Geothermal heating and cooling takes advantage of the constant temperature of the earth. In the summer, heat is extracted from the building and is discharged into the ground. In the winter, heat is extracted from the ground and discharged into the building. The heat is concentrated through the use of a heat pump. Unlike air source heat pumps that work against highly variable air temperatures, geothermal heat pumps both heat and cool using a constant earth temperature.
In New England, the ground temperature is approximately 55 F degrees year round at depths greater than 20 to 30 feet below grade. Typically, for vertical geothermal installations, the borings are 300 to 600 feet deep. For horizontal installations, the trenches are 6 to 8 feet deep and hundreds of feet long. Even in horizontal installations, the ground temperature at this shallower depth offers enough capacity to adequately heat and cool your home year-round.
The table below demonstrates the temperature differences that an air source heat pump and a geothermal heat pump must overcome. Although both technologies are technically heat pumps, the geothermal system experiences a much smaller temperature difference (delta T) than the air source heat pump. The geothermal system, in effect, has less work to do to achieve the desired indoor air temperature than the air source system. This means that geothermal heat pumps are more efficient at the same outdoor temperature.