How ground-source heat pumps work (2024)

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by Chris Woodford. Last updated: August 3, 2024.

Must we burn the whole Earth to keep warm? Looking at the last couple of hundred years of human history, you might think that'swhere we're heading. The Industrial Revolution—and all that'shappened since—was powered by combustion: burning coal, then oiland natural gas, to drive everything from steam engines and power plants to cars, boats, and planes. The consequences, apart frompollution, include a rapidly warming planet that could eventuallyprove too hot to handle. There's lots of talk about renewable energy—solar cells, wind turbines, and things like energy harvestedfrom the sea—but it's taking time to come onstream. There isanother way of producing renewable energy that most people don't knowabout at all. Heat pumps "mine" the warmth buried justunderground and ferry it into buildings, a bit like air conditionersworking in reverse. Simple, efficient, and effective, and widely used inScandinavia for decades, they could provide much of our home heatingneeds in the future. How exactly do they work? Let's take a closerlook!

Photo: This air-source heat pump looks very much like an air conditioner—and that's because it works in a similar way. It's described as a "1.5 ton" unit, which means it provides roughly 5.5kW of heating or cooling (18,000–19,000 btu per hour). Photo by Dennis Schroeder courtesy of US Department of Energy/National Renewable Energy Laboratory (DOE/NREL) (photo id #68913).

Where does the heat inside Earth come from?

Car engines and power plants make useful energy by combustion—the process of burning a fuel like coal or gas with oxygen from the air to release chemical energy locked inside it. But you don'tnecessarily have to burn a fuel to make heat energy. As I'm typing thesewords, I'm sitting by a window with sunlight streaming in and warmingmy legs (my body is soaking up and storing passive-solar energy). The same thing is happening right across Earth: when the Sun shines on theland, all that energy has to go somewhere, so it warms the groundunderneath.

Although we're used to the idea that temperatures varyfrom day to day, and day to night, it's much less obvious that thetemperature a little way underground is both more constant and, inwinter at least, significantly higher than it is at ground level:in the top 5m (16ft) or so of Earth's surface, the temperature stays a steady 10–20°C (50–60°F). [1]In other words, there's solar energy "buried" underground that wecan "mine" with a heat pump—a machine that extracts energy fromthe ground (or sometimes from water or even air) and pumps it insidea cold building to heat it up.

Geothermal or ground source?

One quick note about terminology. Although these machines aresometimes called "geothermal heat pumps," that's a slightlymisleading description. Geothermal energy is the heat that comes fromdeep inside Earth that we can see spurting to the surface insteamy geysers or bubbling lagoons in places like Iceland; we canharness such wild, extreme energy only in a relatively small numberof very specific locations, typically using geothermal power plantsto turn it into electricity. Heat pumps, on the other hand, aretapping into more modest amounts of solar energy that have warmed theEarth from the outside in—and we can tap into that pretty muchanywhere. To avoid confusion, it's better to talk about ground-sourceor geo-exchange pumps and reserve the word geothermal for the geysersand lagoons.

Photo: Geysers like this occur when rainwater and melted snow drips down through hot rocks inside Earth then blasts back up again as boiling steam. Geologists have estimated there's about 70–80 gigawatts of this geothermal energy inside our planet, which is equivalent to the energy made by about 70–80 really big nuclear power plants! [2]Ground-source heat pumps aren't tapping into this "deep heat," however, but into gentler warmth nearer the surface that comes from the Sun. Picture of "Old Faithful" Geyser in Yellowstone National Park by Carol M. Highsmith. Credit:Gates Frontiers Fund Wyoming Collection within the Carol M. Highsmith Archive, Library of Congress, Prints and Photographs Division.

What is a heat pump?

Photo: An air-source heat pump, seen from the outside, looks much like an air conditioner.Photo by Molly Rettig courtesy of US Department of Energy/National Renewable Energy Laboratory (DOE/NREL) (photo id #136954).

A heat pump is like an air conditioner or refrigerator working in reverse. A refrigerator extracts heat from a metal box (the chillercompartment where you store your food) by circulating a very coldfluid through a convoluted pipe that loops around inside it. Thefluid expands and picks up heat, cooling down the box and warming up by an equalamount, before getting pumped outside and round the back of therefrigerator. There, it's compressed (squeezed into a smaller volume)so it gives up its heat to your kitchen, cooled, and pumped backinside the refrigerator so it can repeat the process again. Heypresto, heat gets pumped from the inside of the box to the outside,making the chiller compartment cooler by making your kitchen warmer.A heat pump is very similar only instead of removing heat from a coolchiller box, it takes it from the warm soil and rock beneath yourhome or garden; and instead of releasing that heat into your kitchen,it releases it more generally into your home, usually through anunderfloor heating system.

The magic thing about a heat pump is that it can put much moreheat into your home than the energy you use to make it spin around.That sounds like a horrible violation of the famous law of physicsknown as the conservation of energy (which says that you can't createor destroy energy), but it isn't. The pump doesn't create anyheat: it simply moves heat out of the ground. The relatively modestamount of electricity that powers the pump enables it to move a muchbigger amount of heat energy into your home. That's why heat pumpsare so efficient: in effect, they give you most of your heat energyfor free.

You can probably see from this that we'd need four key bits tomake ourselves a heat pump:

1. There's the pipe that goes into the ground to extract heat. That's called the ground loop.
2. The pump that forces fluid down through the loop and back up again.
3. Just like in a refrigerator, there's a compressor that effectively squeezes the heat out of the fluid when it's been pumpedinside your home.
4. There's something that releases that heat so you can feel the benefit of it inside your home.

Types of heat pumps

You'll often hear people talk about "ground-source heat pumps"(which pick up heat buried underground) because those are the mostcommon—and they're the most common because, all round, they're themost practical and efficient. Soil and rock underground is at afairly constant temperature from day to day, so getting heat out ofit is predictably efficient.

We can also use heat pumps to extractheat from water (something like a lake might seem cold but, as it'shotter than absolute zero, it still contains heat we can pull out) oreven the air. Water- and air-source pumps (as these are called) aremuch less common than ground-source ones. That's because most of usdon't have lakes in our backyard (ruling out water-source pumps) andbecause air tends to fluctuate in temperature much more than theground, making it less efficient as a source of heat (in other words,making ground-source pumps more attractive). It's worth noting thatit's possible to have hybrid heat pumps that take heat both from theground and the air.

Artwork: Three common types of ground-source heat pump system based on closed loops: 1. A horizontal loop, where the pipes run near the ground surface but over a large area; 2. A vertical loop has much deeper pipes recovering more heat from deeper, warmer ground;3. A horizontal loop using a lake or pond as its heat source.

Closed loop, ground-source

Ground-source heat pumps use a closed loop of pipe and, because itextracts a certain amount of heat, it has to come into contact with acertain amount of ground to do so. There are two ways of achievingthat. Either the loop can run deep underground in a relatively smallarea (so it's a vertical pipe) or it can run much closer to theground (about 1.5–2m or 4–6 ft below the surface) over a much larger area (making it a horizontal pipe).[3]Generally speaking, the bigger your building, the more heat you need,and the bigger or deeper the loop needs to be to extract that heat.Some heat pump loops are really huge. Vertical loops can run100m (330ft) below ground (the length of an athletic sprint track) orsometimes even deeper, while horizontal loops might stretchjust over a meter deep and as far as 200m (650ft) across (they tend to belonger than vertical loops because, being closer to the surface,they're likely to be cooler during winter).Horizontal pipes are sometimes installed as a "slinky" (curved or coiled)to save space.[4]

How does a ground-source heat pump work?

In practice, most ground-source heat pumps actually have two closed loops and aheat exchanger that passes heat from one to the other. So there's avery large loop that goes down into the ground filled with water andanti-freeze, and a much smaller loop that takes heat from this loopand passes it on to the heating system.

Here's how it works:

1. The ground loop (shown here as a coiled slinky) is filled with a working fluid containing propylene glycol(or another antifreeze) that absorbs heat from the ground.
2. The working fluid is pumped into a heat exchanger (dark gray) and flows around the red-blue closed loop.
3. Inside the heat exchanger, the working fluid gives up its heat to a different fluid, a volatile (easily evaporating) refrigerant (shown in light blue), which flows around a second closed loop, entirely inside your home.The refrigerant fluid boils and evaporates to form a low-temperature, low-pressure vapor.
4. The vapor passes into a compressor unit (red), which increases its temperature and pressure.
5. After exiting the compressor, the vapor gives up its heat (to your space heating radiators) and warms your home.It may also heat your hot water using a separate heat exchanger.By giving up its heat, the vapor condenses back to a liquid at lower temperature and high pressure.
6. The liquid passes through an expansion unit that reduces its pressure, turning it back intoa low-pressure, low-temperature liquid ready for the cycle to repeat.
7. Meanwhile, the fluid from the other loop returns to the ground to pick up more heat.

Open loop, water source

If you're trying to extract heat from something like a lake, pond,or river, there's no reason why one of the loops can't be open. Inother words, you could pump water (containing heat) from the lakeitself, extract the heat with a heat exchanger and a second loop,then return the water (cooled somewhat, because you've removed heat)back to a different part of the lake some distance away (so there'senough of a temperature difference to make heat extractionefficient). A system like this is called an open-loop or sometimes asurface-water heat pump. If the lake water isn't suitable for pumping(perhaps because it's too silted up), it's possible to sink aclosed-loop into it instead and extract heat in the same way as weextract it from the ground.

Air source

Heat pumps can also take heat from the air outside a building andpump it inside—so they're effectively working like air conditionersin reverse, with the primary purpose of heating a buildingrather than cooling it, but using much the same principle. Duringwinter, air temperatures are much more variable than groundtemperatures (the air heats up and cools down far more quickly andchanges more erratically than either the surface or deep level groundbeneath your home), so air-source heat pumps tend to be lessefficient and less dependable than ground- or water-source ones.Obviously, it's much easier to extract useful heat from the groundthan from the windy air above it, which could be ten degrees colder.

Photo: A simple air-source heat pump has two parts, which fit either side of a walla bit like a basic air-conditioning unit. Picture by Dennis Schroeder courtesy of US Department of Energy/National Renewable Energy Laboratory (DOE/NREL).

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How does an air-source heat pump work?

Short answer: As I said above, almost exactly like a refrigerator! Where a refrigerator picks up heat from inside the cooling compartment and dumps it in your kitchen, a heat pump picks up heat from outside yourhome and dumps it inside—using almost exactly the same process.

Here, quite simplified, is how it works. The whole process is based on circulating a volatile (easily evaporating) fluid around a closed loop, one part of which is inside your home and the other part is outside.

1. Outside your home, the pump sucks in air (containing heat)—and the working fluid evaporates: it turns from a low-pressure, low-temperature liquid into a low-temperature, low-pressure gas.
2. The gas gets pumped into the compressor unit (there's one at the bottom of your refrigerator,round the back). The compressor squeezes the gas so its pressure and temperature increase.
3. As the gas passes through the inside of your home, it gives off much of its heat, warming yourhome, and cools back down into a high-pressure, somewhat lower temperature liquid.
4. The liquid is allowed to expand through an expansion unit (equivalent to the expansion valvein your refrigerator), so it turns back into a low-temperature, low-temperature liquid.

Advantages and disadvantages of heat pumps

Advantages

Heat pumps have a lot of things going for them. Widely used allover the world for decades, they're tried-and-tested machines withrelatively few moving parts, so they're long lasting, reliable, veryquiet, and very low maintenance. Other than the electricity needed topower the pump, they consume no fuel, so they produce little or nocarbon emissions (none at all if the pump is powered 100 percentrenewably by something like photovoltaic solar panels or a wind turbine) and no pollution(there's also no risk of carbon monoxide poisoning, as there is withany form of combustion heating). No fuel means no fuel to havedelivered, pay for, move around, or store securely (unlike withboilers that burn solid fuel such as biomass, oil, or coal).

What about some hard numbers?As we've already seen, the magic thing about a heat pumpis that it moves several times more energy than it consumes;in other words, it's well over 100 percent efficient (300–400 percentis typical) compared to the very best condensing gas boilers, whichare 90–99 percent efficient. One common measurement of theefficiency of heat pumps is called the coefficient of performance(CoP), and it's simply the amount of heat you get out of your systemdivided by the electrical energy you put into it to drive the pump(the percentage efficiency divided by 100).Typical CoP values are in the range 3–4.[5]

According to the US Department of Energy, heat pumps give annual energy savings of about 30–60 percent.They're several times more efficient than the best gas furnaces (gas central-heating boilers) and 75 percent more efficient than oil furnaces (oil boilers). That means they pay for themselves in 5–10 years(obviously depending on the type of system, what it's replacing, andother factors like how well your house is insulated to retain theheat the pump extracts) and last two to three decades (with theground loop often warranted for as much as 50 years), so thelong-term economic case for putting them in new buildings is verycompelling.[6] And owners seem to love them: according to one detailed academic study by the Open University,75 percent of those surveyed (85 percent of private householders; 58 percent of social-housing occupants)thought their heat pump was "better or much better" than their previous heating system.

Chart: Europe has seen a steady growth in sales of heat pumps, although there was a slight decline between 2022 and 2023. Statistics and data courtesy of European Heat Pump Association.

Heat pumps are especially popular in Europe. According to theEuropean Heat Pump Association,by 2024, a total of almost 24 million pumps had been installed in 21 European countries. In the 17 years between 2005 and 2021, the total number of installed pumps in the region went up almost 17 times. Having said that, Europe now covers a very large area and a huge diversity of different countries, and some are notably more enthusiastic than others. In Norway, Finland, Sweden, and Estonia, there are between 400 and 600 pumps per 1000 households;at the opposite end of the spectrum, Slovakia, the UK, and Hungary have just 30, 15, and 12 pumpsper household. It's also important to put these figures in context. There are roughly 200 million households in the 27 members of the European Union. Although the EHPA's statistics use a different definition of "Europe," you can see that heat pumps are still used by only a fraction of all European homes (perhaps 10 percent, as a rough estimate, based on these imperfect figures).

Takeup is rather lower in the United States. According to the International Energy Agency there were just 1.7 million heat pumps in the United Statesin 2020 (roughly a tenth as many as in Europe), with 40 percent installed in homes and the rest in commercial orother buildings.

Disadvantages

You need land where you can sink a loopand the construction process can be expensive and disruptive (whichis another reason why heat pumps are better for new builds). In the UK, where I live, you generally don't need planning permission forresidential heat pumps (air, water, or ground), unless you live in a listed (heritage)building or in a conservation area—but it's as well to check first.

Heat pumps don't produce such high temperatures as gas boilers and furnaces, so they're often used with low-temperatureunderfloor heating. Ordinary radiators are likely to produce significantly lower temperatures from a heat pump than you'd get with a conventional gas boiler, so they'll take longer to heat upyour home from cold. Unlike electric and natural gas heating, whichcan be used in almost any building, heat pumps depend on the localgeology and climate, so you need to factor that in; and obviously,you can't use them just for one apartment on the 93rd floor!Another important point is that heat pumps do need to be properly installed. One UK study published in 2010 found that 80 percent of heat pumps were badly installed andmassively underperforming—sometimes consuming more energythan they actually produced.

Some air-source heat pumps can be noisy—they are mechanical pumps with moving parts—butthey aren't that different from air conditioners.One typical user quotes 45–55dB, which is about the same noise level as moderate traffic.

But all forms of heating have their pros and cons. Taken in theround, heat pumps are a very attractive long-term alternative (or addition) togreen technologies like solar hot water panels, photovoltaic solar panels, andpassive solar.

Who invented the ground-source heat pump?

Photo: Lord Kelvin's thermodynamics research—the science of moving heat—underpins modern heat-pump technology. Photo by T. & R. Annan & Sonscourtesy of US Library of Congress.

Whom do we have to thank for this brilliant idea? The theory of heat engines (machines that cangenerate mechanical energy by heating fuel) largely began with French engineer Nicolas Sadi Carnot (1796–1832),who laid the foundations for the modern science of thermodynamics (how heat moves).British scientist Lord Kelvin (William Thomson,for whom the Kelvin temperature scale is named)built on Carnot's work and figured out the theory of heat pumps (how a machine can move heat energyfrom one place to another).

Kelvin's work was concerned with the theoretical movement of heat; efficient, practical heat pumps only really appeared in the early decades of the 20th century when electric power became widespread, giving us such indispensable inventions as the electricrefrigerator and air conditioner. Mexican-born Swiss engineer Heinrich Zoelly invented the modern,electrically powered ground-source heat pump in 1912. After that, numerous inventors improved on theidea. Around 1930, Douglas Warner developed an air conditioning system using the ground as a heat sink(which is effectively a ground-source heat pump idea working in reverse), for whichhe was granted US Patent US 1,957,624: Air conditioning with ground cooling and solar heat in 1934.According to geologist David Banks, Robert C. Webber of Indianapolis developed one of the first, practical, working heat pumps around 1945, after experimenting with using waste energy from his refrigerator to heat his home. Taking the idea a step further, he buried a 152-m copper coil underground and used that to extract heat instead. He sold the rights to this idea to a group of local businesses in 1948, but I can find no record of a patent for it either in his name or that of the company (Webber Engineering Corporation).

How it works

The early heat pump I've chosen to illustrate here was patented by Marvin Smith and Emory Kemler of Muncie Gear Works, Inc. in the late 1940s. Smith and Kemler's 1940s water-well heat pump is not the world's first geothermal pump, but it's the earliest one for which I can find a decent diagram. Like most heat pumps, it can run as either a heater or a cooler. Here, I've colored and numbered to show how it works as a heat pump.

Although this diagram looks confusing at a glance, it's simpler than it appears: there are two separate fluid loops connected by a pair of heat exchangers (the red and blue boxes near the top). The lower loop removes heat from the water well at the bottom, while the upper loop carries that heat to whatever the pump is heating.

Artwork: An early ground-source heat pump designed by Marvin Smith and Emory Kemler, from US Patent #2,461,449: Heat pump using deep well for a heat source. Artwork courtesy of US Patent and Trademark Office.

In a bit more detail, here's how it works: (1) Heat is extracted from a deep water well, passes up through valves (2) to a heat exchanger (3), before returning through the circuit to a cooler, higher part of the same well (4). The heat exchanger passes the heat to a fluid in a closed loop, where it gives up its heat to the "receiver" (6, the room or whatever else is being heated), before passing back around the loop to pick up some more. You can read a detailed explanation in US Patent #2,461,449: Heat pump using deep well for a heat source.

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On other sites

• Consumer's Guide: Geothermal Heat Pumps: from US Department of Energy EERE (Energy Efficiency andRenewable Energy) department.
• International Ground Source Heat Pump Association (IGSHPA): A non-profit organization designed to promote geothermal heat pump (GHP) technology.
• Geothermal energy: A backgrounder for young people from the US Department of Energy.

Statistics

• Heat Pumps: A market outlook for the next few decades compiled by the International Energy Agency.
• [PDF] Geothermal Heat Pumps: Overview of Market Status: February 2009: An old (but still useful) US Department Energy report summarizing the current status of the geothermal market and the potential for future growth. [Archived via the Wayback Machine.]

Books

• Heat Pumps for the Homeby John Cantor. Crowood, 2020. A handy practical guide written by a experienced heat pump engineer.
• Heat Pumps by Eugene Silberstein. Cengage, 2015. An up-to-date and detailed technical guide.
• Heat Pump Technology by Hans Ludwig von Cube and Fritz Steimle. Butterworth Heineman, 1981/2013. A modern reprint of a general, quite theoretical introduction toheat pumps and their applications. Not a great source for up-to-date practical details, but still quite good for the basicconcepts and thermodynamic theory.
• An Introduction to Thermogeology: Ground Source Heating and Cooling by David Banks. Blackwell, 2012. An excellent primer that covers everything from the history of heat pumps and how they work to the various different types and their environmental impact.
• Geothermal Heat Pumps: A Guide to Planning & Installing by Karl Ochsner. Earthscan, 2008.
• Geothermal Heat Pumps: Installation Guide by Steve Ewings. Globalgreenhousewarming.com, 2008. Mainly a US-focused guide, but with some information about the UK, Canada, and Australia.
• Smarter heating by David MacKay, includes a simple introduction to heat pumps, and comes from his book Sustainable Energy Without the Hot Air.

Articles

• Why Mainers Are Falling Hard for Heat Pumps by Cara Buckley, The New York Times, March 2, 2024. Heat pumps are now outselling gas furnaces in the United States—and Maine is most enthusiastic.
• The 'exploding' demand for giant heat pumps by Chris Baraniuk, BBC News, May 29, 2023. How about using giant pumps to power entire city districts or small towns?
• Heating up the global heat pump market by Jan Rosenow et al, Nature Energy, September 7, 2022. A great review of the current state of play. Also explains why maintaining current rates of heat pump sales will be a challengein the coming years.
• What are heat pumps and why is the UK government pushing them? by Jillian Ambrose, The Guardian, October 19, 2021. A brief introduction to why British people have suddenly started talking about heat pumps.
• One Thing You Can Do: Consider a Heat Pump by Tik Root and Nadja Popovich, The New York Times, October 16, 2019. Why mechanical engineers consider heat pumps "a no brainer."
• Heat pumps extract warmth from ice cold water by Richard Anderson. BBC News, 10 March 2015 and Plas Newydd: Heat from the sea to warm historic house by Roger Harrabin. BBC News, 22 May 2014. These articles describe two very different projects for harvesting energy from cold water using heat pumps.
• Digging Up Energy Savings Right in Your Backyard by Lorraine Kreahling. The New York Times, March 7, 2011. A US perspective on heat pumps.
• UK 'heat pumps' fail as green devices, finds study by Adam Vaughan, Guardian, 8 September 2010. A study finds that a majority of heat pumps are badly installed and don't perform efficiently.
• Hot Earth to heat homes: BBC News, 22 November 2001. Reports on a 70m (230ft) closed vertical loop system in Nottingham, England.
• Ground-source heat pump: still a smart buy? by Jack Horst, Popular Science, October, 1987. I include this old article mainly for historical interest, although it does have a good little box and illustration explaining how heat pumps work.

Scholarly articles and papers

• An estimation of the European industrial heat pump market potential by A. Marina et al, Renewable and Sustainable Energy ReviewsVolume 139, April 2021. This paper explores the market for industrial pumps, estimated at 23GW and 4174 units.
• Domestic heat pumps in the UK: user behaviour, satisfaction and performance by Sally Caird et al, Energy Efficiency 5, 283–301 (2012). An interesting survey of heat pump users–and what they think of their systems.

Patents

• US Patent #2,461,449: Heat pump using deep well for a heat source by Marvin Smith and Emory Kemler of Muncie Gear Works, Inc. February 8, 1949. The early ground-source pump illustrated above.

1. ↑Figures for ground temperature vary around the world, and for various other reasons too. I've taken my 10–20 degrees from measurements in Nicosia, Cyprus quoted in[PDF] Measurements of Ground Temperature at Various Depths by Georgios Florides and Soteris Kalogirou.
2. ↑According to theWorld Bank, global geothermal potential stood at 70–80 GW in 2017, of which about 15 percent is currently exploited.The US Department of Energy estimates that typical nuclear plants produce 1GW of power.
3. ↑An Introduction to Thermogeology: Ground Source Heating and Cooling by David Banks, Wiley, 2009, p.128 quotes 1.2–2m.
4. ↑Faber & Kell's Heating & Air-conditioning of Buildings by Doug Oughton, Steve Hodkinson, Taylor & Francis, 2008, p.388 quotes "up to 100m deep depending on geology, the energy output required from the system and the space available."Mechanical and Electrical Equipment for Buildings by Walter T. Grondzik et al, Wiley, 2011, p.370 quotes 120–200m (400–650ft) of pipe installed 1–2m (3–6ft) deep.
5. ↑Heat Pumps by Eugene Silberstein, Cengage, 2015 quotes a range of COP values, roughly 3–6, for heat pumps operating in different heating and cooling modes.
6. ↑Figures in this paragraph come from the US Department of Energy.

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