GeoXchange 101

How Geothermal Heating and Cooling Systems Work

Geothermal heating and cooling systems rely on the fact that the temperature below the frostline remains at a relatively constant temperature of 50-60°F (Fahrenheit) throughout the year. The ground is warmer than the outdoor air in the winter and cooler than the outdoor air in the summer. Thus, the ground is a heat source in winter and a heat sink in summer.

The geothermal heating and cooling system includes three principal components:

  • Geothermal heat exchange located in the ground
  • Geothermal heat pump located inside the building
  • Distribution system (air handler, ductwork, radiant heat, radiators, etc.)

Geothermal Heat Exchange

There are two basic kinds of geothermal heating and cooling exchange systems: water-source and direct exchange.

The Water-Source Exchange system consists of either a water supply and return (open loop) or a closed loop containing water or a water/anti-freeze mix.

Open loop systems exchange heat water-to-water, circulating it from a well, or local pond or lake, through the geothermal heating and cooling system of the building. Then it is sent back to the pond or lake or through another discharge well. High-density polyethylene (HDPE) pipe is used to convey the water. The water sources, of course, derive their heat from the ground.

Closed loop systems exchange heat water-to-ground, circulating a water/anti-freeze mix through the geothermal heating and cooling system of the building to the ground and back using HDPE pipe. To maximize contact with the earth, the pipes are coiled and buried either vertically or horizontally near the building. There is no connection to any ground source water.

The Direct Exchange (DX) system exchanges heat refrigerant-to-ground. The copper pipe filled with refrigerant is buried below ground in direct contact with the earth, or in water baths, which exchange heat with the water and then the ground. Since copper is an excellent heat conductor, the tubes are narrower, fewer loops are needed and less space is necessary.

Whether water-source or direct exchange, the pipes circulate fluid that absorbs heat from the ground in winter when the air is colder than the ground. In summer, the fluid releases heat to the ground when the air is warmer than the ground.

Geothermal Heat Pump

The heart of this geothermal technology is the heat pump. This is alternately known as the water-source heat pump, ground-source heat pump and earth-coupled heat pump. It “exchanges” heat with the fluid-filled pipe.

The compressor in the heat pump boosts the temperature gradient so that in winter, it amplifies that heat and transfers the warmed refrigerant to the distribution subsystem. In summer, it magnifies the cooling and transfers the cooled refrigerant to the distribution subsystem.

Distribution System

The heat distribution system is similar to a conventional HVAC system. It consists of a thermostat, compressor and air handler, which circulates the heated or cooled air from the heat pump throughout the building using conventional ductwork. Geothermal exchange can also provide water heating for hydronic radiant floors and/or domestic hot water, using a desuperheater.