How Does Geothermal Heating Work? The Full Cycle, Explained

A geothermal heating system is two separate loops of fluid and one clever machine sitting between them. One loop is plastic pipe buried in your yard, filled with water (usually with antifreeze mixed in). The other is a sealed refrigerant circuit inside the heat pump itself. The machine's whole job is to grab low-grade warmth from the first loop, concentrate it, and hand it to your house.

That's it. No combustion, no fuel line, no flue. Most of the heat in your living room in January was in the dirt under your lawn a few minutes earlier — the rest is the electricity the compressor, pumps, and blower turned into heat on the way.

Those two loops are easy to confuse, and most bad explanations of geothermal — including some from people selling it — come from collapsing them into one. Two details also catch new owners off guard: vent air that feels lukewarm to the hand, and an "auxiliary heat" indicator that does not necessarily mean anything is wrong. If you want the broader picture first, start with what geothermal energy actually is and our overview of how geothermal heat pumps work.

The One-Sentence Version

Water circulating through buried pipe picks up heat from the ground, carries it indoors to a heat exchanger, and a refrigerant cycle inside the heat pump concentrates that heat and releases it into your home's air or floors.

That sentence contains a fact that routinely gets garbled, including by people selling these systems: the fluid in the ground is water or a water-antifreeze mix, not refrigerant. Refrigerant stays in a separate sealed circuit inside the equipment. The two fluids pass heat to each other through the metal wall of a heat exchanger and never mix.

There is one exception, and it's worth naming so you don't get confused reading manufacturer literature. DOE describes a "much less common" closed-loop design called a direct exchange (DX) system, which circulates refrigerant directly through buried copper tubing. If you're looking at a conventional system rather than a DX unit, the loop in the ground carries water.

Why does the distinction matter to you as a homeowner? Because it changes what's serviceable. The buried loop is plastic pipe and water. The refrigerant circuit, the part that needs a certified technician, sits indoors where a tech can reach it.

The Ground Is the Battery

Winter air where geothermal makes sense can swing from below zero to the fifties within a single week. The ground does not care.

DOE puts it precisely: at about 30 feet below the surface, temperatures in most areas of the United States remain relatively constant year-round, between about 50°F (10°C) and 59°F (15°C). DOE's 2011 heating and cooling guide adds a second framing at a much shallower depth: the earth stays fairly constant at a U.S. average of 55°F (cooler in the north, warmer in the south), varying less than 20 degrees over the year at 5 feet down. That 55°F is a national average, not a promise for your backyard. The exact temperature varies with local geology and climate, but it changes far less over the year than outdoor air does.

"Mild" is the operative word. Fifty-degree dirt cannot heat your house directly. What it can do is feed a heat pump a steady, generous source to pull from, which is exactly what an air-source heat pump lacks on the coldest night of the year, when it needs heat most and the air has the least to give.

To tap that reservoir, installers bury a closed loop of pipe in one of two main layouts, per DOE's guide:

• Horizontal loops run through trenches 5 to 10 feet deep. They need yard space but keep drilling costs down.
• Vertical loops go into boreholes 75 to 500 feet deep, backfilled with bentonite or other grout so the pipe stays in firm thermal contact with the earth.

The right choice depends on your lot, your soil, and your budget. We compare them in detail in horizontal vs. vertical ground loops, and if you've seen systems that use well water instead of a sealed loop, that's a different design covered in open loop vs. closed loop.

However the pipe is arranged, its job is the same. Fluid leaves the heat pump cool, wanders through ground that is warmer than it is, and comes back carrying heat. DOE's own system diagram labels the two ends of this journey plainly: "cooler antifreeze out of heat pump, warm antifreeze into heat pump." For a deeper look at the machine on the other end of those pipes, see our ground-source heat pump guide.

The Heating Cycle, Step by Step

The refrigeration cycle works like a refrigerator in reverse. Your fridge removes heat from a cold box and releases it into your kitchen; a geothermal heat pump removes heat from cool loop fluid and releases it into your house. Same physics, bigger scale.

Follow one lap around the system in heating mode:

1. The loop gathers heat. Cool water-antifreeze leaves the heat pump and circulates through the buried pipe. The surrounding ground is warmer than the fluid, so heat flows in. The fluid returns to the house carrying that warmth.

2. The first hand-off. Inside the unit, the warm loop fluid flows through the primary heat exchanger, a water-to-refrigerant heat exchanger. On the other side of the metal is the refrigerant, which at this point in its cycle is colder than the incoming fluid. Heat crosses over, and the refrigerant absorbs enough of it to boil into a vapor. This is the evaporator side of the cycle. The loop fluid, now cooler, heads back underground for another lap.

3. The compressor does the heavy lifting. Refrigerant vapor carrying ground heat enters the compressor, which squeezes it hard. Compressing a gas raises its temperature, so the vapor comes out much hotter than anything the ground could provide on its own. This is the step that turns mild warmth into useful heat, and it's why the system needs electricity: the compressor is the muscle.

4. The second hand-off. The hot refrigerant flows through the secondary heat exchanger, where it releases its heat and condenses back to a liquid. DOE describes both ways that heat reaches you: air blown over the exchanger and pushed through ducts, just like a central furnace, or fluid warmed and sent through tubing in the floors for radiant heat. Ducted versions are called water-to-air systems. The radiant version, a water-to-water system, heats a hydronic loop rather than blowing air at all.

5. The reset. The liquid refrigerant passes through an expansion device, which drops its pressure abruptly. Dropping pressure drops temperature, and the refrigerant emerges cold again, ready to soak up the next batch of ground heat at the primary exchanger. The cycle repeats as long as the thermostat calls for heat.

A note on where all this hardware lives. In a packaged unit, the whole refrigerant circuit sits in one indoor cabinet. Split configurations also exist, connecting indoor components with refrigerant piping between sections. Either way, the refrigerant circuit is not buried. Only the water loop goes in the ground. DOE's component diagram also shows an optional desuperheater, an add-on heat exchanger some owners use toward water heating.

The components DOE labels in its packaged water-to-air example, for the curious: reversing valve, expansion device, primary heat exchanger, secondary heat exchanger, compressor, desuperheater, and blower. Seven parts, no burner. Other configurations differ — a water-to-water unit has no air blower because it heats a hydronic loop, and split systems distribute the refrigeration components differently.

The Reversing Valve: Why the Same Box Also Cools

"Because a heat pump is equipped with a reversing valve, it can both heat and cool your home." That's DOE's phrasing, and the mechanism is as tidy as it sounds.

The reversing valve changes the direction the refrigerant flows through the circuit. When flow reverses, the two heat exchangers swap jobs:

• Heating mode: the ground-side exchanger is the evaporator (absorbing heat from the loop fluid) and the home-side exchanger is the condenser (releasing heat into your house).
• Cooling mode: the roles flip. The home-side exchanger becomes the evaporator, pulling heat out of your indoor air, and the ground-side exchanger becomes the condenser, rejecting that heat into the loop fluid, which carries it underground and leaves it there.

In summer the system runs the transfer the other way, moving indoor heat into the ground. The same stable ground temperature that supplies heat in January absorbs it in July, using the same heat pump and the same buried loop. That's the case we lay out in geothermal heating and cooling, and the summer half of the story gets its own full treatment in our geothermal cooling guide.

If you searched "how does geothermal heating and cooling work," this valve is the entire answer to the "and cooling" part. Nothing else about the system changes.

Why the Air Feels Different From a Furnace

Put your hand over a supply vent in a furnace-heated house and you feel a blast of hot air. Do the same in a geothermal house and the air feels warm, maybe barely warm. New owners sometimes mistake the cooler airflow for a malfunction, but it is normal heat pump operation.

DOE's heating and cooling guide, citing NRCan 2009 data, gives the numbers for heat pumps generally: furnaces deliver air to the living space at between 130°F and 140°F, while heat pumps provide air at about 80°F to 115°F. Because the air is cooler, more of it has to move to deliver the same amount of warmth. DOE quotes that comparison in its air-source section rather than as a geothermal-specific measurement, so treat it as the shape of the difference rather than a spec for any particular unit — your own supply temperature depends on the equipment, the airflow, and the entering water temperature.

Notice that even the low end of that range is well above room temperature, and every degree of it is heating your house. But 95°F air moving across a hand that expects 135°F reads as "cool," the same way 70°F pool water feels cold on a hot day. Your body judges by contrast, not by thermometer.

What this means day to day: air moves through the ducts in higher volume, and the system warms the house steadily rather than in short hot blasts. Some homeowners describe furnace heat as a wave and heat pump heat as a tide. If you're weighing that difference against what you're used to, our comparison of geothermal vs. traditional HVAC walks through it from the homeowner's chair.

So when your hand says "this air is lukewarm," believe your thermostat instead. If the house is holding temperature, the system is doing exactly what DOE's numbers say it should.

What Auxiliary Heat Actually Is

Auxiliary heat is often explained using air-source heat pump behavior, which does not transfer to geothermal systems.

DOE's 2011 guide notes that the heating efficiency of standard, central air-source heat pumps of that era dropped when outside temperatures fell below about 35°F, which is why those systems often needed a backup heat source. That statement is about air-source machines, whose heat supply is the outdoor air itself. When the air gets cold, their source gets stingy at exactly the moment demand peaks.

A geothermal system's source is the ground, and as covered above, the ground at loop depth doesn't track the outdoor air. A cold snap that hammers an air-source unit's supply barely registers on a ground loop. Loop temperatures do drift over a season as heat is drawn out, but they don't swing with the weather. This is the central advantage of putting your heat source underground.

So why do geothermal systems have auxiliary heat at all? Because of a sizing decision, not a performance cliff.

A home's heating demand peaks for only part of the year, and sizing equipment and loop for that peak means paying for capacity that sits idle the rest of the time. Some designs therefore select a heat pump below the peak load and let electric resistance elements cover the coldest stretches. The temperature where the heat pump alone stops keeping up and the elements start helping is called the balance point. It isn't simply chosen off a menu — it falls out of where your home's load curve crosses the equipment's available capacity, which is why the load calculation, the equipment selection, the loop, and the controls all move it.

The practical takeaways:

• Seeing "aux heat" on your thermostat during a hard cold snap can simply be the system working as designed.
• Auxiliary heat running constantly, or in mild weather, is a different story and worth a service call.
• When you get quotes, ask each installer where they're setting the balance point and how the auxiliary stages are controlled. It's a revealing question, because it forces a real sizing conversation.

One more durability point from DOE while we're on the subject of cold weather: because the compressor of a ground-source heat pump is located inside the home, it is subject to much less wear and tear than the outdoor compressor fans of air-source heat pumps. Your neighbor's heat pump sits in the sleet. Yours sits next to the water heater.

What This Means When You're Shopping

Understanding the mechanism changes the questions you ask. If you're at the getting-quotes stage, the physics above translates into a short checklist:

1. Which loop type, and why? Horizontal trenches (5 to 10 feet deep) and vertical boreholes (75 to 500 feet, grouted) solve the same problem at different price points and lot requirements. An installer should justify the choice from your site, not their default. Loop decisions drive a large share of the project price, which we break down in the geothermal installation cost guide.

2. Water-to-air or water-to-water? If your house has ducts, you're likely looking at water-to-air. If you have (or want) radiant floors or another hydronic setup, water-to-water systems heat the water loop directly. Some homes use both. Know which one is being quoted.

3. Packaged or split? Packaged units keep the refrigerant circuit in one cabinet. Splits connect indoor components with refrigerant piping. Neither puts refrigerant in the ground. It's a layout question for your mechanical space.

4. Where's the balance point? Per the auxiliary heat section above, this is the sizing conversation. An installer who can't answer it crisply hasn't done the load math on your house.

5. What are the efficiency numbers, and at what conditions? Ratings for ground-source equipment have their own alphabet soup, and the test conditions matter as much as the headline figure. We decode them in geothermal efficiency ratings explained.

On incentives, keep your math conservative. The 30% federal residential credit that helped fund many past installations expired for systems placed in service after December 31, 2025, so don't let a quote lean on it. State and utility programs vary and change, so verify anything you're counting on as of the month you sign. None of this is tax advice, and it describes the landscape as of August 2026. Run the actual numbers past a tax professional.

One caveat on sources: the DOE heating and cooling guide cited throughout this article is dated August 2011. The physics of the cycle hasn't changed and won't. Equipment specifics from that era, like the air-source performance figures mentioned above, should be read as historical context rather than a description of today's product catalogs.

Frequently Asked Questions

Is there refrigerant in the pipes buried in my yard?

In a conventional system, no. The buried loop carries water or a water-antifreeze mixture, and the refrigerant stays in a separate sealed circuit that is not underground. The exception is the direct exchange (DX) design, which DOE describes as much less common: it circulates refrigerant through buried copper tubing. Unless your paperwork says DX, assume water.

Does geothermal heating work in very cold climates?

The system's heat source is the ground, not the air. DOE reports that at about 30 feet down, temperatures in most of the United States hold relatively constant year-round between about 50°F and 59°F. An outdoor cold snap raises your home's demand for heat, but it doesn't shrink the ground's supply the way it shrinks an air-source heat pump's.

Why do geothermal systems have backup heat if the ground stays warm?

Because of sizing, not weakness. Installers often size the heat pump below the home's absolute peak load, since that peak only occurs a few hours a year, and let electric resistance elements cover those hours. Auxiliary heat during a deep cold snap is the design working. Auxiliary heat running in mild weather is a reason to call for service.

Can the same system cool my house in summer?

Yes. A reversing valve changes the direction of refrigerant flow, which swaps the roles of the two heat exchangers. In summer the system pulls heat out of your indoor air and rejects it into the loop fluid, which carries it into the ground. The details are in our geothermal cooling guide.

Does geothermal work with radiant floor heat, or only ducts?

Both. DOE describes two delivery paths: warming air that's blown through ducts like a central furnace, or warming fluid that flows through tubing in the floors for radiant heat. The ducted version is a water-to-air system, and the radiant version is a water-to-water system.