Geothermal Heat Pumps
Q — My old electric furnace is on its last legs and has needed service
lately. I was considering installing a geothermal heat pump this time. With its
higher installation cost, does this make sense for me?-Paul G.
A — A geothermal heat pump is probably the most efficient heating and
air-conditioning system available for any home today. In the heating mode, the
heat blowing out of the registers is often warmer than with a standard air-to-air
heat pump, so comfort during the winter is better.
Throughout its life, installing a geothermal heat
pump usually makes economic sense for the average home, but your electric co-op
can do a payback analysis for you. If your house is extremely energy efficient
and already has low utility bills, even cutting them in half with a geothermal
heat pump may not justify its higher cost.
A geothermal heat pump both heats and cools your
home like a common air-type heat pump, but that is where the similarities end.
In the heating mode, a geothermal heat pump pulls free heat from a ground loop,
well water, lake water, etc., for super-high efficiencies.
During the summer, the energy savings can be as much
as 60 percent. Some geothermal models have efficiencies as high as a seasonal
energy efficiency rating (SEER) of 22, compared to an old central air conditioner
at about 8. The dehumidification factor is also often better for improved indoor
air quality, less allergy problems, and better comfort.
When operating in the air-conditioning mode, most
geothermal heat pumps offer the option of using a desuperheater device. This
device takes the waste heat from your house that is normally exhausted outdoors
and diverts it to your water heater for free hot water during the summer.
Geothermal heat pumps are energy efficient because
they use the earth as the source of heat during the winter, and during the summer
they exhaust indoor heat to the earth. Since the ground temperature several
feet below the surface stays relatively constant year-round, a geothermal heat
pump uses less electricity to heat and cool your house.
During the winter, even though it feels cold outdoors
when it is 30 degrees, there is still heat energy in the air that a standard
heat pump can "pump" into your house. At the same time, the ground
temperature may be 55 degrees. It makes intuitive sense that it is easier to
pull heat from the ground at 55 degrees than from the air at 30 degrees. The
colder it gets outdoors, the greater the benefit of a geothermal heat pump becomes.
To install a typical ground-loop system, a series
of small pipes is placed in deep narrow trenches dug in your yard with a backhoe.
Drilling deep vertical holes can also be used. The pipes are connected to a
heat exchanger inside the heat pump. An antifreeze solution runs through the
pipes to capture heat during the winter or exhaust heat during the summer.
Another efficient design (called DX) does not use
an antifreeze solution in pipes or a heat exchanger. Instead, thin copper tubing
is placed in the ground and the actual refrigerant (freon or R410A) runs directly
through it. This requires less overall ground-loop length and is ideal for smaller
yards. The inefficiencies of using an extra heat exchanger are also eliminated.
For ultimate comfort and efficiency, select a two-stage
model with a variable-speed blower. This allows it to constantly fine-tune the
heating and cooling (and electricity usage) to the varying needs throughout
the day.
