Geothermal Furnace: What It Really Is (a Heat Pump, Not a Furnace)

Type "geothermal furnace" into a search box and you get two kinds of page: ones that use the phrase without explaining it, and ones that lecture you for using it. Neither helps. You typed it because you have a furnace, you have heard geothermal can replace it, and you want to know what goes in its place.

The short version: there is no piece of equipment called a geothermal furnace. What you are picturing — the tall cabinet in the basement or utility closet that connects to the ducts and heats the house — is a water-to-air geothermal heat pump. It fills the same spot and does the same job, but it is a different kind of machine, and the difference is not pedantry. It changes how the unit is sized, how it is rated, how it runs day to day, and how it should be compared to the gas furnace it replaces.

What people mean by "geothermal furnace"

Almost everyone who uses the phrase means the indoor unit of a forced-air geothermal system. Ask a manufacturer and they will not call it a furnace. WaterFurnace, the company whose name does the most to keep the word alive, labels its category "Geothermal Heat Pumps" and sorts its residential units into "Forced Air All-In-One," "Indoor/Outdoor Split," "Forced Air / Hydronic Combo," and "Radiant, Hot Water & Hydronic." The forced-air all-in-one is the one the searcher is picturing. WaterFurnace describes it as a single unit providing forced-air heating, air conditioning, and optional water heating, and says one all-in-one ground-source unit replaces the traditional indoor furnace and the outdoor air-conditioning equipment together.

So the object is real. The name is the problem. The correct term for the cabinet is a packaged water-to-air geothermal heat pump: "water-to-air" because it takes heat from the ground-loop fluid and delivers it to your ducts, "packaged" because the whole refrigeration circuit lives in one cabinet. Our guide to geothermal air handlers sorts out the packaged, split, and fan-coil configurations; the furnace-versus-heat-pump distinction changes how the system is sized, rated, and operated.

What a furnace actually is

"Furnace" has a definition, and it is narrower than "big box that heats the house through ducts."

The federal definition, as DOE summarizes it on its consumer-furnaces rulemaking page, is a single-phase electric appliance — or an electric appliance used with natural gas, propane, or heating oil — designed as a residence's principal heating source and falling within the equipment-type and input-capacity limits in 10 CFR 430.2. ASHRAE's terminology database defines a forced-air or gravity furnace as "an electric, gas, or oil-burning appliance" that supplies heat through ducts, using air as the heating medium.

The federal definition works through inputs, configuration, product classes and capacity limits rather than through a sentence about how heat is made. For that, ASHRAE's heating-equipment reference is explicit: a fuel-burning furnace burns gas or oil inside a combustion chamber and heat exchanger, and an electric furnace runs current through resistance heating elements. Either way, on ASHRAE's description, the furnace generates heat inside its own cabinet — converting fuel or electricity into heat at the point of use.

That is why furnaces are rated in AFUE. Annual Fuel Utilization Efficiency, as defined in the federal test procedures at 10 CFR Part 430 Subpart B, is the annual useful heat delivered to the heated space divided by the annual fuel-energy input, expressed as a percentage. Its ceiling is, by construction, just under 100 percent: you cannot get more heat out of a fuel than the fuel contains.

Why a geothermal heat pump is not a furnace

No federal rule or ASHRAE glossary contains the sentence "a geothermal heat pump is not a furnace." You do not need one. Line up the definitions above against what a geothermal heat pump does and the conclusion builds itself.

Start with the operating principle. DOE's plain description of heat pumps is that they use electricity to move heat rather than generating it through combustion or resistance. A geothermal unit in heating mode, per DOE's geothermal page, removes heat from the fluid circulating through the ground loop, concentrates it through a refrigeration cycle, and transfers it into the building. The heat was already in the ground; the machine relocates it. That is the opposite of a furnace, which makes heat from a fuel.

Everything that follows from combustion is missing. WaterFurnace's owner's manual states that its geothermal units do not burn fossil fuels and have no flame, fumes, or combustion. A WaterFurnace brochure points out that geothermal equipment can be installed in places fossil-fuel furnaces cannot, because with no combustion there is no exhaust to vent — no flue. DOE's building-science guide to combustion furnaces notes that a combustion furnace must receive combustion air and discharge its flue gases safely; a heat pump needs neither. WaterFurnace's 5 Series literature adds the point homeowners care about most: the unit produces no carbon monoxide because it burns nothing. That claim has limits: a gas water heater, a fireplace, or a car in an attached garage can still create a CO risk.

Then look at how the equipment is rated, because rating programs are where the industry's real categories live. AHRI — the Air-Conditioning, Heating, and Refrigeration Institute, the body that certifies HVAC performance ratings — runs a program titled "Geothermal – Water-Source Heat Pumps (WSHP)." The ratings it verifies are heating COP and cooling EER, tested under ISO 13256-1 for water-to-air and brine-to-air units. AFUE is not one of its certified geothermal ratings. And the reason is not simply "it burns no fuel" — federal procedures do calculate AFUE for electric furnaces, which burn nothing either. The reason is classification: a geothermal heat pump is a different covered product, tested and rated under a heat-pump procedure rather than a furnace one. The rating follows the equipment category, not the fuel.

So: a furnace generates heat inside the cabinet, from fuel or from resistance, and is rated in AFUE. A geothermal heat pump moves heat with a refrigeration cycle, burns nothing, vents nothing, and is rated in COP and EER under a different program and test standard. It sits where a furnace sat. It is not one.

The WaterFurnace problem

If the equipment is so clearly a heat pump, why does "geothermal furnace" persist? A large part of the answer is one company's name.

WaterFurnace International's name is on a great many of the cabinets people are picturing, and a homeowner who has one in the basement, or a neighbor who does, reasonably concludes that the thing is a kind of furnace — a water furnace, a geothermal furnace. The brand does not describe its products that way. Its category page says "Geothermal Heat Pumps," and its 7 Series product page describes forced-air heating, air conditioning, and optional water heating from one unit. "WaterFurnace" is a proper noun. The equipment class is a geothermal, or ground-source, heat pump.

This matters when you shop. Searching "geothermal heat pump" rather than "geothermal furnace" gets you the manufacturers' actual literature, the AHRI directory, and the rating conditions that let you compare one unit to another. If the refrigeration-cycle part is new to you, our explainer on how geothermal heat pumps work is the right next read.

COP vs AFUE: why you cannot compare them directly

The place the furnace mental model does the most damage is efficiency, because the two numbers look comparable and are not.

AFUE, again, is annual useful heat delivered divided by annual fuel energy in, as a percentage. COP — coefficient of performance — is, in DOE's definition for its federal-purchasing program, heating output divided by electrical input, both in the same energy units. (EER, the cooling counterpart, is cooling output in Btu per hour divided by electrical input in watts.) A furnace with a 99 percent AFUE supplies roughly 0.99 units of seasonal heat for every unit of fuel energy. A heat pump with a COP of 4.0 delivers four units of rated-condition heating output for every unit of electrical input to the heat-pump equipment, because most of those four units came out of the ground, not out of the power line.

Put the ranges next to each other and it looks like a rout:

• Furnaces. The current federal minimum for non-weatherized and mobile-home gas furnaces is 80 percent AFUE, per DOE's consumer furnace rule. DOE's December 2024 federal-purchasing comparison used 95 percent AFUE as the northern ENERGY STAR level and 99 percent as the highest available model. A 95 percent federal minimum has been adopted, but its compliance date is December 18, 2028; as of September 2026 the floor is still 80 percent.
• Geothermal heat pumps. DOE's December 2024 federal-purchasing examples used a COP of 3.1 for a less-efficient model, 3.6 for ENERGY STAR, and 4.8 for the best available. WaterFurnace currently lists up to 5.2 COP for its 7 Series forced-air all-in-one unit.

A COP of 4.8 can fairly be described as about 480 percent efficient in the output-over-input sense — DOE describes geothermal efficiencies in the 300 to 600 percent range on exactly that basis. What it is not is "480 percent AFUE." The two metrics use different test conditions, different system boundaries, and different energy carriers, so the percentages are not interchangeable even when both are written with a percent sign. AFUE is a seasonal figure counted against fuel; COP is a rated-condition figure counted against electricity at the heat-pump equipment. The federal calculations behind a gas or oil furnace's AFUE handle the furnace's own auxiliary electricity — blower and controls — separately from the fuel number, so even the furnace side is not purely "energy in, heat out." A COP does not tell you what the ground loop does in February, and an AFUE does not tell you what the blower costs.

The honest comparison is in dollars and in emissions, and both need inputs the ratings do not contain: your local electricity and gas prices for operating cost, plus an assumption about how the electricity is generated for source energy or emissions. Our guide to geothermal efficiency ratings walks through the rating conditions, and our geothermal vs natural gas comparison does the dollar math. The direction is clear — three to five units of heat per unit of electricity versus a machine that can never exceed one — but the comparison has to be done in fuel bills, not percentages.

What the machine in your basement actually is

Set the word aside and look at the cabinet.

AHRI describes water-to-air equipment as including an indoor air-conditioning coil and air-moving means, a compressor, and a refrigerant-to-water heat exchanger. In plain terms: a blower that pushes house air across a coil, a compressor that runs the refrigeration cycle, and a second heat exchanger where the refrigerant trades heat with the ground-loop fluid. Bosch describes its packaged units as occupying the indoor mechanical-equipment role normally filled by the furnace and the indoor cooling coil, in vertical, horizontal, and counterflow configurations — the same orientations furnaces come in, so the unit can drop into the same spot with air entering and leaving the same way.

Notice what is inside that a furnace does not have: a compressor, a refrigerant circuit, a water-side heat exchanger. And what is missing that a furnace has: a burner, a combustion chamber, a flue. It is closer to your central air conditioner than to your furnace — in effect, an air conditioner that can run in reverse, with loop water instead of outdoor air on the far side.

DOE notes that conventional ductwork is generally used to distribute the air from a geothermal heat pump, which is why the furnace comparison is so tempting: same ducts, same thermostat on the wall, same registers. The unit is a heat pump; the delivery system is your existing forced-air system, subject to the airflow check below.

Can it replace a gas furnace in a cold climate?

Yes. DOE states that a properly designed geothermal system can serve as the home's primary heating and cooling system in any US climate.

The qualifier is "properly designed." "Any climate" does not mean any unit off the shelf. The heat pump's capacity, the temperature of the fluid entering it from the ground loop, the house's design heat loss, the ducts' capacity, and the backup strategy have to be engineered together. A unit that is fine for one house, one loop, and one climate can be short in another.

That backup strategy is where the furnace mental model needs its biggest adjustment. Many ducted geothermal installations include supplemental electric-resistance coils in the cabinet — "auxiliary heat" on the thermostat. Bosch's residential geothermal applications manual (January 2013) describes these coils as supplying the portion of the building's heat loss that the compressor cannot meet at low-temperature design conditions. They run at or below the system's balance point (the outdoor condition at which the heat pump's output just matches the house's heat loss), during unusual load increases, and for longer periods as outdoor conditions become colder. They also supply emergency heat if the compressor fails.

Two things follow. First, a geothermal system does not stop heating in a cold snap; it keeps running, and if the design calls for it, the resistance coils fill the gap. Second, those coils run at an effective COP of roughly 1 — one unit of heat per unit of electricity — where the heat pump itself runs substantially higher, as ClimateMaster's June 2021 homeowner article explains, so a design that leans on them heavily costs more to run. Whether a given system needs auxiliary heat at all is a design decision rather than a universal rule: some require it for peak load or emergency operation, others may be sized or configured without it. If you are weighing geothermal against an all-resistance option, see our geothermal vs electric furnace comparison.

What a furnace-shaped mental model gets wrong

Here is where calling it a furnace costs real money. Each of these mistakes follows naturally from "it's just a new furnace."

Sizing it off the old nameplate

A gas furnace has a Btu input on its label, and the instinct is to buy a geothermal unit "the same size." That is wrong twice over: the old furnace may have been sized by rule of thumb, and a furnace's input rating and a heat pump's rated output are different quantities anyway. DOE's building-science guidance is that the home's heating and cooling loads should be calculated with ACCA Manual J, and that the equipment and ducts should then be selected from those loads. DOE's federal-purchasing guidance for geothermal is blunter: proper assessment of peak heating and cooling loads is critical to geothermal system design. Our geothermal sizing guide walks through what a proper load calculation involves.

Assuming the ducts are fine

They may not be. The EPA's 1997 geothermal manual gives typical forced-air geothermal supply-air temperatures of roughly 85 to 110 °F, substantially below what comes out of a gas furnace. It is an older figure and equipment has changed since, but the direction is not in dispute, and every manufacturer's airflow guidance still follows from it. To deliver the same heat with cooler air, the system has to move more of it, and EPA notes this may require a larger blower and duct system than conventional heating. WaterFurnace's 7 Series installation guide (September 24, 2024) instructs installers to confirm that existing ductwork can carry the required airflow, enlarge ducts that are too small, and repair leaks — and it specifically flags replacement of heating-only systems as a case where larger ductwork may be required. A house that never had central air, with ducts sized only for hot furnace air, is the classic case. The airflow numbers and the packaged-versus-split trade-offs are in our geothermal air handler guide. The point here is that "we'll reuse the ducts" is a claim to be verified, not assumed. The same 85–110 °F band is why a geothermal register feels lukewarm to a hand used to a furnace: not a fault, just more air at a lower temperature.

Expecting short blasts

A single-stage furnace heats in bursts: fire, blow hot air, shut off, wait. (Two-stage and variable-speed furnaces run longer and gentler, so this is a contrast with the common case, not with every furnace.) WaterFurnace says its variable-capacity units are designed to run more often at low speeds, producing stable temperatures rather than furnace-like blasts followed by shutdown. Long, quiet run times are the design intent. Judge the system by whether the house holds temperature, not by how often the blower stops.

Using deep thermostat setbacks

With a furnace, turning the thermostat down at night and back up in the morning is a standard way to save. With a geothermal heat pump it can backfire. ClimateMaster's June 2021 article warns that recovery from a setback may require the supplemental electric-resistance heat, and that maintaining a steady setpoint generally improves geothermal operating efficiency. Resistance heat delivers roughly one unit of heat per unit of electricity; the heat pump delivers several times more. The habit that saved money with the furnace can spend it with the heat pump.

Cost and lifespan vs a gas furnace

Installed cost

EnergySage reports $15,000 to $40,000 or more for a geothermal heat-pump installation. Carrier's 2026 furnace-replacement guide puts an installed gas furnace at $3,800 to $12,000. Those are not the same purchase. The geothermal figure normally includes heating, cooling, and the ground-loop infrastructure. The furnace figure covers heating equipment and installation, and does not necessarily include a replacement central air conditioner. A fair comparison sets the geothermal quote against furnace plus air conditioner, plus the operating-cost gap over the years you will own the house, with your own utility prices plugged in. The EnergySage figure is a market estimate, not a government benchmark.

The federal tax credit

Many older articles still show a federal credit that expired after 2025. The Residential Clean Energy Credit under IRC §25D was 30 percent for qualifying property installed from 2022 through December 31, 2025. The IRS page, as accessed September 14, 2026, states that the credit is not available for property placed in service after December 31, 2025, and the 2025 Form 5695 instructions label the change a "Termination of credits," stating that residential clean-energy credits cannot be claimed for expenditures after that date. A residential geothermal system placed in service in 2026 does not qualify for §25D, and any quote that still folds a 30 percent federal credit into a 2026 price is wrong. This is not tax advice; check with a tax professional about your own situation, and see our federal geothermal tax credit guide for the history and current status.

Lifespan

DOE's August 2021 consumer guide gives a 20-plus-year average life expectancy for the heat pump itself and 25 to 50 years for the underground infrastructure (DOE's phrase, which is broader than the loop pipe alone). DOE's current heat-pump page gives a newer estimate: up to 24 years for the inside components and 50-plus years for the ground loop. Lennox puts a typical gas furnace at 15 to 20 years with proper care. So the indoor unit lasts somewhat longer than a furnace, and the loop — the expensive part of the first install — is expected to outlast the cabinet by a wide margin.

Frequently asked questions

Is a geothermal furnace the same as a geothermal heat pump?

"Geothermal furnace" is not a recognized equipment category or rating classification, though plenty of dealers and owners use the phrase. What it refers to is a geothermal heat pump, usually the packaged water-to-air kind that sits where a furnace did. A furnace, on ASHRAE's description, is a gas, oil, or electric-resistance appliance that generates heat inside the cabinet. A geothermal heat pump moves heat from the ground with a refrigeration cycle, burns nothing, and is rated by AHRI in COP and EER, not AFUE.

What is the AFUE of a geothermal heat pump?

It does not have one. AHRI's geothermal/water-source heat pump program certifies heating COP and cooling EER under ISO 13256-1; AFUE measures fuel utilization and does not apply to equipment that uses no fuel. For scale, DOE's December 2024 purchasing examples — modeled on a 36,000 Btu/h unit — ran from COP 3.1 to 4.8, and WaterFurnace lists up to 5.2 COP for its 7 Series at closed-loop part-load conditions, where the same unit's closed-loop full-load COP is materially lower. Always ask which condition a COP describes.

Does a geothermal heat pump need a flue or a chimney?

No. There is no combustion, so there are no exhaust gases to vent and no combustion air to bring in, and the unit itself produces no carbon monoxide — though other combustion appliances in the house still can.

Is the 30 percent federal tax credit still available for geothermal in 2026?

Not for residential property. The IRS states the Residential Clean Energy Credit is not available for property placed in service after December 31, 2025, and the 2025 Form 5695 instructions describe the change as a termination of the credit. Consult a tax professional for your own situation.