Heat-Pump Water Heaters

A heat pump relies on the well-understood refrigerant cycle to move heat from one place to another. The same basic principle used for a refrigerator can also be used to heat water. While an electric-resistance water heater (the standard electric storage water heater that is sold by the millions each year) converts electricity directly into heat, a heat pump water heater uses electricity to extract heat from the room and deliver it—at a higher temperature—into the water. In doing so, the “efficiency” jumps from somewhat less than 100% (for electric resistance water heating) to more than 200%.

Heat-pump water heaters can either be self-contained, with a heat-pump unit that’s directly integrated with the storage tank, or modular, with an add-on heat-pump unit that connects to a standard storage-type water heater. In either case, a fan pulls air from the room into a heat exchanger core where its heat is transferred to refrigerant in copper coils—this occurs in the evaporator coil. The refrigerant is colder than the circulating air, so heat is transferred to the refrigerant, evaporating it. A compressor mechanically compresses the refrigerant gas, raising its temperature. The warm refrigerant gas then passes through a condensing coil, where it is condensed by the (cooler) circulating water from the storage tank, heating the water in the process. The refrigerant liquid then passes through an expansion valve where the temperature drops and the cooler refrigerant enters the evaperator coil to repeat the cycle.