In the right situation and location, heat-pump water heaters offer improved efficiency and increased energy savings compared to standard tank-style units. Here’s how to tell if your household would benefit from this technology.
Using electricity to run a compressor, a heat-pump water heater (HPWH) extracts heat from air, often in the basement, and transfers that energy to heat water in a tank. It’s like a refrigerator, only running backward: A refrigerator moves heat from its interior to the room it’s located in. An HPWH transfers heat from the room to a storage tank, and does it with remarkable efficiency. In fact, an HPWH is able to transfer more energy (up to 2.5 times) than the electricity it consumes. Today’s HPWHs are much less expensive and more efficient than their predecessors.
A heat pump has three basic components: an evaporator, a compressor, and a condenser. Liquid refrigerant is exposed to room-temperature air in the heat pump’s evaporator coil. As it picks up energy from the room air, the refrigerant changes from a liquid to a gas within the evaporator coil. The refrigerant passes into the compressor pump, which compresses the gas, increasing its temperature. The heated refrigerant then runs through a condenser, which is either a coil that is wrapped around the integral tank, or a pumped heat exchanger that feeds a separate water storage tank. As the hot refrigerant gives up its heat to the colder water, the refrigerant cools and condenses back into a liquid and passes through an expansion valve, where its pressure is reduced and the cycle starts over.
By taking advantage of the unique characteristics of the refrigerant, the heat pump can extract energy from the surrounding room air (slightly chilling that air). How much energy is extracted depends on the ambient air temperature and the tank temperature.
This transformation of one unit of electrical energy into three units of heat is the heating advantage that is unique to HPWHs. This increase in the heating effect is known as the coefficient of performance (COP). Most HPWHs have COPs between 2 and 4, depending on room temperature and humidity, and water temperature. By comparison, a conventional electric water heater is considered to have a COP of 1.
The U.S. Department of Energy considers the COP of HPWHs to be about 2, which accounts for storage losses as heat is lost through the walls of the storage tank. The temperature of the room in which the HPWH is housed, the insulation of the storage tank, and hot water usage frequency all impact the HPWH’s COP. For our purposes, we are only considering the actual heating performance. However, it is critical that, with any heat source, the heat storage tank be well-insulated to minimize heat loss.
The energy to feed the heat pump can come from passive solar gain, heat from a conventional heating system, the warming effect of soil surrounding a basement wall, or any other heat source. The resulting cooling of the room by the HPWH is advantageous in the summer, but a disadvantage in the winter. If the unit is installed in a basement, the larger the basement, the less impact it will have on the living area. Additionally, in basement installations in humid regions, a significant benefit of an HPWH is that it also removes some moisture from the air, reducing and usually eliminating the need for a separate dehumidifier. Since HPWHs operate as dehumidifiers, a condensate drain must be provided.