Geothermal Heat Pumps Explained: How Ground-Source Systems Actually Work

If you already understand how heat pumps work in general, geothermal is a short step. Everything in the refrigerant loop is the same: compressor, condenser, expansion valve, evaporator, reversing valve. The only real difference is where the outdoor half of the machine gets its heat. Instead of a fan blowing winter air across a coil, a pump pushes water through plastic pipe buried in your yard.

That one change ripples through everything else, though. It changes the cost, the install timeline, the efficiency, the noise, the way the system is rated, and how long the pieces last. This guide walks through the whole picture so you can tell whether a ground-source system is worth investigating on your property, or whether a good cold-climate air-source unit does the same job for a fraction of the money.

Why the ground beats the air

Sunlight heats the surface of the earth, but soil is a slow, heavy conductor. Heat takes weeks to move down through it, and by the time it does, the season has changed. The practical result is that the deeper you go, the flatter the temperature curve becomes. At a couple of feet the soil still swings with the weather. By three to ten feet the swing has shrunk to a few degrees. By thirty feet or so it is essentially gone, and the ground sits at roughly the annual average air temperature for your location.

The U.S. Department of Energy describes that shallow ground temperature as ranging from about 45°F to 75°F across the country, warmer in the South and cooler in the North. In much of the middle of the U.S. it lands near 50°F to 60°F. Whatever your local number is, the important part is that it does not care what the weather is doing today.

Now think about what a heat pump actually has to do. It moves heat from a colder place to a warmer place, and the work it takes is set by the size of that temperature lift. In January, an air-source heat pump in Minnesota might be pulling heat from 0°F air and delivering it to a 105°F supply-air stream. That is a lift of more than 100 degrees. A geothermal unit on the same night is pulling from a loop running maybe 30°F to 40°F and delivering to the same supply air. The lift is roughly a third smaller, and the machine uses correspondingly less electricity for the same heat.

Summer flips it. In August the air-source unit has to reject heat into 98°F air. The ground-source unit rejects it into a loop that might be running 75°F to 85°F. Rejecting heat into something cooler is easier, so cooling efficiency climbs too. This is why geothermal systems tend to show their strongest advantage in places with big seasonal extremes in both directions, like the upper Midwest, the Mountain West and parts of the Northeast.

One nuance worth knowing: the loop temperature is not the undisturbed ground temperature. When your system runs hard for weeks, it cools the soil immediately around the pipe in winter and warms it in summer. A properly designed loop is sized so that this drift stays within a workable band across a full season, and so that heating and cooling roughly balance over the year. An undersized loop is the single most common cause of a geothermal system that underperforms its brochure.

The two loops, and how they meet

Almost every confusion about geothermal clears up once you see that there are two separate fluid circuits that never mix.

The ground loop is a long run of high-density polyethylene pipe buried in your yard or sunk in a borehole. It is filled with water, usually mixed with a food-grade antifreeze such as propylene glycol or methanol in cold regions. A small circulating pump, sometimes called a flow center, pushes that fluid out into the ground and back. Nothing about this loop is exotic. It is plastic pipe, fusion-welded joints, water and a pump. It contains no refrigerant and no moving parts underground.

The refrigerant loop lives entirely inside the indoor cabinet. It is the standard vapor-compression cycle: a compressor squeezes refrigerant vapor hot, a condenser gives that heat up, an expansion valve drops the pressure, and an evaporator absorbs heat as the refrigerant boils. A reversing valve swaps the direction for cooling, exactly as in an air-source system.

The two loops meet at a heat exchanger, typically a brazed-plate or coaxial coil where the ground-loop water flows on one side and refrigerant on the other, separated by metal. In heating mode, water coming back from the ground at, say, 38°F flows across the plate and gives up a few degrees of heat to refrigerant that is colder still. The refrigerant boils, the compressor takes it from there, and the heat ends up in your ducts. The water goes back into the ground a few degrees cooler to pick up more.

The ground loop doesn’t heat your house. It just hands the heat pump a much easier starting point than a January night ever could.

— the core idea, in one sentence

On the indoor side, the unit is usually a water-to-air package that looks like a large air handler in a basement or mechanical closet, connected to ductwork. There is no outdoor condenser at all. The alternative is a water-to-water unit, which makes hot or chilled water for radiant floor loops, fan coils or a buffer tank. Water-to-water pairs beautifully with low-temperature radiant floors, because a heat pump making 100°F water is far more efficient than one making 130°F water.

A third, less common design is direct exchange (DX) geoexchange, where refrigerant itself circulates through buried copper tubing and there is no water loop or intermediate heat exchanger at all. DX systems can be efficient and use shorter bores, but they put refrigerant underground, which raises leak and corrosion questions and limits your choice of contractors. Most U.S. residential installs are closed-loop water systems.

The four loop types and what each needs

The loop is where your site does the choosing. Three of the four common configurations are closed loops, meaning the same fluid circulates forever. The fourth pulls actual groundwater through the system and returns it.

Horizontal closed loop. Pipe is laid in trenches dug across the yard, generally at least four feet deep and often around four to six feet. DOE describes common arrangements such as two pipes at six feet and four feet, or two pipes side by side at five feet in a two-foot trench. Some installers use “slinky” coils, where the pipe is looped like a spring to fit more length into less trench. Horizontal is usually the cheapest loop to install where there is room, because a backhoe is a lot less money than a drill rig. It needs open, diggable land, typically a few thousand square feet for a normal house, and the trenching tears up whatever is above it.

Vertical closed loop. A rig drills boreholes roughly four inches across, generally 100 to 400 feet deep and spaced about 20 feet apart. A U-bend of pipe goes down each hole and the hole is grouted shut. Vertical loops need very little surface area, work on small suburban lots, and are less affected by soil moisture and surface conditions because they reach into stable, deeper ground. They cost more per ton because drilling is expensive, and the price swings hard depending on whether the rig is punching through soft soil or granite.

Pond or lake closed loop. If you have a pond, lake or large water body on or next to your property that meets minimum depth, volume and quality requirements, coils of pipe can be sunk to the bottom and anchored. This is often the cheapest loop of all because there is no excavation, just pipe and a supply trench from the house to the water. The constraints are real, though: the water body needs enough depth that it does not freeze to the coils and enough volume that your system does not measurably change its temperature. Local permitting for work in or near water is usually involved.

Open loop and standing column well. An open-loop system draws water from a well or other supply, runs it through the heat exchanger, and discharges it to a second well, a surface discharge or a return field. A standing column well is a variation common in hard-rock regions such as New England: a single deep well acts as both source and return, with water drawn from the bottom and returned near the top. Open systems can be very efficient because they use groundwater at its natural temperature. They are also the fussiest. You need a good, sustained water yield, acceptable water chemistry, and local rules that permit the discharge. Hard, iron-rich or mineral-laden water can scale or foul the heat exchanger, and pumping water costs electricity that eats into the efficiency advantage.

Why it’s rated in COP and EER, not SEER2 and HSPF2

Air-source heat pumps are rated with SEER2 and HSPF2, seasonal numbers that average performance across a whole synthetic season of changing outdoor air temperatures. That averaging exists because outdoor air temperature is the variable that dominates an air-source unit’s performance, and it changes constantly.

Ground-source equipment does not have that problem. Its source temperature is a loop, and a loop can be held at a defined test condition. So geothermal units are tested to ISO 13256-1 (water-to-air) and ISO 13256-2 (water-to-water) at fixed entering-water temperatures, and reported as two simple ratios:

• COP — coefficient of performance, heating output divided by electrical input. A COP of 4.0 means four units of heat delivered per unit of electricity consumed. It is a plain ratio with no units.
• EER — energy efficiency ratio, cooling output in BTU/h divided by electrical input in watts. Divide EER by 3.412 if you want the cooling COP.

Because the test conditions differ, you cannot line a geothermal COP up next to an air-source HSPF2 and declare a winner. They are answering different questions. What you can do is compare geothermal units to each other, and to the ENERGY STAR thresholds.

Those are floors, not targets. Well-regarded variable-speed residential geothermal units on the market typically publish closed-loop ratings somewhere in the EER 20s and COP 4s at full-load rating conditions, with part-load numbers higher still. Open-loop ratings run higher than closed-loop ratings for the same box, which is why the ENERGY STAR bar is set higher there — groundwater at 50°F is simply an easier source than a closed loop that has been drawn down to 35°F in February.

One more wrinkle: pump power. The energy used by the ground-loop circulating pump is included in the ISO ratings only up to an allowance for internal pumping. If your installer specs an oversized pump for a long or restrictive loop, that extra draw runs whenever the system runs and can quietly cost you a meaningful chunk of the efficiency you paid for. Variable-speed circulators help a lot here and are worth asking about.

The desuperheater and near-free hot water

Here is a genuinely nice feature that air-source systems mostly do not offer. When the compressor discharges refrigerant, that vapor comes out very hot — well above the temperature actually needed to heat your house. A desuperheater is a small extra heat exchanger placed in that hot discharge line, plumbed to your water heater. It skims the hottest slice of that heat and sends it to the tank.

In cooling season this is close to free. The system is trying to get rid of heat anyway, so any of it that goes into your hot water tank is heat you no longer have to pay to reject into the ground, and hot water you no longer have to pay your water heater to make. In heating season it is not free, because the heat diverted to the tank is heat that did not go into the house, but it is still produced at the heat pump’s COP rather than at the water heater’s efficiency.

Manufacturers and installers commonly claim a desuperheater can cover a substantial share of a household’s annual water heating, with figures in the range of roughly a quarter to a half of the load frequently quoted. We have not measured this, and the real number depends heavily on how much your system runs, your hot water habits and tank size. Treat it as a real benefit of uncertain size rather than a headline saving.

Cold snaps: geothermal vs air-source

This is the difference people feel. When a polar outbreak drops outdoor air to -5°F, an air-source heat pump faces two problems at once: its capacity falls just as the house needs the most heat, and its COP falls because the temperature lift got enormous. Good cold-climate air-source models handle this far better than older equipment, holding most of their rated capacity down to 5°F and continuing to run well below zero. But physics still applies, and below the balance point something has to make up the difference.

A geothermal system does not see the cold snap directly. Its loop is below the frost line, and loop temperature changes over weeks, not hours. Capacity and COP on the coldest night of the year are close to what they are on a merely chilly night. There is no defrost cycle either, because there is no outdoor coil collecting frost — which also means no mid-winter pauses in heating and no steam plume in the yard.

Note what this table does not say. It does not say geothermal is the better buy. It says geothermal is the better machine, and that the gap between them has narrowed a lot as cold-climate air-source equipment has improved. Whether that machine is worth several times the money is a separate question, and it turns almost entirely on cost — which is why we split it into its own guide.

Noise and how long everything lasts

Removing the outdoor unit removes the loudest thing about a heat pump. There is no outdoor fan whirring at 3 a.m., no compressor humming next to a neighbor’s bedroom window, nothing that a homeowners’ association or a municipal noise ordinance can object to. For tight lots, urban infill and properties where an outdoor condenser would sit under a window, that is a real advantage. Our guide to heat pump noise covers what the decibel figures on air-source spec sheets actually mean.

Geothermal is not silent, though — it just moves the sound indoors. The compressor and blower live in your basement or mechanical room, and they make the kind of noise a large air handler makes. Most owners describe it as unobtrusive, but if the unit sits under a bedroom in an uninsulated joist bay you will hear it. Ask about vibration isolation pads, flexible duct connectors and a lined supply plenum. Circulating pumps add a low hum that is usually inaudible beyond the mechanical room.

Lifespan is the other structural difference, and it is lopsided in a useful way.

• The ground loop is buried HDPE pipe with heat-fused joints and no moving parts. Manufacturers and installers commonly warrant or rate loop pipe for 50 years or more, and the material itself is expected to last longer than that. We can’t independently verify multi-decade field data, but the design intent is clear: the loop is meant to be a one-time investment.
• The indoor equipment — compressor, blower, controls, heat exchanger — is a normal piece of HVAC machinery. Industry figures commonly cited put it around 20 to 25 years, better than the 12 to 15 years often quoted for air-source equipment, mainly because it lives indoors out of the rain, sun, ice and road salt.

You buy the loop once and the heat pump two or three times. That asymmetry is the whole financial argument for geothermal, and the whole reason a bad loop is so expensive to fix.

— the practical takeaway on lifespan

The flip side of that asymmetry is risk. A poorly designed or badly installed loop is buried under your yard. Correcting it means excavating or drilling again. This is why loop design credentials matter more than brand loyalty when you are choosing a contractor, and why it is fair to ask for references on loops installed five and ten years ago rather than last season.

This is a whole-system decision, not an appliance swap

Replacing a furnace with an air-source heat pump is largely a like-for-like exchange. A crew arrives, the old equipment comes out, new equipment goes in, and most homes are back running the same day or the next. Our installation walkthrough describes that process.

Geothermal is not that. It is a small construction project with an HVAC job attached. Depending on the loop type you may be dealing with a drilling rig or an excavator on your property, permits for wells or water discharge, utility locates, spoil and mud management, restoration of lawn and landscaping, and a schedule measured in days to weeks rather than hours. The loop contractor and the HVAC contractor may be two different companies whose work has to be coordinated.

There are also decisions that come along for the ride:

1. Load calculation first, always

   Loop size follows load. A room-by-room Manual J determines both the equipment tonnage and the footage of pipe. Guessing here is expensive twice: once for the oversized loop and again for a system that short-cycles.

2. Duct evaluation

   Geothermal supply air, like any heat pump’s, is cooler than furnace air. Ducts sized for a 140°F furnace often need more airflow to deliver the same heat. Undersized returns are the classic problem in retrofits.

3. Electrical service check

   You’re removing a gas or oil appliance and adding compressor, blower, pump and possibly backup strip heat. A panel and service review belongs in the quote, not as a surprise later.

4. Domestic hot water plan

   Decide up front whether you want a desuperheater, a preheat tank, or a separate heat-pump water heater. Retrofitting the plumbing later costs more than doing it during the install.

5. Site restoration in writing

   Who regrades the trenches, replaces the sod, repairs the driveway if a rig cracked it, and hauls away drilling spoil? Get it in the contract with a timeline.

Before you go any further, it is worth working through whether a heat pump of any kind fits your house and your plans. Our is a heat pump right for my home guide covers the envelope, ductwork and climate questions that come first, and the geothermal cost guide handles the money side, including who genuinely comes out ahead and who should stick with air-source.

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