Geothermal heat represents about two percent of the total heating market in the U.S., with more than 1.3 million systems installed. A geothermal system does not burn fossil fuels to create heat. It transfers heat. And that's its charm: To transfer heat takes less energy than to produce heat.
How Geothermal Systems Work
To harness the heat stored in the earth, a geothermal system captures and converts that heat for use in the home. System components include a loop of pipe, a liquid to absorb and transfer heat, and a heat-pump unit to process the heat for use. To capture heat, liquid circulates through a pipe buried in the ground. As it circulates, it absorbs the earth's stored heat, which remains constant at 50 to 60 degrees Fahrenheit 10 feet below ground level.
The heated liquid enters a heat pump unit. In this unit, the heat from the piped liquid is absorbed by a liquid refrigerant sealed in the unit. That refrigerant evaporates and is compressed, which raises its temperature to about 100 degrees Fahrenheit.
Now a gas, the refrigerant passes through a heat exchanger where the heat is removed and pumped into the house. With the heat removed, the refrigerant cools. It returns to its liquid state and continues to circulate, continually absorbing and using the heat transferred from the earth through the piped liquid.
Efficient, Low-Maintenance Heating
Geothermal heat pumps have become quite efficient. Their heating efficiency is indicated by the coefficient of performance, or COP, the ratio of heat provided per British thermal unit (Btu) of energy input.
Energy Star-rated heat pumps have a heating COP starting at 2.8, meaning for every unit of energy used to power the system, 2.8 units of heat are supplied.
Geothermal systems are simple to maintain. A properly installed and buried loop can last nearly 50 years. The mechanicals are installed indoors and typically require only periodic checks and filter changes.
Geothermal heating systems operate as either closed-loop or open-loop configurations. Determining which loop to use depends on site factors like soil composition, landscaping, and underground utilities.
A closed loop uses the liquid solution in a sealed piping loop installed horizontally or vertically underground.
Horizontal loops are used when there is enough usable land available. Pipes are installed in trenches dug about six feet deep and 100 to 600 feet long, depending on the size of the system.
Vertical loops are the only choice when there is limited space available, if the homeowner does not want landscape disturbed, or where many rocks would be encountered when digging. To install the pipe, small-diameter holes 100 to 400 feet deep are bored using well-drilling equipment. Vertical loops are connected to the house via a horizontal underground pipe. When boring for vertical loops, well-drilling codes apply.
A sealed system can also be placed at the bottom of a pond if there is a pond available on the property. Pond loops may be the most economical option because much of the excavation cost can be eliminated.
An open-loop system uses the heat from well water rather than heat from the earth. Groundwater, which also remains at a relatively constant temperature year-round, is carried into the heat pump unit and the heat is extracted in a method similar to the closed-loop system.
The water does not circulate but makes one pass and is eliminated. It might be released into a ditch, drainage tiles, or a pond. It might also be returned to the water table through a return well drilled into the ground. With concerns about declining and polluted aquifers, however, it's important to check local conditions and codes before deciding on this type of system.
Costs and Payback
Initial installation and equipment costs for geothermal heat pumps vary with the maturity of the local market, type and size of the system, and the site. There is no doubt that the system will cost more at the start than a conventional fossil-fuel furnace.
If a home does not have ductwork, a homeowner may need to add that into the cost. However, a small home that uses baseboard heat may be able to forego duct installation.
Rough estimates put a geothermal ground-source system at $1,000 to $2,500 per ton of capacity. A ton of capacity, according to the Department of Energy's Federal Energy Management Program, translates to 12,000 Btu per hour. In heating climates, it is estimated that a ton of capacity is needed for every 550 square feet.
The cost appeal of geothermal heat is in the operating payback. The system delivers more energy per unit consumed than conventional systems, up to 400 percent efficiency compared with 75 to 90 percent efficiency for fossil-fuel furnaces.
According to Jim Bose, executive director of the International Ground Source Heat Pump Association at Oklahoma State University in Stillwater, OK, an existing home with an older furnace could expect an efficiency improvement of around 50 percent by switching to geothermal. A new home with the best fossil-fuel furnace would expect an improvement of 30 percent.
Get the Most from Your Geothermal System
Don't expect a new system to solve your heating problems unless you reduce your heating load. Seal all leaks. Check to be sure the weather stripping is in good shape and the duct system is not leaking. Eliminate drafts. Consider having a heat loss/heat gain/leakage evaluation done of the home.
Find a company that is certified, with people trained specifically in geothermal technology, and get more than bid. Ask for references and call those homeowners. Ask to see installations the company has completed.
Discuss the benefits of a hybrid system. A ground-source heat pump can be added to an existing forced-air furnace and use its blower. Dual-source heat pumps are less costly to install and more efficient than the air-source unit alone. Ask about variable-speed blowers and multi-speed compressors on the system to improve comfort and efficiency.
Consider the system's ability to produce hot water. A device called a “desuperheater” can supplement the production of domestic hot water by using the excess heat when the system is operating. For those who want the system to provide for all hot water needs, there are some full-demand systems offered that use a separate unit for domestic hot-water use.