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
    July 6, 2:50 pm

    Al Webster in Dalton, GA,
    These are all great questions similar to those I have had over the past few decades. You need 40F to effectively cool and dehumidify, and cooling the existing system is not wise. My answer is that you should employ option #2: Employ a compressor / condenser that is designed to be liquid cooled. It will be a permanent solution.
    If you’ll view the depiction of a geothermal heat pump (GHP) from this article: http://goo.gl/O8EBip , you’ll see that GHP’s are engineered specifically for your needs, and can come equipped with DHW heaters, and even radiant heating/cooling capabilities.
    Did this answer your question?

  2. Al Webster
    Dalton, GA
    July 4, 12:06 pm

    I have the opportunity to design a residential cooling system using a natural spring with a constant temperature of 58 F. My question is this…. Should I:
    1) use the existing central hvac and cool the gas with a heat exchanger?
    2) employ a compressor / condenser that is designed to be liquid cooled?
    3) Attempt to pump the 58 degree spring water directly through a coil in the air handler ?

  3. Jay Egg
    June 30, 11:38 am

    Cathy in Greenville, MI,
    Geothermal HVAC will certainly work well in locations that are below sea level. Elevation is not as important as the thermal conductivity of the soil into which the closed loop piping is placed. This article gives a quick overview of the types of systems and loops; http://goo.gl/MjF6CD …more in depth information may be found in the textbook links at the end of this National Geographic article.
    Did this answer your question?

  4. Cathy
    Greenville, MI
    June 27, 10:48 am

    I just recently was in a discussion with my brother regarding geothermal HVAC. He stated it wouldn’t work in certain areas because it has to be above sea level. I say he is wrong, it doesn’t matter, because the system is entirely underground. Who is right?

  5. Jay Egg
    June 26, 3:17 pm

    Kavita in MD,
    This is such a great question to ask; I’ve run into it before…
    In our first geothermal book, Geothermal HVAC, Green Heating and Cooling (McGraw-Hill Professional, 2010, p16), Brian Clark Howard and I shared a dialogue from an article wherein a commenter said that switching from a gas furnace to ground sourced heating is increasing the burning of fossil fuels, because the power plant producing the electrical power is probably burning coal, natural gas or other fossil fuels. The argument seems valid until you understand what we call the “Negatherm Factor”.
    “Negatherm” is a term that was coined to refer to energy that would or could have been used from fossil fuel consumption, but was never used.
    When a consumer is using a fuel heating appliance, he is consuming fossil fuel. When that consumer switches over to an electric appliance, he may be eliminating his CO2 footprint if the electrical power is coming from one of the renewable sources. And with an interconnected grid, we are all increasingly partakers of renewable energy.
    Fuel prices, whether for natural gas, propane, or coal may go down, and they may go up. Electricity prices also fluctuate, but since electric power comes from coal, natural gas, solar, wind, nuclear, hydro (and the list goes on…), electricity is not subject to the supply and demand “commodity like” fluctuations of any one individual fuel source.
    Right now, natural gas is favorable to many. It’s abundant, clean burning and cheap. Investors will be taking profits eventually, but they’re patient. This report from Sustainable Plant says, we will end up paying more sooner or later: http://www.sustainableplant.com/2012/04/why-u-s-natural-gas-prices-won-t-stay-down/
    A good financial practice is to buy something before the demand for it peaks. Earth coupled systems are “all-electric” powered. They heat, cool, generate domestic hot water, heat pool, spas, driveways, and much more. Ground sourced systems are the most efficient heating and cooling systems available.
    Ground coupled heating costs about the same per BTU as natural gas at the current price point of about $0.60/therm. But natural gas is for combustion heating, and does not provide cooling. Here’s a thought to ponder:
    We need cooling and heating. A geothermal heating and cooling system is environmentally friendly, and superior to other types of systems in efficiency, comfort, longevity, and safety. So, when prices go up for fossil fuels, financially we’re already poised to reap the rewards.
    Read this article for more information: http://www.greenbuildermedia.com/blog/geothermal-hvac-and-the-negatherm-factor-heating-cooling-natural-gas

    Did this help you with your question?

  6. Kavita
    MD
    June 23, 5:15 pm

    I am thinking of installing Geothermal in the new home, but in my community, there is natural gas availability, and given the current natural gas prices, it is difficult to think about geothermal system, that uses electricity to function. Won’t my cost be much lower if I use natural gas to heat/cool rather than geothermal?
    Thanks for any input you may have

  7. Jay Egg
    June 18, 2:57 pm

    Daniel in New Orleans,
    Your questions are similar to many who have faced a problem with corrosion. The hidden enemy is likely electrolysis. Electrolysis can occur in any water or conductive fluid can cause a charge build-up at sharp point (like a lightning-rod). If the charge concentration then passes from the metal edge into the water stream, it is possible that sharp edge can become a “sacrificial anode”. Keep in mind that copper-oxide is an electrical rectifier so a DC component can essentially plate-away that sharp edge. As the edge gets plated away, it creates new sharp a critical anode surface, creating a self-degrading condition.
    Three conditions must exit for an electron seeking its ground potential, and electrolytic corrosion. These conditions are not unique to the geothermal heat pump, but can occur in any installation with:
    A high resistance electrical ground
    Conductive water or fluid returning to earth potential (ground)
    A 115 volt device
    Read this blog for a little more information on this: http://goo.gl/VPOkxj
    Please let me know if this helped you out?

  8. Jay Egg
    June 15, 3:46 pm

    Linda in OK,
    These are all great questions! Let me answer them in order;
    You may certainly add a geothermal heating and cooling systems to your house. Here’s some additional information on geothermal http://goo.gl/O8EBip .
    The zones that you are currently using should be usable with your new geothermal system. That is because in most cases, the existing ductwork and dampers may be used with the new geothermal heat pumps (GHP’s).
    The pool may be used as the primary heat sink, but you will still need the earth coupled portion for excess heating and cooling needs. This article http://goo.gl/08ZrZa gives more information on this application.

  9. Daniel
    New Orleans
    June 15, 2:53 pm

    Jay, I recently got called to perform some water sampling on a plantation. They are having problems with corroding pipes. They replaced the pipes which are copper with new copper piping. The problem is starting again. There is some major corrosion going on at an accelerated rate. The plantation has a geothermal heat pump that is used for heating and cooling as well as heating their water. For some reason I am thinking that the unit has something to do with it. I had the water sampled for pH, copper, sulfide, sulfur, and chloride at where the water comes into the house and at the storage tank in their attic that the hot water is stored from the unit before it goes through an additional hot water heater. The results didn’t show anything out of the ordinary. I’m thinking that electrolysis is to blame but am not a pro in these matters. Have you ever heard of severe copper pipe corrosion in association with a geothermal heat pump before?