Geothermal exchange is a method of heating or cooling a building that uses the thermal energy constantly given off by the Earth. At depths greater than six feet, ground temperature is relatively constant, ranging between 45 and 70 degrees Fahrenheit, depending on your latitude. These systems are more efficient because, whether it’s winter or summer, the ground temperature is usually more moderate than the outdoor temperature, so less heating or cooling is required. That means the system doesn’t have to work as hard to make your home comfortable. The EPA estimates that geothermal transfer uses 30-70% less energy for heating and 20%-50% less energy for cooling.

Varieties of Systems

There are three main components to a geothermal exchange system: the ground loop, the flow center and the indoor heat pump equipment. The cost of each component can vary according to the size of the building you’re heating, but the variety and cost of the ground loop, which is the most expensive part of the system, will also depend largely on the characteristics of the terrain on which you wish to build.

If you can build next to a pond or lake, that is probably your most economical choice. You will dig shallow (at least 6 feet deep) trenches for pipes that will lead to the water source and connect to pipe coils at the bottom of the pond or lake. This system flows the naturally heated water from the source, through the pipes, to the heat pump and through your home. After it has run through your home, the water is returned to the water source to be heated again. In these systems, known as open-loop systems, the pipes don’t have to be as long as they do in closed-loop systems, so the costs of digging or drilling and installing pipes are greatly reduced.

If you cannot build next to a water source, you will need to use a closed-loop system, which collects heat by running fluid through pipes in the ground. The heat of the earth is transferred through the pipes to the fluid - usually water or a water-antifreeze mix - and travels with the fluid to the heat pump, which pumps it through your home. The water flows through the pipes and back underground to be heated again.

There are two main types of closed-loop systems: vertically drilled or horizontal trenches. Horizontal trenches tend to be less expensive and can be used when there is a good deal of open land around the building in question - the amount of space usually found in the suburbs or in rural areas. In this case, trenches can be built parallel to the ground, at depths as shallow as 6 feet, and pipes between 100 and 400 feet in length are laid in the trenches.

Where space is more limited, your best bet is vertical piping. You will need to drill holes for the pipes to depths of 75 to 300 feet (difficulty and expense will increase as you drill deeper).

Costs and Benefits

The cost of installing any ground-loop system will also depend on geological conditions and local labor rates. Your other costs will also vary, based mainly on the size of your house. The cost of the internal duct system should not vary according to whether you use an open-loop or closed-loop system. In all, you can expect to pay $4,000 - $11,000 more for a geothermal system than you would for an air source heat pump, and to save $350 - $1475 per year on energy costs, for an average payback period of 7-12 years.

Geothermal systems come with a range of optional features that can enhance your use, including variable speed blowers and multiple-speed compressors, so you can adjust the degree of heating or cooling you receive, and desuperheaters. Desuperheaters use excess heat, in either winter or summer, to heat your water for use in the home.

For More Information

For more information on how geothermal exchange works, and to find contractors in your area, visit the website of the Geothermal Heat Pump Consortium: http://www.geoexchange.org