Illustration of a geothermal heating and cooling system that handles multiple loads for a community. Illustration by Sarah Cheney.

Illustration of a geothermal heating and cooling system that handles multiple loads for a community. Illustration by Sarah Cheney.

Imagine a home in which the temperature is always comfortable, yet the heating and cooling system is out of sight. That system performs efficiently but doesn’t require extensive maintenance or knowledge on the part of the owners.

The air smells fresh; you can hear the birds chirping and the wind rustling lazily through the trees. The home shares energy with the earth similar to the way the roots of the trees exchange the essentials of life to their leaves and branches. Sounds comfortable, doesn’t it?

Geothermal heating and cooling makes that vision a reality. Geothermal HVAC (heating, ventilating, and air conditioning) brings a building in harmony with the earth beneath, taking advantage of subterranean temperatures to provide heating in the winter and cooling in the summer.

How Geothermal Heating and Cooling Works

Outdoor temperatures fluctuate with the changing seasons but underground temperatures don’t change as dramatically, thanks to the insulating properties of the earth. Four to six feet below ground, temperatures remain relatively constant year-round. A geothermal system, which typically consists of an indoor handling unit and a buried system of pipes, called an earth loop, and/or a pump to reinjection well, capitalizes on these constant temperatures to provide “free” energy.

(Note that geothermal HVAC should not be confused with “geothermal energy,” the process by which electricity is generated directly from the heat inside the earth. That takes place on the scale of utilities and uses different processes, normally by heating water to boiling.)

The pipes that make up an earth loop are usually made of polyethylene and can be buried under the ground horizontally or vertically, depending on the characteristics of the site. If an aquifer is available, engineers may prefer to design an “open loop” system, in which a well is drilled into the underground water. Water is pumped up, run past a heat exchanger, and then the water is returned to the same aquifer, through “reinjection.”

Diagram of how geothermal HVAC systems work

Diagram of how geothermal HVAC systems work. Illustration from Modern Geothermal HVAC

In winter, fluid circulating through the system’s earth loop or well absorbs stored heat from the ground and carries it indoors. The indoor unit compresses the heat to a higher temperature and distributes it throughout the building, as if it were an air conditioner running in reverse. In summer, the geothermal HVAC system pulls heat from the building and carries it through the earth loop/pump to reinjection well, where it deposits the heat into the cooler earth/aquifer.

Unlike ordinary heating and cooling systems, geothermal HVAC systems do not burn fossil fuel to generate heat; they simply transfer heat to and from the earth. Typically, electric power is used only to operate the unit’s fan, compressor, and pump.

A geothermal cooling and heating system has three main components: the heat-pump unit, the liquid heat-exchange medium (open or closed loop), and the air-delivery system (ductwork) and/or the radiant heating (in the floor or elsewhere).

Geothermal heat pumps, as well as all other types of heat pumps, have efficiencies rated according to their coefficient of performance, or COP. It’s a scientific way of determining how much energy the system moves versus how much it uses. Most geothermal heat pump systems have COPs of 3.0 to 5.0. This means for every unit of energy used to power the system, three to five units are supplied as heat.

Geothermal systems require little maintenance. When installed properly, which is critical, the buried loop can last for generations. The unit’s fan, compressor, and pump are housed indoors, protected from the harsh weather conditions, so they tend to last for many years, often decades. Usually, periodic checks and filter changes and annual coil cleaning are the only required maintenance.

Geothermal HVAC Spreads

Geothermal HVAC systems have been used for more than 60 years in the U.S. and beyond.

They work with nature, not against it, and they emit no greenhouse gases. (As mentioned earlier, they use a smaller amount of electricity to run, because they are coupled in with the earth’s average temperature.)

Geothermal HVAC systems are becoming common features of eco-friendly homes as part of the growing green building movement. Green projects accounted for 20 percent of all newly built homes in the U.S. last year. By 2016, a Wall Street Journal article predicted that green housing will grow from $36 billion a year to as much as $114 billion. That’s approaching 30 to 40 percent of the entire housing market.

But a lot of information out there on geothermal heating and cooling is based on outdated information, or outright myths. In our new book Modern Geothermal HVAC Engineering and Control Applications (Egg/Cunniff/Orio -McGraw-Hill 2013), co-authors Greg Cunniff, Carl Orio and I bust many of these myths.

Geothermal HVAC Myths Busted

1.     Geothermal HVAC systems are not considered a renewable technology because they use electricity.

Fact: Geothermal HVAC systems use only one unit of electricity to move up to five units of cooling or heating from the earth to a building.

2.     Photovoltaic and wind power are more favorable renewable technologies when compared to geothermal HVAC systems.

Fact: Geothermal HVAC systems remove four times more kilowatt-hours of consumption from the electrical grid per dollar spent than photovoltaic and wind power add to the electrical grid. Those other technologies can certainly play an important role, but geothermal HVAC is often the most cost effective way to reduce environmental impact of conditioning spaces.

3.     Geothermal HVAC needs lots of yard or real estate in which to place the polyethylene piping earth loops.

Fact: Depending on the characteristics of the site, the earth loop may be buried vertically, meaning little above-ground surface is needed. Or, if there is an available aquifer that can be tapped into, only a few square feet of real estate are needed. Remember, the water is returned to the aquifer whence it came after passing over a heat exchanger, so it is not “used” or otherwise negatively impacted.

4.     Geothermal HVAC heat pumps are noisy.

Fact: The systems run very quiet and there is no equipment outside to bother neighbors.

A technician inspects a geothermal HVAC air handler

A technician inspects a geothermal HVAC air handler. Photo courtesy of Jay Egg

5.     Geothermal systems eventually “wear out.”

Fact: Earth loops can last for generations. The heat-exchange equipment typically lasts decades, since it is protected indoors. When it does need to be replaced, the expense is much less than putting in an entire new geothermal system, since the loop or well is the most pricey to install. New technical guidelines eliminate the issue of thermal retention in the ground, so heat can be exchanged with it indefinitely. In the past, some improperly sized systems did overheat or overcool the ground over time, to the point that the system no longer had enough of a temperature gradient to function.

6.     Geothermal HVAC systems only work in heating mode.

Fact: They work just as effectively in cooling and can be engineered to require no additional backup heat source if desired, although some customers decide that it is more cost effective to have a small backup system for just the coldest days if it means their loop can be smaller.

7.     Geothermal HVAC systems cannot heat water, a pool, and a home at the same time. Fact: Systems can be designed to handle multiple loads simultaneously.

8.     Geothermal HVAC systems put refrigerant lines into the ground.

Fact: Most systems use only water in the loops or lines.

9.     Geothermal HVAC systems use lots of water.

Fact: Geothermal systems actually consume no water. If an aquifer is used to exchange heat with the earth, all the water is returned to that same aquifer. In the past, there were some “pump and dump” operations that wasted the water after passing over the heat exchanger, but those are exceedingly rare now. When applied commercially, geothermal HVAC systems actually eliminate millions of gallons of water that would otherwise have been evaporated in cooling towers in traditional systems.

10.  Geothermal HVAC technology is not financially feasible without federal and local tax incentives.

Fact: Federal and local incentives typically amount to between 30 and 60 percent of total geothermal system cost, which can often make the initial price of a system competitive with conventional equipment. Standard air-source HVAC systems cost around $3,000 per ton of heating or cooling capacity, during new construction (homes usually use between one and five tons). Geothermal HVAC systems start at about $5,000 per ton, and can go as high as $8,000 or $9,000 per ton. However, new installation practices are reducing costs, to the point where the price is getting closer to conventional systems under the right conditions.

Factors that help reduce cost include economies of scale for community, commercial, or even large residential applications and increasing competition for geothermal equipment (especially from major brands like Bosch, Carrier, and Trane). Open loops, using a pump and reinjection well, are cheaper to install than closed loops.

Jay Egg is the co-author of the new book Modern Geothermal HVAC Engineering and Control Applications (McGraw-Hill 2013), with Greg Cunniff and Carl Orio. He co-wrote the book Geothermal HVAC, Green Heating and Cooling in 2010 with National Geographic’s Brian Clark Howard. Jay consults with the geothermal HVAC industry. He previously served as an installer of the technology through his company EggGeothermal.

Comments

  1. Jay Egg
    August 20, 3:48 pm

    Bud in Omaha, NE
    You’ve been thorough in your description of the issues with regard the newly installed geothermal heating and cooling system. It is likely from your description that the thermal conductivity of the soil relative to the length of pipe in the ground is the culprit. I recommend that you access some published data as you work with your contractors to remedy this.
    In chapter 5 on page 141 of Modern Geothermal HVAC Engineering and Controls Applications http://goo.gl/cyaEM6 , there is a chart that shows the thermal conductivity of dry sand is significantly diminished when compared to saturated sand. Additionally, a thermal conductivity test as mentioned on pp 139-154 will help to determine the linear feet of heat exchanger need in the ground for you application. There are also other types of heat exchangers that may be employed depending upon you lithology.
    I would welcome a direct dialogue via email to help you through this challenge. Just like a car needs a certain size of tire for a certain rim; your Geothermal Heat Pump (GHP) system needs a ground loop that matches you heating and cooling needs. Email: jayegg.geo@gmail.com

  2. Bud Shaw
    Omaha, NE
    August 17, 3:05 pm

    Having troubles with a newly installed Geothermal system involving two 4-ton water-to-air heat exchangers. The area’s recognized experts installed the vertical ground loop that they ensured me would suffice for at least 8-tons. However, one of the two heat exchangers regularly locks out because of overheating (return water is > 100 deg F), and the other, which includes a desuperheater connected to our standard gas water heater, only occasionally locks out, but for the same reason. The installers have been out multiple times to check the equipment, and also had the manufacturer representative come out to confirm that all of it is working as it should. They conclude that the ground loop is not doing adequate heat exchange and have direcedt me to the well-drillers (whom I paid independent of the HVAC contractor). The driller, in turn, believes his ground loop is perfectly adequate. We have five 200-foot deep holes for each exchange unit. The soil here is sandy and the driller did not hit water at the 200 feet depth. The driller plans to send someone out next week to look things over.

    So, neither party seems to know what to do to help me. I don’t have back-up cooling, of course, and I worry about the lack of heating we may experience this winter. I’m looking for advice about whether others have experienced problems like this and how best to seek a resolution.

  3. Jay Egg
    August 12, 7:21 am

    Jane Hinds in Birmingham,
    The scenario that you’ve painted here would be a great opportunity for a “Lake-Loop” system. Typically, coils of HDPE pipe can be fabricated into effective exchangers that may be carefully placed/weighted to sit on the lake floor, preferably at the deeper area that you mentioned. You can see a depiction of this in the color drawing at the top of this article. Deeper water provides more stable temperatures, and will also protect the loops. Conversely, there are companies that manufacture “Lake-Plate-Exchangers” that transfer heat more effectively.
    With the size of this lake, it sounds as if the capacity will be adequate to provide heat for a geothermal pool heat pump that you can use to heat your pool. Much of this is covered in some detail with great pictures in the book, “Modern Geothermal HVAC Engineering and Control Applications” http://goo.gl/cyaEM6 on pp. 105-132. When you decide to move forward, please consult the IGSHPA directory http://www.igshpa.okstate.edu/directory/directory.asp to find a good installer/designer for your system.
    I hope that you will keep me updated on your progress!

  4. Jay Egg
    August 10, 7:34 am

    Alton in Hiawassee, GA,

    I think that you can pull this off, and get a geothermal upgrade in your current residence! Each particular area may have differing geological formations, so the same system that you used successfully in Michigan may or may not be suitable for your place in Georgia. The best this to do is check the International Ground Source Heat Pump Association (IGSHPA) website for certified geothermal installers and designers near you http://www.igshpa.okstate.edu/directory/dresult.asp . They will likely point you in the right direction. If you’re uncertain, email me back and I’ll give you further guidance.

  5. Jane Hinds
    Birmingham AL 35243
    August 8, 12:39 am

    I own a small (113 acre) farm with a small 8 acre spring fed lake which I would like to use to heat and cool a modestly sized new house of about 900 square feet, I wish to build in spring of 2016. The house would be located about 40 feet above the lake, and about 100 feet from the lake. The shallow end of the lake (near the house is a mere 8 feet deep), the deeper end (about 24 feet) is about 4 to 600 feet away. It would be nice to use it to heat a medium sized pool since the lake is inhabited with very large water moccasins. Also I hope to raise catfish in the shallow end. I would prefer not to use a buried ground loop but a loop going into the lake if it is possible.
    Comments and approximate price appreciated. I can probably afford this but am as much interested in using it as an example of an earth saving education for others as I am in saving money for myself. I have quite a following so the effect would be good “for the earth”. thank you. Jane H.

  6. Jay Egg
    July 29, 7:16 am

    Bill in Joliet, Illinois,
    I applaud your convictions! I was in much the same situation 25 years ago myself. I have some recommendations for you:
    As you know, DIY is great as long as you have a good foundation, and as an electrician and a tradesman, you likely have the stuff you’ll need. The next thing you’ll need to do is attend a workshop from the International Ground Source Heat Pump Association (IGSHPA) http://goo.gl/EZl2C8 . This will set you on a firm foundation so that you are armed with the information you’ll need to do your project properly. I’m certain that you would recommend similarly (electrical training) to someone wanting to do an electrical project.
    There are several good books out there. Geo-Power http://goo.gl/YMYHsW and Modern Geothermal HVAC… http://goo.gl/cyaEM6 are a couple that you may consider.
    Please keep me informed on your progress!