Heat Pump Water Heaters

Heat pump water heater (HPWH) systems extract energy content from a heat source, usually air, to efficiently heat water. Depending on cold water and ambient air temperatures and on patterns of hot water use, HPWHs do the same job as standard electric water heaters but use half the electric energy. These systems use a motor to run a compressor that draws a gaseous refrigerant through an evaporator, raising its pressure until it liquefies in the condenser (Figure 1). This familiar process heats the condenser and cools the evaporator. In wringing the heat from air, HPWHs both cool and dehumidify the air that passes through them, thus helping to meet space-conditioning needs during cooling seasons.

Figure 1: The heat pump cycle
In a heat pump water heater, air is cooled as it passes through the evaporator’s fins (with the help of a fan to improve efficiency), while at the same time, water is heated as it passes through the condenser’s heat-exchange surfaces.

But if HPWHs are so efficient, why aren’t more businesses purchasing them? An uninformed design community, historical reliability issues, and high initial costs are the three main culprits. For the most part, the only engineers with the knowledge to properly design these systems work for the manufacturers themselves. When HPWHs first entered the market, poorly designed and unreliable products made consumers skeptical of the technology. Also, the complexities of HPWHs make them more difficult to install than standard water heaters, raising the price of installation as well as the opportunity for installation errors. These difficulties continued throughout the 1990s and early 2000s for both residential and commercial systems, which led to a profound decline in the use of HPWHs. Largely as a result of this decline, a continuing barrier to HPWH sales is that very few HVAC distributors carry them, and many contractors are unaware that they even exist and therefore do not advocate for them. However, although commercial systems remain underutilized, residential systems are making a comeback since the US Environmental Protection Agency and Department of Energy’s Energy Star program released its 2009 Water Heater Specifications, increasing the availability of integrated systems and making them simpler to choose and install. And don’t let the name mislead you—residential HPWHs can supply adequate hot water for many types of small commercial facilities.

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