GEOTHERMAL SYSTEMS

What Is It and How Does It Work?

Geothermal systems take advantage of stable soil temperatures year round to trade warmth with the ground in order to both heat and cool buildings. Even in climates with extreme weather conditions—both hot and cold—soil temperatures remain fairly constant and moderate just a few feet below the ground. Ground temperatures are cooler than air temperatures during the summer, and warmer than air temperatures during the winter, so a heat-exchange unit can use this stable condition to both heat and cool your home.

Also known as geothermal heat pumps or ground source heat pumps, these systems are made up of a loop of pipes buried in the ground, and a heat exchange unit. Water is passed through the pipes and through the ground. During hot summer weather, the water deposits heat in the ground and drops to the temperature of the soil before returning to the heat-exchange unit. This cool water is then used to provide cool air to the house. During cold winter weather, cold water passes through the pipes and picks up warmth from the ground as it makes its way back to the heat-exchange unit. Then, rather than using a lot of energy to warm up very cold air in order to heat the house, it only needs to warm the air a small amount.


Why Should I Consider It?

Ground source heat pumps use 25 to 50 percent less electricity than conventional HVAC systems. This can lead to big energy savings, even when considering the larger upfront cost. This type of system can be installed as a retrofit or as part of a new design. Much of the components of a ground source heat pump have extremely long life cycles, due to being isolated from extreme temperatures and weather. The piping elements can last up to 50 years, while heat pumps can last 20 years or more.

What Are the Options?

There are different ways to configure a ground source heat pump, including varieties of both closed-loop and open-loop systems. Horizontal ground source closed-loop heat pumps involve piping that is spread out in a yard or below a parking surface, at least 4 feet below the surface. This type of installation is useful if you have a wide area that you can dig into in order to lay the pipes, and if the frost level does not penetrate too deep into the ground. A total of 400 to 500 linear feet of pipe must be laid for each ton of capacity of the system, but a coiled pipe that resembles a slinky can be used to reduce the length of the trenches.

Vertical ground source closed-loop heat pumps use narrow, deep holes to bury the piping. This configuration is a good consideration if there is a limited amount of horizontal space available, or if the frost level extends relatively deep into the ground, making it cost-prohibitive to dig extensive trenches that are deep enough. The holes are generally approximately 20 feet apart and 100 to 400 feet deep. Less piping is required for vertical installations than horizontal because the temperature of the soil decreases as it gets farther from the surface.

If your home is adjacent to a controlled body of water, a pond or lake closed-loop heat pump can be a cost-effective way to implement a ground source heat pump. In this case, piping is buried in a trench until it reaches the water, where it is coiled below the surface. The depth of the water must remain at least 8 feet. Water conducts heat more quickly than soil, so this type of installation requires a smaller length of pipe.

Open-loop ground source heat pumps require abundant groundwater and may be prohibited in some areas. Groundwater source open-loop heat pumps require the digging of two wells into the groundwater. Well water is then drawn into the heat pump system from one well, circulated through the heat exchanger, and then discharged into the other well.

Standing column ground source heat pumps operate in a similar manner, except they only need one well. In this case, water is drawn from the top of the well and circulated into the heat pump system. After use, it is pumped to the bottom of the well to be discharged.


How Is It Installed?

Ground source heat pumps should be installed by a qualified professional. You may be able to obtain a list of installers from your local utility company, but you can also contact the International Ground Source Heat Pump Association or the Geothermal Heat Pump Consortium for a list of installers in your area. However, there are a few steps you can take to evaluate your site either before meeting with an installer or during an initial consultation.

The characteristics of your site’s geology will have an impact on heat transfer, so you will need your qualified ground source heat pump installer to evaluate the properties and composition of the soil and rock around your site. If the soil has good heat transfer, you will not need as much piping. If there is not enough soil, or if the soil is too shallow, you may have to consider a vertical installation.

If you are considering an open-loop system, your site's hydrology is a critical factor. There must be an adequate source of groundwater, and the depth, volume, and water quality must all be evaluated. Before installing an open-source system, it is important to make sure your ground source heat pump will not have negative impacts such as aquifer depletion and groundwater contamination.

The availability of land around your site is another important consideration. If you are building a new home with sufficient surrounding land, a horizontal ground loop will probably be the most economical option. If you are building on a tight site, or are adding a ground source heat pump to an existing site, a vertical ground loop may be the only option.

What Are the Potential Benefits?

The benefits of installing a ground source heat pump to provide for your heating and cooling needs include reduced operating and maintenance costs.

Are There Health and Safety Concerns?

Open-loop ground source heat pumps can deplete groundwater, so it is important to only consider this method in areas with ample groundwater. Closed-loop systems that use an antifreeze solution in place of water should be carefully installed and inspected to ensure that they do not leak.

How Much Does It Cost?

The purchase price of a ground source heat pump will likely be higher than that of a conventional HVAC system. For example, an average geothermal system may cost around $2,500 per ton, or $7,500 for a typical 3-ton unit. An equivalent HVAC unit should cost about $4,000. However, since ground source heat pumps are able to provide more energy than conventional HVAC systems compared with the amount of energy consumed, they use less energy to heat and cool your home and cost less money to operate. Depending on the specific conditions at your home, your ground source heat pump may pay for its cost premium over a conventional HVAC system within 10 years.

What Else Should I Know?

If you are considering a ground source heat pump for your home, you should look for an Energy  Star–qualified model. Energy Star ground source heat pumps are at least 45 percent more efficient than unqualified models.


Where Do I Start?

Examining the land around your home is a good place to start, since you will need to determine what kind of ground source heat pump can be installed on your property. If you are planting a large yard or garden, install your horizontal piping network first. If space is tight or the freeze level of the soil too deep, you may still be able to fit a vertical ground source heat pump close to your home.

Where Can I Get More Information?

U.S. Department of Energy
Energy Star
International Ground Source Heat Pump Association
Geothermal Heat Pump Consortium