During the summer, the geothermal heat pump operates as a standard central air conditioner: It removes heat from the house and vents it to the outside. A liquid refrigerant is pumped through an evaporator coil of tubing. The liquid expands as it moves through the coil, changing to its gaseous state as it absorbs heat from the air surrounding the coil. A blower then pushes air around the cooled coil through ducts and into the house. The gas, now carrying considerable heat, moves through a compressor and begins the liquefying process. It then moves to a condenser coil outside the house, where the compressed gas releases its heat and returns to a liquid state.

During the winter, the geothermal heat pump reverses this process, extracting heat from the cold air outside and releasing it inside the house. The geothermal heat pump is very efficient when the outside temperature is around 45° to 50° F, but it becomes less efficient as the temperature drops. When the outside air temperature is very low (below 20° F), an auxiliary electric heater must be used to supplement the geothermal heat pump’s output.