Geothermal Energy: Mother Nature’s HVAC

Geothermal Energy: Mother Nature’s HVAC

Have you ever explored a cave and noticed how it got cooler the farther in you went, but it never got really cold? Well, if you have then you have been using one of the most cost effective methods for heating and cooling your home available today. You were using the Earth’s ability to serve as a heat reservoir, and that is the basis for heating and cooling your home or building using geothermal methods.


There are many ways to heat or cool the air in a home or other building. Most of us use a heat pump as the foundation of our heating/ventilation/ air conditioning (HVAC) system which runs on electricity to make things hot or cold. Heat pumps work by concentrating the heat in whatever transport medium they use. Most residential and commercial buildings that use heat pumps use air as the transport medium.

The geothermal map of North America is a heat flow map depicting the natural heat loss from the interior of Earth to the surface.

To generate cool air they compress the air taken from the house and outdoors to concentrate it. The heat is then removed from the air through the radiators in the heat pump unit located outside of the building. The remaining air, which is now cooler, then recirculates back into the house. To generate warm air they take air from inside and outside of the building to compress it, just like they do for cooling. Then, instead of removing the concentrated heat from the air they recirculate it back into the house. This is the same technique that in-window and central air conditioning units use to create cooler air.

When available space at ground level is at a premium, like here at the Muldoon River Center in Annapolis, MD, the use of a vertical orientation for piping allows the builder to install an effective geothermal heating and cooling solution.

Although it is the most common method, due in large part to its ease of installation, heating and cooling with electricity is expensive and inefficient. Also, although effective, this method of heating and cooling the air in our buildings does have its limitations and costs. In very cold weather that reaches down to near or below freezing, air based heat pumps are not as effective because there just isn’t much heat left in the air to concentrate and recirculate back into the building. Also, the colder the air is the more the heat pump has to run to get the inside temperature to rise up to the settings on the thermostat, which is what leads to higher electric bills in the winter.

The use of multiple coils of piping, as shown in this ground coupled heat exchanger, maximizes the conduction of thermal energy between the ground and the anti-freeze solution in the pipes for a relatively small area. Heat is absorbed by the coils and then returns to the heat pump via the straight length of piping.

Through good building design we can maximize the insulation to cut down on electricity needs, and we can use the right mix of materials and room layout to make the best use of the heat from the sun and cooling from shade. We can also integrate other electricity generation solutions such as solar power, wind power, and water power…or even the intense heat found deep underground in magma that can be used to generate steam for generating electricity. But electricity isn’t the only way, or even the best way, we can generate hot and cold. As Yoda said in Star Wars, “…there is another.”

The ground, as noted at the beginning of this article, is a great thermal insulator. It also has the ability to hold heat and keep it at a constant temperature if you go far enough down below the surface. Once you go below the frost line in your area, approximately six to ten feet below the surface, the temperatures stays at around 55 degrees year round regardless of how hot or cold it is above ground. Geothermal heating and cooling systems take advantage of this phenomenon by using this moderate temperature as a source of heat in the cold weather and as a source of cooling in the hot weather.


A geothermal heating and cooling system has two major components: the ground source heat pump and the transfer system made up of pipes in the ground filled with a liquid that either absorb the heat from the relatively warmer ground or radiate the heat out into the relatively cooler ground.


The better solution for many of us is to use a ground source heat pump. A ground source heat pump, the engine that drives the heating and cooling in a home with a geothermal heating and cooling solution installed, works in a manner similar to a typical air sourced heat pump, but instead of manipulating the heat in the air it manipulates the heat in the ground.

A ground sourced heat pump, like this one installed in a new home, uses the heat stored in the ground six or more feet below the surface and below the frost line to condition the air inside of the building.

The ground source heat pump system will transfer that 55 degree temperature up using a liquid medium to cool the warmer air at ground level in the summer and warm the cooler air at ground level in the winter. This is not only more efficient but it also requires less electricity because you don’t need to compress the medium to concentrate the transport medium. You also don’t have as far to move the temperature, up from the 30’s in the winter or down from the 90’s in the summer.


Heat from the subterranean earth battery is drawn up using piping of plastic or metal that are filled with a mixture of water and anti-freeze. The heat pump brings it up and then uses it as described above to condition the air that it re-circulates into the house. The pipes that make up the transfer system can be laid out in one of two ways; in a vertical orientation or in a horizontal orientation.

If a passive solution is preferred, dirt and sod can be used to insulate the building and keep its internal temperature at the same 55 degrees found below the frost line, six feet down.

A vertical orientation is much more expensive to install as it requires digging deep holes to put the piping in and then filling in around the pipe with a thermal conductor to help transfer the heat from the ground to the antifreeze mixture. Unfortunately, it is sometimes the only option available when there isn’t much available space at ground level. The pipes are installed in long U-shaped loops designed to maximize contact with the ground and to absorb as much warmth as possible or radiate as much warmth as possible, depending on the season.

A horizontal orientation is the cheaper approach, but it also requires the access to more land at ground level than the vertical orientation, which can be a limitation at some sites. Pipes are laid out in long loops running back and forth from the heat pump, or in long series of spirals each having a return line back to the heat pump. With more surface area exposed to the ground than the vertical orientation this approach is very efficient at transferring heat between the ground and the piping.


As mentioned at the beginning of this article you can use passive approaches to using the thermal battery located below the frost level. If you need to escape the heat or cold on the ground floor of your house you can, possibly during a power outage or if your electric bill has grown too large, simply by retreating to your basement where the temperature will be in the 50’s or 60’s. Just make sure to close the basement door so the temperature at ground level doesn’t cause the temperature in the basement to change as the two areas try to reach a thermal equilibrium. Many people just use their basement for storage, but you can also finish it off and turn it into a family room where you can relax during normal times or live during emergencies, like power outages or extreme weather events.

Sources of geothermal heat like this subterranean hot spring have been used for centuries by cultures in all parts of the world as a free source of heat and energy for warming homes, bathing and even cooking food.

Actually building into the side of a hill or down into a below ground level site or covering an existing structure with dirt and sod are additional ways to take advantage of the earth’s insulating properties and to use the thermal battery to condition your home’s environment.

So next time you are thinking about how to heat your home or cut down on your frustratingly high electric bill, think outside of the box or maybe six feet below the box, and see how some non-conventional methods might just be the ticket for what you are trying to accomplish.

Geothermal Tax Credits

If you need another reason to think about geothermal technology for your heating and cooling needs, most states, in addition to the 30 percent federal government tax credit, now offer tax incentives for homeowners who use geothermal technology and materials in their new or existing construction projects.


Editors Note: A version of this article first appeared in the March 2015 print issue of American Survival Guide.

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