How Does Geothermal Heating Work? Ground-Source Basics

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Geothermal heating uses a ground-source heat pump to pull heat out of the earth through buried loops of pipe and move it into the house. It does not burn anything and it does not tap volcanic heat; it takes advantage of the fact that the ground a few feet down stays at a fairly steady temperature all year, warmer than winter air and cooler than summer air. The same system runs in reverse for cooling. For homeowners, the appeal is low operating cost, quiet operation and long equipment life; the catch is a high installation cost driven mostly by the loop field. This guide explains how geothermal heating works, the loop types, the indoor unit, efficiency, typical costs, what changes on the Colorado Front Range, and what a home inspector can and cannot check. It summarizes U.S. Department of Energy and ENERGY STAR guidance as of 2026; a geothermal designer’s site assessment takes priority for any specific property.

Part of our Heating, Cooling & Ventilation guide — start there for the full picture →

What Geothermal Heating Is

The U.S. Department of Energy describes geothermal heat pumps (GHPs), also called ground-source heat pumps, as systems that use relatively constant underground temperatures to heat homes in winter and cool them in summer. According to DOE, temperatures at about 30 feet below the surface stay between roughly 50°F and 59°F year-round across most of the country. In most areas that means the soil is warmer than the air in winter and cooler than the air in summer.

A heat pump is a machine that moves heat from one place to another using electricity. A refrigerator does this; so does a central air conditioner. ENERGY STAR’s explainer on heat pumps notes that because a heat pump moves heat rather than generating it, it uses less energy to warm a home than a conventional electric or gas system. A geothermal system is simply a heat pump that trades heat with the ground instead of the outdoor air. Our guide to what a heat pump is covers the basic refrigeration cycle in more detail.

DOE lists three parts in every system:

• An underground heat collector (the ground loop), a series of connected pipes buried vertically or horizontally near the building that circulates a fluid to absorb or release heat.
• A heat pump, installed indoors, that concentrates heat from the loop fluid and delivers it to the house, or removes heat from the house and sends it to the ground.
• A distribution system, usually conventional ductwork, though some systems heat water for radiant floors or other hydronic emitters.

How Geothermal Heating Works, Step by Step

In heating mode, a typical closed-loop, water-to-air system works like this:

1. Fluid circulates through the ground loop. A small circulating pump moves water, or a water-and-antifreeze mix, through buried plastic pipe. As it travels, the fluid picks up heat from the surrounding soil or rock.
2. The fluid returns to the heat pump. Inside the indoor unit, it passes through a heat exchanger where it hands that heat to the refrigerant. The refrigerant, at low pressure, boils into a gas.
3. The compressor raises the temperature. Compressing the refrigerant gas raises its pressure and temperature to a level useful for heating a home.
4. The indoor coil releases the heat. A blower pushes house air across the hot coil, and warm air goes out through the ducts. The refrigerant condenses back to a liquid.
5. An expansion valve resets the cycle. The refrigerant’s pressure drops, it cools sharply, and it heads back to the loop heat exchanger to pick up more heat.
6. The cooled loop fluid goes back underground to be rewarmed by the earth, and the loop repeats continuously.

Cooling mode

A reversing valve flips the direction of refrigerant flow. The indoor coil becomes the cold side, pulling heat and moisture from house air, and the loop carries that heat into the cooler ground. DOE notes that when ambient temperatures are warmer than the ground, the heat pump removes heat from the building and deposits it underground. Because the ground in summer is usually much cooler than the outdoor air, a geothermal unit rejects heat more easily than an outdoor air conditioner on a 95°F afternoon.

Hot water from a desuperheater

Many residential geothermal units include a desuperheater, a small extra heat exchanger that skims some of the very hot refrigerant gas leaving the compressor and uses it to preheat water for the household water heater. It does the most work in cooling season, when that heat would otherwise go into the ground, and contributes less in heating season. DOE notes that some systems can supply hot water, and ENERGY STAR’s current geothermal category includes water-to-water units that can provide space conditioning, domestic water heating or both.

Ground Loop Types

The loop field is the part of the system that varies most from one property to another, and it drives most of the cost. DOE’s fact sheet illustrates shallow horizontal, deep vertical and pond collectors. Open-loop well systems are a fourth option where groundwater and rules allow.

Closed loops are sealed, so the same fluid circulates for years. They are filled with water or an antifreeze solution sized for the expected loop temperatures, and they are typically made of high-density polyethylene with heat-fused joints. Open loops avoid the cost of a large loop field but depend on the well, the water chemistry and the regulator.

Efficiency: COP, EER and What the Numbers Mean

Geothermal heat pumps are rated by two main numbers:

• COP (Coefficient of Performance) is heating output divided by electrical input. A COP of 4 means four units of heat for every unit of electricity. Geothermal units generally post higher heating COPs than air-source units because the loop fluid entering the unit in winter is far warmer than January air.
• EER (Energy Efficiency Ratio) is cooling output in Btu per watt-hour at a set test condition. Higher is better.

ENERGY STAR states that certified geothermal heat pumps use 61% less energy than a standard model, and estimates savings of nearly $830 per year and more than $9,500 over a 15-year product life. Those are program averages, not a promise for any one house; real savings depend on the fuel being replaced, utility rates, the home’s insulation and how well the loop was designed.

One advantage that matters on cold mornings: an air-source heat pump loses capacity as outdoor temperatures fall, while a well-designed ground loop delivers relatively stable entering-water temperatures through a cold snap. DOE notes that geothermal systems do not depend on outdoor air temperature the way air-source systems do.

Geothermal Versus Air-Source Heat Pumps and Furnaces

DOE says geothermal systems have been shown to be quieter, last longer and need less maintenance than air-source systems, and are typically more expensive up front, with the extra cost often returned through energy savings. Our comparison of a heat pump versus a furnace covers the furnace side in more depth.

Typical Costs and Cost Drivers

Geothermal is the most expensive common home heating system to install. The ranges below are typical national figures for 2026 and should be treated as rough starting points, not quotes.

• Whole-home closed-loop system, installed: typically about $18,000 to $45,000, with large or complex homes running higher.
• Loop field share: often the largest single line item, sometimes half or more of the total on a vertical-bore job.
• Indoor unit replacement later: typically about $7,000 to $15,000 if the existing loop is reused.
• Common repairs: a circulating pump typically runs several hundred dollars to about $1,500; a loop leak search and repair can range from under $1,000 to several thousand.

The biggest cost drivers are loop type, total bore footage or trench length, soil and rock conditions, house size and heating load, whether ductwork must be added or modified, a desuperheater or water-to-water option, electrical upgrades and permits. Our guide to geothermal heat pump installation cost walks through these factors in more detail.

Federal tax credit status

Geothermal heat pumps have been eligible for a federal Residential Clean Energy tax credit. ENERGY STAR’s geothermal tax credit page lists 30% for property placed in service after December 31, 2021, and before January 1, 2026, and requires ENERGY STAR certified equipment. DOE’s tax credit page notes that the One Big Beautiful Bill Act, enacted July 4, 2025, revised dates and provisions affecting these credits and directs readers to IRS.gov and Treasury.gov. Anyone planning a system now should confirm current eligibility with a tax professional and current IRS guidance rather than relying on older marketing materials. Utility and local incentives may still apply.

Front Range Factors That Change the Picture

Geothermal works anywhere in the country, but local conditions decide what it costs and which loop makes sense.

• Rock and bedrock. Foothill and mountain properties west of the metro often sit on shallow granite and other hard rock. That makes horizontal trenching difficult and pushes projects toward vertical bores, and hard rock slows drilling. On the plains, expansive clay and cobbly alluvial soils are common, and each affects drilling and trenching differently.
• Lot size. Typical suburban lots in Douglas, Arapahoe, Jefferson and Adams counties rarely have room for a horizontal loop large enough for a whole house, so vertical bores are the usual retrofit path. Rural parcels in Elbert or El Paso counties may have the acreage for trenches.
• Permits and water rules. Vertical boreholes and open-loop wells may require state and county permits, and in Colorado groundwater use is closely regulated. Check with the county building department and the state’s water-well authority before committing to a loop design.
• Heating-dominated climate. Front Range homes heat far more than they cool. A loop sized mainly for cooling can be undersized for heating, and repeated heavy winter extraction can gradually cool the ground around a short loop. Proper loop design accounts for that imbalance.
• No outdoor unit to damage. With nothing exposed outside, hail, drifting snow and wind-blown debris are not concerns for the heat pump itself, which is a genuine advantage in this region.

DOE recommends contacting a geothermal designer or a local professional engineer, not just an installer, to assess whether a home is a good candidate.

Lifespan and Maintenance

A geothermal system has two lifespans. The indoor heat pump is mechanical equipment with a compressor, blower, pumps and controls; industry estimates commonly place it around 20 years or more with good maintenance, and ENERGY STAR uses a 15-year product life in its savings estimate. The buried loop is plastic pipe with fused joints and no moving parts, and installers commonly cite loop lifespans of 50 years or more. That split is why a replacement 20 years from now usually means swapping the indoor unit and reusing the loop.

Maintenance is modest but not zero:

• Change or clean the air filter on the manufacturer’s schedule.
• Keep the condensate drain clear.
• Have a technician check loop pressure, flow and fluid condition, plus the compressor and controls, periodically.
• For open loops, watch for scaling and iron fouling in the heat exchanger and have the well equipment serviced.
• Know where the loop field and headers are buried before digging for fences or landscaping.

Homes with hydronic geothermal often pair it with in-floor tubing; our guide to radiant floor heating explains how that distribution works.

Signs a Geothermal System Has a Problem

• Electric bills climb or the auxiliary electric heat runs often. This can point to low loop flow, a refrigerant problem or an undersized loop.
• The unit locks out with a fault code, often for low pressure, high pressure or loop temperature. Repeated lockouts call for a technician.
• Falling loop pressure on a pressurized closed loop suggests a leak in the loop or the indoor piping.
• Gurgling or grinding near the circulating pumps, or wet spots under the unit.
• Weak cooling and poor dehumidification in summer.
• On open loops, reduced well flow, staining or rising pressure drop across the heat exchanger.

What a Home Inspector Checks, and the Limits

This point surprises many buyers. The ASHI Standard of Practice states that the inspector is not required to inspect heating systems or cooling systems using ground-source, water-source, solar and renewable energy technologies. A general home inspector cannot see the loop, measure loop flow or test antifreeze concentration.

Within those limits, an inspector will typically describe the system and energy source, operate it with normal controls if conditions allow, note the brand and approximate age of the indoor unit, check visible ductwork, filters, condensate drains and electrical connections, and look for leaks or corrosion at the indoor piping and pumps. When a home has geothermal, the usual recommendation is a separate evaluation by a geothermal contractor.

Buyers should also ask the seller for the loop design, bore or trench layout, installation date, installer name, warranty transfer terms and service records. Our heating and cooling hub collects related guides on heat pumps, furnaces and boilers.

When to Call a Pro

Call a geothermal-qualified HVAC contractor for repeated lockouts, a loss of loop pressure, heavy reliance on backup electric heat, water leaks at the unit, or before buying a home with an older system. For a new installation, start with a designer or engineer who will perform a heating and cooling load calculation and a site assessment before anyone quotes loop footage. Never open sealed loop piping or the refrigerant circuit yourself.

If you are buying a Front Range home with a ground-source heat pump and want the rest of the house checked thoroughly, contact our team to schedule an inspection.

References

• Geothermal Heat Pumps — U.S. Department of Energy
• Tax Credits, Incentives, and Technical Assistance for Geothermal Heat Pumps — U.S. Department of Energy
• Geothermal Heat Pumps — ENERGY STAR
• Geothermal Heat Pumps Tax Credit — ENERGY STAR
• How Does a Heat Pump Work? — ENERGY STAR
• ASHI Standard of Practice for Home Inspection — American Society of Home Inspectors

Buying a Front Range home with geothermal or another heat pump system? Get in touch with our team to arrange an inspection.