The Minnesota basement
There’s a couple in Saint Paul who installed a ground source heat pump in 2018. Their natural gas bill the year before was $2,400. The year after the install, their electricity bill went up by $700 and their gas bill went to zero. Net savings: $1,700/year. The system cost them $28,000 after rebates. They’re at year 7 of a 10-year payback. Their neighbor with a 95% efficient gas furnace is still spending $2,200/year on heating. The Minnesota couple is on track to pay off the system, then save thousands a year for the next 15+ years until the ground loop reaches end of life.
Geothermal heating (technically: ground source heat pumps) is the most efficient way to heat a building, full stop. The catch is the upfront cost — you have to drill or excavate to bury a loop of pipe in the ground. Whether the math works for you depends entirely on your climate, your current heating fuel, and how long you’re staying in the house.
What it actually is
“Geothermal” in residential context almost never means tapping volcanic heat or magma. It means using the constant temperature of the ground (50–60°F at 6 ft below the surface in most climates) as a heat source in winter and a heat sink in summer.
The system has three main parts:
- The ground loop: Buried pipes containing a water-glycol mixture. The loop can be horizontal (trenches 4–6 ft deep, hundreds of feet long), vertical (boreholes 100–400 ft deep), or in a body of water (pond or lake).
- The heat pump: A unit inside the house that uses a refrigeration cycle to move heat. In winter, it pulls heat out of the ground loop fluid and pushes it into the house. In summer, it does the reverse.
- The distribution: Forced-air ducts, radiant floor tubing, or hydronic radiators. Same as any heating system.
Why it’s so efficient
Conventional heating burns a fuel to generate heat. 90% efficient gas furnace converts 90% of the gas energy to heat. An electric resistance heater converts 100% of the electricity to heat. Both create heat.
Heat pumps don’t create heat. They move it. For every 1 unit of electricity a ground source heat pump uses, it moves 3–5 units of heat from the ground into your house. The “coefficient of performance” (COP) is typically 3.5–5.0. This is why electric bills go up modestly when you switch to geothermal — you’re using electricity, but very little of it.
For comparison: a 95% efficient gas furnace has effective heating “efficiency” of 0.95. A ground source heat pump has effective heating efficiency of 3.5–5.0. It moves 4x more heat per unit of energy than the furnace creates.
Where it makes sense
Cold climates with high heating bills: The math is best here. Minnesota, Maine, Vermont, Quebec, anywhere with 6,000+ heating degree days per year. Annual heating savings are large enough to pay back the steep upfront cost.
Off-grid or propane-heated rural homes: Propane is expensive ($3–5/gallon). Replacing propane with geothermal can save $2,000–4,000/year.
New construction: The ground loop excavation can happen as part of site work, reducing installation cost by 20–40%. Best time to install geothermal.
Homes you’ll live in 10+ years: Payback periods are typically 8–15 years. If you’re moving in 5, the cost likely isn’t recovered through energy savings (though it can add resale value).
Where it doesn’t
Mild climates with low heating needs: Phoenix, San Diego, southern California. The energy savings are too small to pay back the upfront cost.
Already-efficient electric resistance or modern gas furnaces with low local gas prices: The savings are smaller.
Small lots with no room for horizontal loops AND solid rock that makes vertical drilling expensive: Installation cost can balloon.
Homes you’re selling soon: Geothermal increases resale value, but not by the full installation cost. Bridge the gap with energy savings, not appreciation.
Cost & payback
Typical installed cost for a 3-ton ground source heat pump (sized for a 2,500 sq ft home in a cold climate):
- Heat pump unit: $5,000–9,000
- Ground loop (horizontal): $4,000–8,000
- Ground loop (vertical drilling): $8,000–20,000
- Distribution upgrades: $2,000–5,000
- Total: $20,000–35,000 (before rebates)
Federal tax credits (US, 30% under current Inflation Reduction Act provisions) and state rebates often reduce this 30–50%. Many EU countries have similar incentive structures.
Annual savings vs natural gas: $800–2,000 in cold climates. Vs propane: $2,000–4,000. Vs electric resistance: $1,500–3,500.
Payback: 7–15 years in cold climates with expensive heating; 15–25 years in mild climates.
FAQs
Does the ground loop need replacement?
Polyethylene ground loops last 50+ years. The heat pump unit needs replacement at 20–25 years. The loop is the expensive part and the durable part.
Will geothermal work in extreme cold?
Yes. Ground temperature at 6+ ft is constant regardless of surface conditions. Geothermal works in -40°F outdoor weather as easily as in 40°F. This is its main advantage over air-source heat pumps in genuinely cold climates.
Can I add geothermal cooling without changing my heating?
The same system does both. Adding only cooling is possible but unusual; most projects do both.
What’s the noise level?
Quiet. The compressor in the indoor unit is comparable to a refrigerator (~45 dB). No outdoor condenser unit (unlike air source heat pumps).
Does it work with radiant floor heating?
Excellent fit. Radiant floors run at lower temperatures than forced-air ducts, which is exactly where heat pumps are most efficient. The combination is one of the best heating systems available.
What about hybrid systems (geothermal + gas backup)?
Common in retrofits where existing gas service is hard to remove. The gas furnace handles the coldest 1–3% of hours; geothermal handles the rest. Slightly less efficient than pure geothermal but lower upfront cost.
The landing
Geothermal is the highest-efficiency heating technology available, period. The catch is the upfront cost, which is real and significant. In cold climates with expensive heating fuels, the math works. In mild climates with cheap natural gas, it doesn’t. The Saint Paul couple’s $28,000 install will pay them back and then save them thousands a year for two more decades. Their neighbor with the 95% gas furnace is making a different bet. Both bets are reasonable; one is on the rising long-term cost of fossil fuels and the other is on natural gas staying cheap. The solarpunk-aligned answer is also the engineering-aligned answer: if you can afford the upfront cost and you live somewhere that needs real heat, geothermal is the right tool.