Just 6 to 8 feet below the earth's surface, the ground remains at a
relatively constant temperature. The stable, even heat of the earth
can be used as a heat source in the winter and a heat sink in the
summer to provide residential and commercial buildings with heating
and air conditioning, using a geothermal heat pump.
geothermal heat pump is one of the most efficient and
environmentally friendly heating and cooling systems available
today," said Xinlei Wang, a professor in the Department of
Agricultural and Biological Engineering at the University of
A normal heat pump system extracts heat from outdoor air and
transfers it inside, where it is circulated through your home's
ductwork by a fan, said Wang.
"If you want to keep it 70 degrees inside and it's below zero
outside, a heat pump would have to work pretty hard to produce those
temperatures," Wang explained. "But if we use underground heat as
our source, even on the coldest day the temperature is still 45 to
50 degrees 6 to 8 feet below ground. Going from 50 to 70 degrees,
your efficiency will be much higher."
A geothermal heat pump, known as a GHP, works by collecting the
earth's natural heat through a series of pipes, called a loop, that
are installed below the surface of the ground. These loops can also
be submersed in a pond or lake. Fluid in the loop, either water or
an environmentally safe antifreeze solution, circulates through the
pipes in a closed system and carries the heat to the house. An
electrically driven compressor and a heat exchanger concentrate the
energy and, using typical ductwork, release it inside the home at a
In the summer, the loop draws excess heat from your home and
transfers it back into the ground. The system works the same way
your refrigerator keeps your food cool -- by drawing heat from the
interior, not by blowing in cold air.
There are several loop configurations that can be used, said
Wang, but one of the most common is the horizontal ground loop.
"Horizontal ground loops are used where there's sufficient land
available and where trenches are easy to dig," said Wang.
Workers dig trenches 4 to 6 feet deep, 2 feet wide, and then lay
a series of parallel plastic pipes. A typical horizontal loop will
be 400 to 600 feet long for each ton of heating and cooling, and an
average residence requires a 3- to 4-ton unit.
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If you have little yard space or you don't want to disrupt the
landscape, a vertical ground loop can be used, said Wang.
Vertical holes, about 4 inches in diameter, are bored into the
ground 150 to 450 feet deep. A single loop of pipe with a U-bend at
the bottom is inserted before the hole is backfilled. The vertical
pipe is then connected to a horizontal underground pipe that carries
fluid to and from the indoor exchange unit.
A third alternative is the pond loop system. If your property has
a pond or lake with an adequate supply of water, a supply line pipe
can be run underground from your residence to the water. The pipe is
coiled into circles at least 8 feet under the surface to prevent
freezing. The water source must meet minimum volume, depth and
Because of the technical knowledge and equipment needed to
properly install a GHP system, Wang advises finding a qualified
installer to do the job. Your local utility company or the
Geothermal Heat Pump
Consortium should have a listing of qualified installers in your
On average, a geothermal heat pump system costs about $2,500 per
ton of capacity, although a horizontal ground loop system will
generally cost less than a system with vertical loops. And though
more conventional heat pump systems cost about half that amount to
install, Wang said the savings over time more than make up for the
GHP's installation costs.
"Heating efficiencies are 50 to 70 percent higher than other
heating systems, and cooling efficiencies are 20 to 40 percent
higher than available air conditioners," Wang said. "Most of the
components are sheltered from the harsh weather, and the underground
piping is guaranteed to last anywhere from 25 to 50 years, so
there's very little maintenance. These systems generally pay for
themselves in five to eight years."
Many people worry about the reliability or durability of some
alternative energy technology, said Wang, because it's not mature
"But this is one technology that is very mature," Wang concluded.
"People can use it."
[Text from file received from
the University of Illinois
College of Agricultural, Consumer and Environmental Sciences]