Water circulates through loops in deep wells to take advantage of Earth's constant underground temperatures.

The principle behind a GHP system is simple. It uses the constant temperature of the Earth below the surface -- typically around 60 degrees Fahrenheit -- as a highly efficient way to heat or cool water. That water is used to heat or cool air in a building.

Electric heat pumps have been popular in climates that require both heat and air conditioning. GHPs use less electricity and are non-polluting. They've been in use for decades. To date over 750,000 GHP systems have been installed in the US.

GHP systems have two main components: the underground heat exchanger (EHX) and indoor heat pumps. The EHX is installed by drilling boreholes, often 300 to 400 feet deep, and connecting them with a manifold just below the surface. Water is circulated through pipes in the boreholes and then into the heat pumps, where it acts as either a heat sink or a heat source.

A GHP system can reduce HVAC energy bills by 20 to 60 percent, without the added emissions of switching to fossil fuel-burning sources. Maintenance costs tend to be lower because of the simplicity of the equipment.

GHP should not be confused with geothermal steam energy, which is used for heat and electric generation in regions with geothermal activity, such as Idaho and Wyoming.