Heat Pump Water Heaters
Heat pumps seemingly defy the laws of physics. They take heat from a
low temperature source and, with efficiencies of 200 to 300 percent,
raise its temperature. A heat pump works like a refrigerator in
reverse. While refrigerators take heat from inside a box and dump that
heat to a surrounding room, heat pump water heaters take heat from a
room and dump that heat (at a higher temperature) in a tank to heat
water.
Heat pumps have
traditionally been used for space conditioning, but are now also being
used for electric water heating. They are two to three times more
efficient than electric resistance water heaters--meaning that they can
produce the same amount of hot water for one-half to 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.
Details
There are four basic types
of stand-alone heat pump water heaters. 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.
A typical residential HPWH
can heat 15 gallons of water per hour by 80F, with a final storage
temperature between 130F and 150F. 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-quarter the power of a standard electric resistance heater.
Installation
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.
Benefits/Costs
Compared to conventional
electric water heaters, heat pump water heaters 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.
System cost is from $600 to over $2000. Installation cost may be between $300 and $700.
Limitations
Heat pump water heaters are
most cost effective in warm climates with long cooling seasons and in
homes with high water use. They have a high initial cost compared to
other water heaters.
There may be additional
heating energy required during the heating season as a result of the
space cooling effect of the water heater.
Code/Regulatory
Precautions should be taken
to ensure back drafting does not occur when using the exhaust air type
heat pump water heaters in homes that have gas appliances or fireplaces.
Availability
There are several manufacturers of residential HPWH and they are readily available.
