Geothermal Cooling: How Geothermal Air Conditioning Actually Works

By Geothermal Insider · Published July 24, 2026

Most people understand geothermal as a heating system — pulling warmth out of the ground in winter. But the same box is one of the best air conditioners money can buy, and for a reason that's easy to miss: a geothermal system doesn't cool your home by making cold. It cools by moving your home's heat somewhere it will actually go — into the earth, which stays around 55°F while the air outside bakes at 95°F.

That single difference is why geothermal cooling keeps its efficiency on exactly the days a normal AC is gasping. Here's how it works, what the real efficiency numbers are (and why the number on a conventional AC's sticker is the wrong one to compare against), and a summer bonus most cooling articles never mention.

How Geothermal Cooling Actually Works

A geothermal heat pump uses the same vapor-compression refrigeration cycle your refrigerator and your current AC use. The trick is that it can run that cycle in either direction, and the component that flips it is a reversing valve (also called a four-way or changeover valve). Competitors will tell you the system "runs in reverse" for cooling but never name the part that does it — so here's the actual mechanism.

In cooling mode, the reversing valve routes hot, high-pressure refrigerant from the compressor out to the buried ground loop, which now acts as the condenser. The refrigerant sheds its heat into the loop fluid — and from there into the surrounding earth — then condenses, passes through the expansion valve, and returns to the indoor coil, which now acts as the evaporator, absorbing heat from your household air. Your blower pushes air across that cold indoor coil, and the cooled air goes back into the house. As the U.S. Department of Energy puts it, the system "removes heat from the building and deposits it underground."

So there's no separate "geothermal AC unit" to buy. The same box that heats your home in January cools it in July — it just reverses which coil is doing what.

Why the Ground Beats the Air on a 100° Day

Every air conditioner has to dump the heat it removes from your home somewhere. A conventional AC dumps it into the outdoor air through the condenser coil in that big box outside. The problem is obvious once you say it out loud: on a 100°F afternoon, that unit is trying to shove heat into 100°F air. Heat doesn't want to move from hot to hot, so the compressor works harder, draws more power, and its real-world efficiency drops right when you're leaning on it hardest.

Geothermal doesn't have that problem. According to the DOE, the ground at typical loop depth stays around 50–59°F year-round. So on that same 100°F afternoon, a geothermal system is rejecting your home's heat into a roughly 55°F sink instead of 100°F air. The temperature gap it has to fight is far smaller, so it holds its rated efficiency even on peak-load days. The Geothermal Exchange Organization points to field observations — including a 2010 Oklahoma utility study on a 110°F day — where air-source efficiency collapsed while ground-source units held near their rated performance. That's the whole advantage in one sentence: your AC fights the heat; geothermal just relocates it.

The Efficiency Numbers (and Why EER, Not SEER)

If you shop conventional air conditioners, you see SEER — a seasonal efficiency average. For geothermal cooling, the honest metric is EER, the efficiency at a specific peak operating condition. Here's why that distinction matters and isn't just alphabet soup.

SEER is a season-long average, and it flatters equipment that does well in mild weather. But you don't buy air conditioning for mild weather — you buy it for the worst day of the summer. EER measures efficiency at a hard peak condition, and on that peak day a high-SEER air-source unit's actual efficiency drops off, while a geothermal unit barely moves because its ground sink barely moves. Comparing a geothermal EER to a conventional SEER is comparing a bad-day number to a good-average number. EER is the fair fight.

Here are the real figures, anchored to primary sources:

The takeaway: even an entry-level ENERGY STAR geothermal unit (17.1 EER) sits above the top of the conventional-AC range, and best-available geothermal gear reaches 26.1 EER per DOE's federal purchasing guidance. Note also that open-loop systems rate higher than closed-loop (21.1 vs. 17.1 EER at the ENERGY STAR minimum), because groundwater is a colder, steadier source — more on that below.

The Free Hot Water Nobody Tells You About

This is the biggest thing missing from the cooling pages that currently rank, and it's a genuine perk: in summer, a geothermal system can heat much of your domestic hot water for almost nothing.

The component is a desuperheater — a small secondary heat exchanger that skims the hottest part of the refrigerant coming off the compressor and uses it to preheat your hot-water tank. Think about what's happening in cooling mode: the system is pulling heat out of your house and about to dump it into the ground. The desuperheater intercepts some of that heat on its way out and puts it into your water instead. The heat was leaving anyway, so capturing it is close to free. WaterFurnace states a desuperheater can supply up to 50% of a home's hot water "at almost no cost," and the DOE notes a desuperheater heats water far more efficiently than a standard electric-resistance element.

Be clear-eyed about what it is: a preheat and supplement, not a replacement for your water heater, and its summer output naturally tracks how much cooling you're doing. But "your air conditioner also makes your hot water in July" is a real, and rarely mentioned, benefit.

Passive vs. Active Cooling

There are actually two ways a ground-source system can cool, and knowing the difference helps you read manufacturer spec sheets honestly.

Active cooling is what essentially all U.S. residential geothermal air conditioning is: the compressor runs, the refrigerant cycle is reversed by the reversing valve, and you get full cooling capacity. This is the mode described above.

Passive cooling (sometimes called "natural" or "direct" cooling) leaves the compressor off entirely. Only the loop's circulation pump runs, sending cool loop fluid through a heat exchanger to shed some household heat directly into the ground. It uses very little energy, but its capacity is limited — one European manufacturer describes on the order of a few degrees of room-temperature reduction. Passive cooling is more common in European systems and mild climates; in a typical hot American summer it's a limited supplement, not a whole-house solution. If a sales pitch leans hard on "passive cooling," ask pointed questions about capacity in your climate.

Open Loop vs. Closed Loop for Cooling

The kind of ground loop you have affects cooling performance, and it's worth understanding before you compare quotes.

A closed loop is a sealed circuit of water and antifreeze buried in vertical bores, horizontal trenches, or a pond. It's the low-maintenance default. In cooling mode it rejects your home's heat into the surrounding soil all summer long.

An open loop pulls water from a well or aquifer, runs it through the heat exchanger, and discharges it. Because that incoming groundwater is cold and steady, open-loop systems rate meaningfully higher in cooling — ENERGY STAR's minimum is 21.1 EER for open-loop versus 17.1 for closed-loop water-to-air. The trade-off is that open loops need an adequate water supply, a discharge path, and attention to water quality. Our open loop vs. closed loop guide walks through which fits which property.

The Honest Part: Cooling-Dominated Climates

Here's where we'll be straighter with you than the competition. Geothermal is often described as if the ground is an infinite, unchanging heat sink. Over a long, hot cooling season, in a place like the Deep South or Florida, that's not quite true.

If you're rejecting heat into the ground month after month with relatively little heating season to pull it back out, a closed loop can gradually warm the surrounding soil — a slow drift toward thermal imbalance. As the ground near the loop heats up, the ground-versus-air efficiency edge narrows somewhat. This is a real engineering consideration in cooling-dominated climates, and it's solvable: a properly sized (often larger or deeper) loop field, or a hybrid design that pairs the ground loop with a supplemental cooling tower to shed the extra summer heat. None of this means geothermal doesn't work in hot climates — it works well — but it does mean loop sizing matters more there, and any installer who waves off the question isn't being straight with you.

Comfort, Dehumidification, and Operating Cost

Beyond raw efficiency, geothermal tends to cool differently in a way you can feel. Ground-source systems generally run longer, steadier cycles rather than the short on-off bursts of an oversized conventional AC. Longer runtimes pull more moisture out of the air, which usually means better humidity control and a more comfortable house at the same thermostat setting.

The important caveat is that this depends on correct sizing. An oversized geothermal unit short-cycles just like an oversized conventional AC, and a short-cycling system dehumidifies poorly. Bigger is not better; right-sized is better. See our sizing guide for how that's calculated.

On operating cost, the fair thing to say is that the cooling-mode savings come from the EER gap described above. You'll see vendors claim figures like "25–50% lower electricity use" — treat those as marketing ranges, not hard guarantees, because the honest primary-source savings figures (from DOE) are calculated for combined heating and cooling, not cooling alone. What's solid is the efficiency differential and the fact that geothermal holds that efficiency on peak days. For the full-system economics, our installation cost guide and payback period guide have the current numbers.

Frequently Asked Questions

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