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
    March 3, 8:54 am

    Kathy in Missouri,
    Good news! You can have a geothermal cooling system with a gas furnace. What is better yet is that you can choose a geothermal heat pump with a gas furnace for supplemental heating.
    I would ask you to consider the fact that a geothermal heat pump can be engineered to handle the entire load efficiently. The key is in putting in the “right sized” and designed exchanger to extract the heat from the earth at the same rate as the earth is able to replenish the heat. A thermal conductivity test should be done to determine this rate of transfer, and then the linear feet of exchanger and borehole are designed accordingly. When a GHP heats a home adequately for a period of time, then loses its capability to heat adequately, that means that something with regard to the design of the ground heat exchanger may be insufficient for the heating load. This could even be something as simple as one or more of the vertical wells not circulating water, an issue that can be checked and remedied in a variety of ways.
    It’s always helpful to have a good reference such as “Modern Geothermal HVAC…” http://goo.gl/6b98mp . Also, I would recommend that you check with IGSHPA http://www.igshpa.okstate.edu/directory/directory.asp for designers and contractors in your area that will be able to assist you on your geothermal quest.
    Did this help you with your comments?

  2. Kathy Falter
    Missouri
    February 25, 4:46 pm

    Why can we not choose between electric or gas to run the heat pump unit. Electricity is not cheap and w/o updating energy plants — Coal or putting in more energy plants — Nuclear in USA, electrical costs is going up. Renewable energy is a joke — windmills. I would consider Geothermal if I had my choice and would choose “Gas”. I know people who have this system and wish they had a secondary heat to keep their house comfortable w/o the huge electrical bill. The AC is great but the heat sucks!!!!!!

  3. Jay Egg
    February 24, 3:07 pm

    Kelvin in Tacoma,
    That is a great question. It certainly depends on the regional information and type of soil, but if you know your median temperature, which closely follows your ground water temperature, you will find that generally the temperature of the earth between 6 and 8 feet deep will swing about 10F plus or minus depending on the season. You can see in this EPA Ground Water Map http://goo.gl/7Ncfjs that the temperature for your area is between 47F and 52F. That means you can be reasonably certain that your swing will be within 37F and 62F seasonally.
    Did this answer your question?

  4. Jay Egg
    February 24, 3:06 pm

    Debbie in PA,
    I had a similar concern mentioned by Al in Central New York. I mentioned to him that I believe the electric backup heat is engaging in the “auxiliary” or “emergency” mode because the ground source heat pump (GSHP) is not functioning at the capacity needed for heating your home.
    Geothermal (ground-sourced) heat pumps can be designed to work flawlessly for the entire winter, providing all of the heating needed for a building of any size. The key is in putting in the “right sized” and designed exchanger to extract the heat from the earth at the same rate as the earth is able to replenish the heat. A thermal conductivity test should be done to determine this rate of transfer, and then the linear feet of exchanger and borehole are designed accordingly. When a GHP heats a home adequately for a period of time, then loses its capability to heat adequately (as you mentioned in your comments that your 4 ton geothermal system has been using 3500 kwh/month in January), that means that something with regard to the design of the ground heat exchanger may be insufficient for the heating load. This could even be something as simple as one or more of your vertical wells not circulating water, an issue that can be checked and remedied in a variety of ways. Your question about the glycol-water ratio could be a factor, but is not likely at the root of the concern.
    I realize that you have had a few different contractors review this. I would recommend that you check with IGSHPA http://www.igshpa.okstate.edu/directory/directory.asp for other designers in your area that may be able to give a second opinion on the heating concern. If you take the time to explain yourself, and log their recommendations from your phone calls, I think you will end up with some that “stand-out” above the others. If you are uncertain at this point, just email with your summary, and I may be able to help you with your options!
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