Geothermal is often referred to as ground source energy because it is energy that comes from underground. It is used to heat and cool homes and buildings. Although geothermal systems need some electrical energy to run, they are very energy efficient because heat is moved rather than generated. They take advantage of the fact that the ground stays at a constant 55 degrees Fahrenheit anywhere from 5 to 12 feet below the surface.
A study by the U.S. Environmental Protection Agency (EPA) found that geothermal systems have the lowest life-cycle cost of all heating and cooling systems. They have higher installations costs than conventional heating systems, but generally require less maintenance and repairs and can often last for generations. See “Is geothermal right for you?” below.
How Ground Source Geothermal Systems Work
- A geothermal system takes advantage of the constant temperatures underground. It is made up of a loop of pipes or coils that are buried underground and a heat pump inside a building at ground level.
- When the weather is cold, water or a refrigerant heats up as it travels through the part of the loop that is buried underground.
- Once it gets back above ground, the water or refrigerant is transferred to a heat pump that warms it further and then transfers the heat to the building.
- The water or refrigerant cools down after its heat is transferred and is pumped back underground where the process is restarted.
- When the weather is hot, the heat pump can run in reverse. It cools the water or refrigerant and transfers the extra heat back into the ground.
- The pump needs electricity to run, but it uses considerably less electrical energy than the heating and cooling energy it produces.
The efficiency of geothermal heat pumps is measured by comparing the amount of energy the system moves to the amount it uses. The rating for heating efficiency is measured as a coefficient of performance (COP). Most geothermal heat pumps have COPs of between 3 to 5, meaning that for every unit of energy used to power the system 3 to 5 units of heat are supplied.
Types and parts
A geothermal heat pump system is made up of three main components – the ground loops that form the heat-exchange medium, a heat-pump unit, and ductwork to deliver the air to the building.
Ground Loop Systems
There are four basic types of ground loop systems:
Open loop systems pump water from a well or other water source directly into a heat pump and circulate it back to the water source. If you have a water source such as a well, pond or lake that remains at 45 degrees you can consider an open loop system. Installing this type of system is one of the most cost efficient ways of installing geothermal.
Closed loop systems use a sealed water system in which water or a refrigerant are continuously circulated. They can be installed horizontally, vertically or into a water source such as a pond.
Horizontal loop systems have trenches dug and polyethylene pipes laid in the ground below the frost line (usually anywhere from 5 to 12 feet underground). Water and a refrigerant are circulated through the pipes and heat is transferred to and from the ground with a heat pump. Typically ¼ to ¾ of an acre is needed to lay pipes horizontally. Trenches are usually spaced 4 or 5 feet apart. If you have available land, horizontal installations are usually simpler and less costly to install than vertical loop systems.
Vertical loop systems can work when there are space limitations, pipes can be installed in a series of holes or bores drilled into the ground vertically. A typical home requires 3 to 5 bores with about a 15-foot separation between the holes that can go to depths of between 100 – 450 feet. Vertical installations are generally more expensive due to the increased cost of drilling vs. trenching, however since they are buried deeper than a horizontal system, they are usually more efficient.
Pond loop system – closed loops can also be laid in the bottom of a sufficiently deep and free-flowing pond or body of water (usually at least ½ acre and 8 feet deep.) Loops are usually submerged on a rack.
Two recent innovations in loops are the Lima-1 system from Limnion that anchors a compact case containing loops into a lake or pond and Slinky coils that concentrate the heat transfer into a smaller volume in horizontal and vertical loops. They require less land area and shorter trenches or bores.
Heat pump – this device extracts heat from the water or refrigerant that is circulated in the ground loops and transfers it into a building. In the warm weather it can be run in reverse to cool a building by transferring the inside heat into the ground. The cooling process works just like an air conditioner. It extracts heat from a building and discharges it outside. However it’s much more efficient because unlike a conventional system that must deal with extremes of cold and hot outside air, a geothermal system gets and discharges heat from a constant underground source that averages 55 degrees in all seasons. For example, if it is 20 degrees outside, a conventional system needs to use energy to heat the air from 20 degrees to 68 degrees (a 48 degree difference), but a geothermal system only needs enough energy to heat from 55 degrees to 68 degrees (a 13 degree difference). It can also provide most of the hot water for a home when a heat exchanger, also known as a desuperheater is added to the system.
Duct System – ducts transfer heated or cooled air throughout a building. If you are retrofitting your home and have a forced air system, you can probably use the same ductwork you already have although generally ducts for geothermal systems are larger.
Is geothermal right for you?
Geothermal system installation is usually more expensive than conventional heating systems and is the most cost effective when done during new construction or as part of a major renovation because:
- The building can be properly insulated reducing the size of the geothermal system needed;
- Equipment can be purchased specially to support a geothermal system;
- Ducts can be designed and installed to support the geothermal system;
- Extra costs can be rolled into your mortgage;
- The property has not been landscaped and there is easy access for digging.
However, many people successfully retrofit their homes. The cost of installation varies widely based on the property’s proximity to water, how and where loops need to be buried and whether or not new ductwork needs to be installed. Note: geothermal and all other energy systems work most effectively when a building is well insulated.
How large a system do you need?
The size of a geothermal heat pump system varies greatly so it’s important to have an assessment done of your heating and cooling needs. Both the size of your heat pump and the size of the field or bore where your loops are to be buried need to be calculated. For more information see item #6 in “Questions for a Geothermal Installer”.
Do I need a supplemental heat source?
Some systems meet the full heating and cooling requirements and some need a supplemental heat source for the coldest days. See item # 7 in “Questions for a Geothermal Installer”.
Savings and Costs
The cost of installing a geothermal heating, cooling and water heating system can be significantly greater than installing a conventional heating or cooling system, however, the savings in both operating costs and maintenance over time should make this investment worthwhile.
Financing your geothermal system
There are many federal and state rebates and incentives plus special loan programs to help you finance a geothermal system. See item # 17 in “Questions for a geothermal installer”.
Select a geothermal installer
It is possible to do a lot of the research and installation on your own; however, there are many advantages to hiring trained professionals.
There are several ways to look for a geothermal installer:
- Check in your phone directory or on the Internet for “Geothermal Energy Equipment and Systems-Dealers” in your county or city.
- Go to the NYSERDA Community website and click Contact Me under Homeowners to submit a request for them to find an installer in your area.
- Review a list of accredited International Ground Source Heat Pump Association installers.
- Go to the GeoExchange website for a list of installers organized by state.
- Check out geothermal installers and contractors on Angie’s List. Note that there is a fee to join Angie’s List.
It is recommended that you get bids from more than one company if possible.
The attached list of important questions to ask a geothermal installer contains more detailed information about technical and other aspects of installing a geothermal system.