Heat-Pump Water Heaters
A new breed of water heater uses time-proven heat-pump technology to cut energy consumption in half.
Synopsis: Although heat-pump water heaters have been around for 25 years, they’ve caught interest in the past few years as energy prices have spiked and as a number of manufacturers have introduced integrated appliances (as opposed to the retrofit units that made up the market previously). Heat-pump water heaters create hot water from warm indoor air and are fueled by either electricity or natural gas. The heat pump warms water in a storage tank by compressing and expanding refrigerant in a closed loop. As by-products, the heat-pump water heater generates cool, dry air and condensate. While heat-pump water heaters can be expensive, the cost can be minimized through federal tax credits and reduced energy consumption.
Heat-pump water heaters have been around for about 25 years, but I didn’t know they existed until energy costs spiked a few years ago. The relatively low cost of energy over the past 30 years is one reason for the technology’s anonymity. Another reason is that until recently, heat-pump water heaters were sold only as retrofit kits. Some homeowners might have been turned off by the Franken-tank appearance of a conversion or leery of energy-saving claims associated with bolting a metal fan box to a water heater.
That has changed recently as some big players—Rheem, GE, Stiebel Eltron—have entered the heat-pump water heater (HPWH) market with integrated appliances. With these companies’ large marketing budgets and distribution channels, and government incentives, you can expect HPWHs to become more commonplace.
Energy factor (EF) is a rating of a water heater’s efficiency based on the amount of hot water produced per unit of energy while accounting for standby losses from the storage tank and for cycling losses.
Traditional electric water heaters have an energy factor that theoretically approaches 1. That means that an electric water heater is able to convert almost all the electricity to heating the water. Oil and natural gas are less efficient; the most efficient combustion tank-type water heater has an EF of 0.67. (A typical gas-fired tankless water heater has an EF of 0.82, but a few have ratings from 0.96 to 0.98.) The downsides to electricity are that it’s expensive in many parts of the country and that it’s only as efficient and clean as the fuel source, which is often coal (see “Taking Issue” in FHB #211, “Heat Pump, Shmeat Pump”). HPWHs, however, are able to generate more energy (in the form of hot water) than they consume, and register EF values between 2.0 and 2.8.
Hot water from warm air
The key to a heat pump’s efficiency is that it doesn’t create heat. Instead, it transfers heat from one place to another. There is nothing mysterious about how an HPWH works. Every refrigerator uses a heat pump to do its job. The difference is that a refrigerator pumps heat out of an insulated box (think of the warm air vented from the bottom of a refrigerator) while an HPWH sends heat into an insulated tank.
By compressing and expanding refrigerant in a closed loop, an HPWH extracts heat from surrounding air and delivers it to the tank. The neat thing about this process is that despite the fact that it’s extracting heat from air that might be only 60°F, it can raise water temperatures as high as 140°F.
This multiplier effect is measured by something known as the coefficient of performance, or COP. HPWHs can achieve COP values between 3 and 5. That translates into 300% to 500% efficiency for every kilowatt of electricity used to power the fan and compressor. There’s a catch, though: COP varies by climatic conditions. Heat pumps are most efficient when the surrounding air is warm and humid. When comparing the efficiency of different models, be sure that COP is calculated under the same conditions. Usually, the manufacturer-listed COP is around 3 and is calculated with a temperature in the 60s and relative humidity in the 70% range.
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