Heat pumps have traditionally been used for space conditioning, but are now also being used for electric water heating. They are usually three times more efficient than electric resistance water heaters--meaning that they can produce the same amount of hot water for one-third the amount of electricity. Stand-alone heat pump water heaters (HPWH), which are described here, are different from heating and cooling heat pump systems that have integrated water-heating capability.
There are four basic types of stand-alone HPWH. The heat pump can be integrated with or separate from the hot water storage tank, and cool exhaust air can be exhausted to the room or to the outdoors (see Figures). Because HPWH take heat from the surrounding room air, they cool and dehumidify a space. This is a benefit during the cooling season and a drawback during the heating season.
An add-on HPWH can be used to convert an electric resistance water heater into a heat pump water heater. The optimum locations would be in a utility room, garage or basement; the unit requires air circulation to work effectively.
A typical residential HPWH can heat 15 gallons of water per hour by 80°F, with a final storage temperature between 120°F and 140°F. At the same time, a HPWH provides some room cooling. During the heating season, this incidental cooling increases space heating needs.
A typical residential HPWH draws less than one-third the power of a standard electric resistance heater. Some HPWH models only require 110v electricity, making them ideal for the "do-it-yourself" home owner.
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Because of their high efficiency, HPWH can significantly reduce energy needs for heating water.
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Trained contractors are recommended.
System cost is from $600 to over $2000. Installation cost may be between $300 and $700. Estimated payback is 2 - 5 years.
Because they operate at a small fraction of the energy for typical water heaters, HPWH can save from a third to half of water heating costs. Heat pump water heaters are most cost effective in warm climates with long cooling seasons and in homes with high water use. There may be additional heating energy required during the heating season as a result of the space cooling effect of the water heater.
Section M1403 of the International Residential Code states that heat pump water heaters must conform to Underwriter’s Laboratories’ standard UL 1995.
In homes that have gas appliances or fireplaces, precautions should be taken to ensure backdrafting does not occur when using the exhaust air type heat pump water heaters.
There have been no PATH field evaluations of HPWH to date. However, the U.S. Department of Energy’s Oak Ridge National Laboratory has conducted long-term research on heat pump water heaters.
Heat pump water heaters that draw room air must be located in a room large enough to prevent over-cooling of the space. Exhaust-air HPWH should not be used in residences that have gas stoves or fireplaces unless there is a separate make-up air supply to avoid backdrafting.
HPWH need to be installed by an experienced contractor.
Warranties range from 1 year to 7 years, sometimes only on certain parts, and vary by manufacturer.
Compared to conventional electric water heaters, HPWH are more energy efficient and draw less power. Some units can provide air conditioning or residential ventilation. HPWH are most applicable where electric rates are high, where hot water use is high, and where space cooling or ventilation needs are high.
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