Even cold air contains a great deal of heat; the temperature at which
air no longer carries any heat is well below -200 degrees Fahrenheit.
But the coldest temperature ever recorded in the lower 48 states was
-70 degrees, recorded at Roger Pass, Montana on January 20, 1954. Obviously
in such weather, a heat pump would have to work pretty hard to produce
68-degree temperatures inside your home.
That's why geothermal heat pumps are so efficient.
Geothermal heat pumps are similar to ordinary heat pumps, but instead
of using heat found in outside air, they rely on the stable, even heat
of the earth to provide heating, air conditioning and, in most cases,
From Montana's -70 degree temperature, to the highest temperature ever
recorded in the U.S. - 134 degrees in Death Valley, California, in 1913
- many parts of the country experience seasonal temperature extremes.
A few feet below the earth's surface, however, the ground remains at
a relatively constant temperature. Although the temperatures vary according
to latitude, at six feet underground, temperatures range from 45 degrees
to 75 degrees Fahrenheit.
Ever been inside a cave in the summer? The air underground is a constant,
cooler temperature than the air outside. During the winter, that same
constant cave temperature is warmer than the air outside.
That's the principle behind geothermal heat pumps. In the winter, they
move the heat from the earth into your house. In the summer, they pull
the heat from your home and discharge it into the ground.
Studies show that approximately 70 percent of the energy used in a
geothermal heat pump system is renewable energy from the ground. The
earth's constant temperature is what makes geothermal heat pumps one
of the most efficient, comfortable, and quiet heating and cooling technologies
available today. While they may be more costly to install initially than
regular heat pumps, they can produce markedly lower energy bills - 30
percent to 40 percent lower, according to estimates from the U.S. Environmental
Protection Agency, who now includes geothermal heat pumps in the types
of products rated in the EnergyStar® program. Because they are mechanically
simple and outside parts of the system are below ground and protected
from the weather, maintenance costs are often lower as well.
As an added benefit, systems can be equipped with a device called a "desuperheater" can
heat household water, which circulates into the regular water heater tank.
In the summer, heat that is taken from the house and would be expelled
into the loop is used to heat the water for free. In the winter, the desuperheater
can reduce water-heating costs by about half, while a conventional water
heater meets the rest of the household's needs. In the spring and fall
when temperatures are mild and the heat pump may not be operating at all,
the regular water heater provides hot water.
How Do They Compare?
Surveys taken by utilities have found that homeowners using geothermal
heat pumps rate them highly when compared to conventional systems. Figures
indicate that more than 95 percent of all geothermal heat pump owners
would recommend a similar system to their friends and family.
Geothermal heat pumps are durable and require little maintenance. They
have fewer mechanical components than other systems, and most of those
components are underground, sheltered from the weather. The underground
piping used in the system is often guaranteed to last 25 to 50 years
and is virtually worry-free. The components inside the house are small
and easily accessible for maintenance. Warm and cool air is distributed
through ductwork, just as in a regular forced-air system.
Since geothermal systems have no outside condensing units like air
conditioners, they are quieter to operate.
How Do They Work?
Remember, a geothermal heat pump doesn't create heat by burning fuel,
like a furnace does. Instead, in winter it collects the Earth's natural
heat through a series of pipes, called a loop, installed below the surface
of the ground or submersed in a pond or lake. Fluid circulates through
the loop and carries the heat to the house. There, an electrically driven
compressor and a heat exchanger concentrate the Earth's energy and release
it inside the home at a higher temperature. Ductwork distributes the
heat to different rooms.
In summer, the process is reversed. The underground loop draws excess
heat from the house and allows it to be absorbed by the Earth. The system
cools your home in the same way that a refrigerator keeps your food cool
- by drawing heat from the interior, not by blowing in cold air.
The geothermal loop that is buried underground is typically made of
high-density polyethylene, a tough plastic that is extraordinarily durable
but which allows heat to pass through efficiently. When installers connect
sections of pipe, they heat fuse the joints, making the connections stronger
than the pipe itself. The fluid in the loop is water or an environmentally
safe antifreeze solution that circulates through the pipes in a closed
Another type of geothermal system uses a loop of copper piping placed
underground. When refrigerant is pumped through the loop, heat is transferred
directly through the copper to the earth.
Types of Loops
Geothermal heat pump systems are usually not do-it-yourself projects.
To ensure good results, the piping should be installed by professionals
who follow procedures established by the International Ground Source
Heat Pump Association (IGSHPA). Designing the system also calls for professional
expertise: the length of the loop depends upon a number of factors, including
the type of loop configuration used; your home's heating and air conditioning
load; local soil conditions and landscaping; and the severity of your
climate. Larger homes requiring more heating or air conditioning generally
need larger loops than smaller homes. Homes in climates where temperatures
are extreme also generally require larger loops.
Here are the typical loop configurations:
Horizontal Ground Closed Loops
This type is usually the most cost effective when trenches are easy
to dig and the size of the yard is adequate. Workers use trenchers or
backhoes to dig the trenches three to six feet below the ground in which
they lay a series of parallel plastic pipes. They backfill the trench,
taking care not to allow sharp rocks or debris to damage the pipes. Fluid
runs through the pipe in a closed system. A typical horizontal loop will
be 400 to 600 feet long for each ton of heating and cooling.
Vertical Ground Closed Loops
This type of loop is used where there is little yard space, when surface
rocks make digging impractical, or when you want to disrupt the landscape
as little as possible. Vertical holes 150 to 450 feet deep - much like
wells - are bored in the ground, and a single loop of pipe with a U-bend
at the bottom is inserted before the hole is backfilled. Each vertical
pipe is then connected to a horizontal underground pipe that carries
fluid in a closed system to and from the indoor exchange unit. Vertical
loops are generally more expensive to install, but require less piping
than horizontal loops because the Earth's temperature is more stable
farther below the surface.