A growing number of U.S. homeowners are taking advantage of renewable energy such as solar and wind, but another option lies just a few feet underground, where the earth maintains a year-round temperature of about 55 degrees Fahrenheit (ºF). Geothermal heat pumps (GHPs), also known as ground-source or geoexchange heat pumps, take advantage of this constant temperature to provide highly efficient heating and cooling with a much lower environmental impact than fossil fuels.

A Solution to Fit Every Home

A GHP system has three main components: an indoor heat pump (which includes a refrigerant loop and compressor), an outdoor "ground loop" of fluid-filled pipe that transfers heat between the ground and the refrigerant loop, and ducts that distribute heated or cooled air throughout the home. In the winter, the ground loop transfers heat from the ground to the refrigerant, evaporating it into a gas. The heat pump then compresses the gas, making it hot enough (about 100ºF to 120ºF) to heat your home. In the summer, the process is reversed. A GHP system with a "desuperheater" can also make hot water for the home from the compressed refrigerant gas.

A given GHP system design requires a loop of a specific length, which can be configured in various ways based on the amount of available land, the home’s heating and cooling needs, and the thermal conductivity of the ground. Pipes can be buried horizontally in shallow trenches, as straight pipe runs or coiled loops ("slinkies") if space permits, or placed vertically in deeply drilled holes if space is at a premium. Most systems circulate a mixture of water and nontoxic antifreeze through a closed loop. Open-loop systems that circulate water to and from a deep pond or well may be an option depending on local regulations. An experienced installer can help you decide which configuration is best (see below).

Is the Price Right?

GHP systems move three to five units of energy for every unit of energy used to run the pump, making them highly efficient; as a result, heating costs can be 40 to 70 percent lower than conventional systems, and cooling costs can be 30 to 50 percent lower, according to U.S. Department of Energy (DOE) figures. The up-front costs, however, may be a hurdle for prospective buyers. The pump costs about $2,500 per ton of heating capacity, which covers approximately 500 to 700 square feet of living space; a typical home requires about three tons of capacity. There are additional costs for ground loop installation, which can vary greatly depending on the loop configuration, and for any changes you make to an existing heat distribution system (such as ductwork). The total cost generally runs $20,000 or more.

The federal government offers homeowners help in the form of a tax credit equal to 30 percent of the installed cost of GHP systems, with no upper limit; some local governments and utilities offer tax credits or rebates as well. Together, these incentives can lower the up-front costs significantly and, when combined with much lower heating and cooling costs, reduce the time it takes a GHP system to pay for itself to as little as five years.

To learn more about how GHP systems work, and to find knowledgeable contractors in your area, visit the DOE’s Energy Efficiency and Renewable Energy website at www1.eere.energy.gov/geothermal/heatpumps.html.