heat pump's hot water generator costs about 80 percent less in a heating dominated climate, and up to 95 percent less in a cooling dominated climate, than if you heat your domestic water with an electric, oil, or propane fired hot water heater. At the present cost of natural gas the savings is about 15 percent less - not as drastic a savings as the others, but saving on your bills is still saving on your bills!
In the summer heating your domestic water is almost free. This is because the heat that is being removed from your home is transferred into your hot water tank. Since this heat from inside of your home was going to be rejected (thrown away) into the earth anyway, putting it into your hot water tank instead is free. The only cost for the summer water heating is the small cost of running the circulating pump that moves the water.