Air-source heat pumps have been used for residential and commercial heating and cooling for many years. Such units rely on air-to-air heat exchange through evaporator units similar to those used for air conditioners.
More recent heat pump technology relies on circulating a refrigerant through buried copper tubing for heat exchange. These units rely on the constancy of the ground temperature below the frost level (about 55° F) for heat transfer and are considerably more efficient than their air-source counterparts. They are known variously by such terms as ground source, earth-coupled, direct exchange or geothermal.
The most efficient ground source heat pumps use ACR, Type L or special-size copper tubing buried in the ground to transfer heat to or from the conditioned space. The flexible copper tube (typically 1/4-inch to 5/8-inch) can be buried in deep vertical holes, horizontally in a relatively shallow grid pattern, in a vertical fence-like arrangement in medium-depth trenches, or as custom configurations suited to the installation.
The ground source or geothermal heat pump has been recognized as having superior efficiency to the standard air units because the ground is almost always at a more favorable temperature than the ambient air. For example, when outside air temperature is 10 F, the ground temperature six feet under the surface may be about 40 degrees F. Again, since the heat pump efficiency is a function of the temperature difference, the geothermal system will be superior. In this case, for example, the temperature difference for the air unit will be 65 F (assuming 75 F inside temperature) but for the ground unit the temperature difference will be 75 - 40 = 35 F.
Geothermal heat pump installations involve placing pipes in the ground and circulating a fluid within the pipes to extract the heat. Arrangements can be pipes laid horizontally in a pit or trench or placed vertically in holes drilled into the earth.
The direct exchange geothermal (DXG) heat pump utilizes buried copper pipe filled with circulating refrigerant to remove heat from the earth. It does not require an additional pump to move the refrigerant through the ground or any intermediate heat exchanger. The direct exchange technology is new and has exciting possibilities. In tests performed to date, the DXG system has reduced residential heating and cooling bills by as much as half. Since the technology is new, system improvements are expected which will produce even greater savings.
The number of manufacturers which can supply commercial and residential ground source units is constantly growing. Contact the CDA to obtain the current listing.
CDA has also set up a special section - DX Geothermal Heat Pump - to provide additional information on these systems.