Geothermal heating works in every climate zone in the United States — zone 1 through zone 7. The reason is simple: roughly 6 to 10 feet below the surface, the earth holds a nearly constant temperature of 45–65°F regardless of what the air above is doing. A heat pump moves energy between that stable ground reservoir and your living space. In a cold Minnesota winter or a scorching Florida summer, the ground source does not change. What does change by climate is how large a loop field you need, how much auxiliary heat to budget for, and how the economics stack up against what you were burning before.
How Geothermal Heating Works: The Temperature Differential Principle
Conventional furnaces burn fuel to create heat. Geothermal heat pumps do something fundamentally different: they move heat that already exists in the ground into your home. Moving energy is far cheaper than generating it, and that gap shows up in your utility bill every month.
The core mechanism is the refrigeration cycle, running in reverse. A fluid (water, or a water-antifreeze mix) circulates through underground pipes called a loop field. In heating mode, that fluid absorbs heat from the relatively warm earth and carries it to a heat exchanger inside the unit. A refrigerant picks up that energy, a compressor concentrates it to a higher temperature, and a second heat exchanger releases it into your home’s air-distribution or radiant system. The cooled ground loop then returns underground to absorb more heat, and the cycle repeats.
The critical insight is the temperature differential between your source (the ground) and your sink (your home). Air-source heat pumps pull heat from outdoor air that can drop to 0°F or below in winter, severely stressing the cycle and degrading efficiency. A geothermal system always operates against a ground source in the 45–65°F range. That smaller, more favorable differential is why ground-source units maintain a coefficient of performance (COP) of 3.0 to 5.0 year-round — meaning for every unit of electricity consumed, they deliver 3 to 5 units of heating energy — while air-source performance falls sharply in cold weather.
In cooling mode the process reverses: the unit pulls heat out of your home and dumps it into the cooler ground, which acts as a heat sink. In summer, even in hot southern states, the ground 6 to 10 feet down stays well below outdoor air temperatures, giving geothermal a consistent advantage as a cooling source too.
For a deeper look at the mechanical components — compressor, reversing valve, desuperheater, and loop configurations — see our guide How Geothermal Heating Works and the companion piece How Ground Source Heat Pumps Work. The piece on ground source heat pumps covers equipment selection in detail.
The 6-Foot Underground Constant
One of the most common questions homeowners ask before installing geothermal is: How cold does it get underground in winter? The answer surprises most people.
At depths of 6 to 10 feet, soil temperature is governed not by today’s weather but by the long-term average of air temperatures over the past year or more. Solar energy penetrates the soil slowly; by the time a cold snap registers at 6 feet, it has been smoothed out across months. The result is a nearly constant temperature that tracks the local mean annual air temperature (MAAT), typically within ±5°F.
Practical values by region:
- Florida, coastal Texas, Louisiana: ~70–72°F at 6 feet
- Georgia, North Carolina, Virginia: ~60–65°F
- Pennsylvania, Ohio, Indiana: ~52–56°F
- Michigan, Wisconsin, Iowa: ~48–54°F
- Minnesota, North Dakota: ~46–50°F
- Northern New England, mountain states: ~42–48°F
Even at 46°F — the coldest end of the range for US geothermal installations — the ground is warmer than outside air on the typical January night in Minnesota. That positive differential is what the heat pump exploits. Deeper vertical wells (150 to 400 feet) access even more stable temperatures and a slight geothermal gradient that adds a few degrees more warmth.
Want the full science behind soil temperature? See our dedicated article How Cold Is It 6 Feet Underground?
ASHRAE Climate Zones Explained
The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) and the International Energy Conservation Code (IECC) divide the US into climate zones based on heating degree days (HDD) and cooling degree days (CDD). The higher the HDD, the more heating a building needs; the higher the CDD, the more cooling. HVAC engineers use these zones to size equipment and design loop fields.
| Zone | ASHRAE Name | Approx. HDD (base 65°F) | Approx. CDD (base 65°F) | Representative States / Regions |
|---|---|---|---|---|
| 1 | Very Hot–Humid / Dry | <1,000 | >6,000 | South Florida, coastal Texas, Puerto Rico, Hawaii |
| 2 | Hot–Humid / Dry | 1,000–2,000 | 3,500–6,000 | Most of Florida & Texas, Louisiana, Mississippi, Georgia (south), Arizona (low-elevation) |
| 3 | Warm–Humid / Dry / Marine | 2,000–4,000 | 2,000–3,500 | Georgia (north), North & South Carolina, Virginia, Tennessee, Arkansas, New Mexico, California (Central Valley), Oregon (coast) |
| 4 | Mixed–Humid / Dry / Marine | 4,000–5,500 | 1,000–2,000 | Maryland, Delaware, Pennsylvania, Ohio, Indiana, Kansas, Missouri, Colorado (front range), Washington (Puget Sound) |
| 5 | Cool–Humid / Dry / Marine | 5,500–7,200 | 500–1,000 | Michigan, Wisconsin, Minnesota (south), Iowa, Illinois (north), New York, Massachusetts, Connecticut, Rhode Island, Oregon (inland) |
| 6 | Cold–Humid / Dry | 7,200–9,000 | 200–500 | Minnesota (north), Vermont, New Hampshire, Maine (south), Montana, Wyoming, Idaho (north) |
| 7 | Very Cold | 9,000–12,600 | <200 | Northern Minnesota, northern Maine, high-altitude Rockies, interior Alaska (lower 48 fringe) |
A note on the 2021 IECC update: roughly 400 counties were reclassified into warmer zones compared to earlier maps, reflecting observed warming trends. If your contractor is using older load calculations, confirm they’re working from 2021 IECC data — it affects loop sizing and equipment selection.
Zone 1 (Very Hot / Hot–Humid): Florida, Coastal Texas, Louisiana
Zone 1 buildings are cooling-dominant. Heating loads are minimal — in Miami, the furnace or heat pump strip might run for fewer than 400 hours a year, compared to 2,000 or more cooling hours. For a geothermal system, this means the annual load profile is almost entirely about rejecting heat into the ground rather than extracting it.
Geothermal delivers real advantages here:
- Smaller loop field than cold climates. Because the heating load is small, the loop does less annual work. Horizontal loops are often practical, and vertical wells can be shorter.
- Dehumidification quality. In hot-humid zone 1, latent cooling (moisture removal) can represent 40–60% of the total cooling load. Ground-source units cool the refrigerant to lower evaporator temperatures than most air-source equipment, which improves moisture removal. The result is lower indoor humidity at equivalent sensible temperatures — a meaningful comfort advantage over window AC or standard central air.
- Consistent efficiency regardless of ambient temperature. Even when outdoor air hits 95°F, the ground loop water is still around 70–75°F — well below air temperature. That favorable differential keeps EER (Energy Efficiency Ratio) high when grid demand and electricity prices peak.
The financial case in zone 1 is built primarily on cooling savings versus central air, plus the dehumidification quality upgrade over window units. Replacing an aging central AC system with geothermal often yields 30–50% reduction in cooling electricity, particularly on older R-22 or inefficient R-410A equipment.
Zone 2 (Hot–Humid / Mixed-Humid): Georgia, North Carolina, Virginia
Zone 2 covers the transitional South: hot summers that demand serious cooling, winters that are mild but not trivial. Atlanta averages about 3,000 HDD; Charlotte sits near 3,500. These buildings need both a good air conditioner and a real heater — geothermal delivers both from one system.
The heating season in zone 2 typically runs November through March, with only the coldest weeks pushing below 25°F. The ground loop at 60–65°F provides ample source energy for those conditions with no auxiliary heat needed in most installations. Summer performance mirrors zone 1 advantages: the loop stays well below outdoor air temperature, keeping efficiency high on the hottest weeks.
Geothermal replaces one of the most common equipment combinations in the South: a gas or propane furnace paired with a separate AC compressor. A single geothermal unit handles both functions, reducing maintenance, mechanical complexity, and capital replacement cost over the system’s 20–25-year life. Homeowners replacing propane in zones 2–3 typically see payback in 7–12 years depending on local propane prices and loop installation costs.
Zone 3 (Warm–Mixed): Tennessee, Maryland, Kentucky
Zone 3 is the great middle ground of the US climate map. Nashville sits at roughly 3,800 HDD; Baltimore at about 4,500. Both heating and cooling are real annual loads, and the equipment has to perform well in both directions.
From a geothermal design perspective, zone 3 is well-balanced. The loop field is sized for a mix of heating and cooling extractions, and the thermal recharge of the ground (a concern in extreme-cold climates) is straightforward. Ground temperatures at 6 feet range from 54–62°F, an excellent source for both seasonal modes.
Zone 3 homeowners replacing electric resistance heat (baseboard heaters, old heat strips) see the clearest immediate operating-cost improvement: geothermal delivers 3 to 5 units of heat per unit of electricity consumed, versus exactly 1 unit from resistance heat. Annual savings of 65–75% on heating electricity are common in this comparison. Natural gas competition tightens the economics, but geothermal still wins over a 25-year horizon in most zone 3 markets.
Zone 4 (Mixed–Cold): Pennsylvania, Ohio, Indiana — Most Listings Here
Zone 4 is where geothermal has its deepest market penetration in the US, and where the financial case against oil and propane is strongest. Pennsylvania, Ohio, and Indiana sit at 5,000–6,200 HDD — heating-dominated, but not so extreme that loop sizing becomes cost-prohibitive.
The replacement math in zone 4 is compelling. A typical 2,400-square-foot home in central Pennsylvania might burn 1,000 gallons of heating oil per year at $3.50–$4.50/gallon — a $3,500–$4,500 annual spend that is entirely weather-dependent and subject to supply shocks. The equivalent geothermal system in the same home typically uses $700–$1,100 of electricity for heating, achieving gross savings of $2,400–$3,400/year.
After a 30% federal tax credit (under the Inflation Reduction Act, which remains in effect for residential systems), a typical zone 4 geothermal installation nets to $14,000–$22,000 depending on loop configuration. At the savings rates above, simple payback runs 5–9 years. Over 25 years, net-present-value advantages of $30,000–$60,000 over heating oil are realistic.
Zone 4 also concentrates the propane replacement market, which is even more compelling: propane has historically traded at a 40–60% premium over heating oil in rural Pennsylvania, Ohio, and Indiana markets. Find zone 4 installers in our state hubs: Pennsylvania geothermal installers, Ohio geothermal installers, and New York geothermal installers.
Zone 5 (Cool–Very Cold): Michigan, Wisconsin, Minnesota, Iowa
Zone 5 is where some homeowners worry geothermal “might not work.” It works. The difference from zone 4 is that loop field design requires more careful Manual J load calculations, and auxiliary heat strips may activate on the coldest 1–3% of winter days.
Ground temperatures in zone 5 at 6 feet depth range from 48–54°F — still far warmer than outdoor air on a Wisconsin January morning. COP remains in the 3.0–4.0 range even in deep winter because the source temperature stays positive. Compare this to an air-source heat pump whose COP can fall to 1.5–2.0 at outdoor temperatures below 15°F.
Key design considerations for zone 5:
- Loop field sizing. Heating loads are larger, so loops must be longer or wells deeper to maintain adequate source temperature through prolonged cold snaps. Vertical wells 200–400 feet deep are common; horizontal systems require larger land area.
- Antifreeze concentration. Loop fluid typically uses 20–25% propylene glycol or methanol to prevent freezing in closed loops.
- Manual J accuracy. An oversized zone 5 geothermal unit with a correctly sized loop is fine. An undersized loop with a correctly sized unit will see loop temperatures fall below acceptable limits during sustained cold, degrading performance. Get the load calculation right before drilling.
- Auxiliary heat budget. Most zone 5 installations include 5–10 kW of electric resistance backup. Annual runtime on aux heat in a well-designed system is typically 50–150 hours — roughly 2–6% of total heating hours. If auxiliary is running constantly, the loop is undersized.
Find zone 5 installers in our hub: Michigan geothermal installers.
For a deep dive into geothermal performance in northern climates, see our sister article Geothermal Heating in Cold Climates.
Zones 6–7 (Very Cold / Subarctic): Northern New England, Mountain States
Zone 6 covers northern Vermont, New Hampshire, Maine, northern Minnesota, and high-elevation Rocky Mountain locations. Zone 7 reaches the most extreme continental US climates — northern Maine, far northern Minnesota, and pockets of high-altitude Montana and Wyoming. Heating degree days in these zones run 7,200 to 12,600+ annually.
Geothermal works in zones 6 and 7, but with important design adjustments:
- Larger loops. Heating demand is high; loops must be substantial. Vertical well fields of 300–500+ feet per ton of capacity are typical. Land area for horizontal systems is large.
- Higher auxiliary heat probability. In zone 7 especially, even a correctly sized system may rely on backup heat for 5–10% of heating hours during extreme cold snaps. Electric resistance strips sized to 100% of design load are standard so the home remains comfortable if the ground temperature drops near the loop’s minimum operating threshold.
- Enhanced antifreeze. Loop fluid freeze points must be calculated for local design temperatures; 30% propylene glycol is common in zone 6–7.
- Economic calculus tilts hard against oil and propane. Zone 6–7 homeowners burning 1,500–2,000 gallons of heating oil or propane per year face energy bills of $5,000–$9,000+ in cold winters. The savings case for geothermal is compelling even with larger loop-field costs, provided the Manual J is accurate and the contractor is experienced with cold-climate sizing.
Vermont, New Hampshire, and Maine have active state incentive programs for cold-climate heat pumps that often stack on top of the 30% federal tax credit. Check current program availability before installation — these programs change annually.
Cooling-Only Mode and Dehumidification
Geothermal is often discussed as a heating technology, but in warm and hot climates it functions as central air conditioning with material advantages over conventional equipment.
In cooling mode, the unit operates exactly like a central AC system from the homeowner’s perspective: air flows across an evaporator coil, heat is extracted, cooled air returns through the ducts. The difference is where that heat goes. A conventional AC compressor rejects heat to outdoor air — which can be 95°F on a hot August afternoon, creating a brutal working condition. A geothermal unit rejects heat into the ground loop, where water temperatures hover around 70–85°F depending on loop size and season. That lower rejection temperature means higher EER ratings: typical geothermal systems deliver EER 14–30, compared to EER 13–18 for premium conventional AC.
Dehumidification advantage. In humid climates (ASHRAE moisture designation “A” — zones 1A, 2A, 3A, 4A), latent cooling load can represent a large fraction of total cooling. Latent cooling is the energy required to condense moisture out of the air. A lower refrigerant evaporator temperature means more effective moisture removal. Ground-source units, operating with consistently favorable source temperatures, sustain the low evaporator temperatures needed for aggressive dehumidification better than air-source equipment that is thermally stressed on hot, humid days. The practical result: at equivalent thermostat setpoints, geothermal homes in humid climates feel less muggy.
Some zone 1–2 homeowners run their geothermal systems in what’s informally called “fan and dehumidify” mode on mild days — the system removes moisture without driving the indoor temperature too far below the setpoint. This improves comfort without overcooling. Standard AC systems struggle with this mode because they’re sized for peak sensible (temperature) load and cycle off quickly, leaving humidity elevated between cycles.
Backup Heat: When, Why, and How to Size It
Many geothermal systems include electric resistance heat strips — typically 5 to 20 kilowatts — installed in the air handler. Understanding when these strips should run (and when they signal a problem) is important for both comfort and operating cost.
When backup heat is normal: Properly sized geothermal systems are designed to meet 95–97% of the design heating load at the 99th-percentile outdoor design temperature for the location. On the coldest 1–4% of winter hours — when outdoor temperatures drop below what the system was designed for — the ground loop temperature may fall toward its minimum, and auxiliary strips activate to supplement. In zones 4–5, this might mean 50–150 hours per year of strip heat, often concentrated in January and February. A well-designed zone 6 system might run 150–250 hours of aux heat annually. This is normal, expected, and budgeted for. The annual electricity cost for this auxiliary heat typically runs $100–$400 depending on zone and electricity rate.
When backup heat signals a problem: If aux heat strips run constantly — not during extreme cold events, but during ordinary cold weather — the loop is almost certainly undersized. A properly sized loop should keep the entering water temperature (EWT) above 25°F in zone 4, above 20°F in zone 5, and above 15°F in zone 6. If EWT is falling below these thresholds during normal operating conditions, additional loop capacity is needed. Constant aux heat runtime means you are paying electric resistance heat prices for a large fraction of your heating, which eliminates most of the geothermal efficiency advantage.
Sizing the backup heater: Industry practice is to size aux strips to cover 100% of the design heating load at the design outdoor temperature. This provides a safety net: if the heat pump ever fails during winter, the strips keep the house safe. During normal operation, the strips run in staged mode — typically in 5 kW increments — only when the heat pump alone cannot maintain setpoint. The stranded cost of this capacity is modest; a 10 kW strip heater adds $400–$700 to system cost but provides complete backup coverage.
For a complete system overview including maintenance schedules for heat strips and loop fluid, see Geothermal Maintenance and Service Manual.
Climate-by-Climate ROI: 25-Year NPV Summary
The table below models a 2,400 sq ft home, new geothermal installation (vertical closed loop), 30% federal tax credit applied, 3% annual fuel escalation, 2% annual electricity escalation, and a $0 system residual value at year 25. All figures are in present-value dollars at a 5% discount rate. Ground-loop cost assumptions vary by zone to reflect depth requirements.
| Climate Region | Zones | Displaced Fuel | Net System Cost (post-credit) | Yr-1 Annual Savings | Simple Payback | 25-Yr NPV Advantage |
|---|---|---|---|---|---|---|
| Hot South (FL, TX gulf coast) | 1–2 | Central AC + electric strip | ~$16,000 | ~$900–$1,300 | 12–17 yrs | ~$8,000–$18,000 |
| Warm South (GA, NC, VA) | 2–3 | Propane + AC | ~$17,000 | ~$1,200–$2,000 | 9–14 yrs | ~$16,000–$30,000 |
| Mid-Atlantic / Midwest (PA, OH, IN) | 4 | Heating oil + AC | ~$18,500 | ~$2,400–$3,400 | 5–8 yrs | ~$35,000–$55,000 |
| Great Lakes (MI, WI, MN, IA) | 5 | Propane + AC | ~$20,000 | ~$2,000–$3,000 | 7–10 yrs | ~$28,000–$48,000 |
| Northern New England / Mountain | 6–7 | Heating oil + electric backup | ~$22,000 | ~$2,500–$4,000 | 6–9 yrs | ~$32,000–$58,000 |
Zone 1–2 NPV is lower because the dominant load is cooling, and air-source AC efficiency has improved substantially. The economic sweet spot remains zones 4–7, where heating-dominated loads combine with high-cost fuels (oil and propane) and ground conditions that favor vertical wells.
Use our geothermal cost estimator to run these numbers for your specific home, climate zone, and fuel type. For full cost and financing guidance, see Geothermal Heat Pump Cost Guide (Pillar #1) and Geothermal Installation Process Guide (Pillar #2).
Frequently Asked Questions
Does geothermal work in cold climates?
Yes. Geothermal heat pumps work reliably in zones 5, 6, and 7 — including Minnesota, Wisconsin, Vermont, and Maine. The ground 6 to 10 feet down stays at 46–54°F year-round in these regions, regardless of outdoor air temperature. That source temperature is far warmer than a January Minnesota night, giving the heat pump a consistent positive differential to work with. Cold climates require larger loop fields and should budget for 50–250 hours of annual auxiliary electric heat strip operation, but the core technology is fully functional. Geothermal is in fact most financially compelling in cold climates, where the savings against heating oil and propane are largest.
Does geothermal work in hot climates?
Yes. In hot climates (zones 1–2), geothermal operates primarily as central air conditioning. The ground acts as a heat sink that remains well below outdoor summer air temperatures, giving the system consistent high efficiency even on the hottest days. An additional benefit in hot-humid climates is superior dehumidification: ground-source units maintain lower evaporator temperatures than air-source equipment, removing more moisture per cooling cycle and improving indoor comfort.
What climate is best for geothermal?
The financial return is highest in cold-to-very-cold climates (zones 4–7) where heating loads are large and the displaced fuels — oil and propane — are expensive. Homeowners in Pennsylvania, Ohio, Michigan, Wisconsin, and Minnesota replacing heating oil or propane with geothermal typically see simple payback in 5–10 years and 25-year net-present-value advantages of $28,000–$58,000. That said, geothermal “works” everywhere — the performance differential versus displaced equipment is meaningful in every zone.
How does geothermal heating work in winter?
In winter, a water-based fluid circulates through underground pipes (the loop field), absorbing heat from the earth. That fluid, warmed to roughly the ground temperature (46–65°F depending on location), passes through a heat exchanger inside the unit. A refrigerant picks up that energy, a compressor concentrates it, and a second heat exchanger releases it into your home’s duct or radiant system. The cooled loop fluid returns underground to absorb more heat. The cycle repeats continuously. Even on sub-zero outdoor days, the ground remains above freezing and continues supplying heat to the loop.
Can geothermal cool a house?
Yes. Geothermal heat pumps reverse the refrigeration cycle for cooling: they extract heat from your home and reject it into the ground. In cooling mode, the ground acts as a heat sink whose temperature is well below outdoor summer air, giving geothermal consistent efficiency advantages over conventional AC. Most residential geothermal units are fully reversible and handle heating and cooling from one compact indoor unit, eliminating the need for a separate outdoor AC compressor.
Does geothermal need backup heat?
Most installations include electric resistance heat strips (typically 5–20 kW) as a backup for the coldest days. In zones 4–5, these strips run roughly 1–6% of total heating hours annually — mainly during the coldest January and February nights. In zones 6–7, backup heat runs more frequently. If backup heat is running constantly during normal cold weather (not just extreme cold events), it is a signal that the loop field is undersized, not a normal operational pattern. A properly designed system runs on the heat pump the vast majority of the time.
How cold is it 6 feet underground?
At 6 feet below the surface, ground temperature tracks the local mean annual air temperature (MAAT) within about ±5°F. In Florida, that means roughly 70–72°F year-round. In northern Minnesota, roughly 46–50°F. In Pennsylvania and Ohio, roughly 52–56°F. These temperatures are essentially constant across all four seasons — the ground at 6 feet feels the same in January as in July. For geothermal systems, this stable temperature is the source (in winter) or sink (in summer) that the heat pump operates against. For full details, see How Cold Is It 6 Feet Underground?
What country gets 90% of its homes heated by geothermal?
Iceland. Approximately 90% of Iceland’s homes are heated by geothermal energy, drawn from the country’s abundant volcanic heat resources via district heating networks. Iceland sits on the Mid-Atlantic Ridge, giving it exceptionally shallow, hot geothermal resources that make utility-scale district heating economically practical. The US residential geothermal market uses a different technology — ground-source heat pumps that extract low-grade heat from shallow soil rather than high-temperature geothermal fluids — but the underlying principle of using the earth’s stored thermal energy is the same.
Does geothermal work below freezing?
Yes. Geothermal heat pumps operate reliably when outdoor air temperatures are far below freezing. The underground loop fluid circulates at ground temperature — typically 46–65°F in the continental US — regardless of the outdoor air temperature. The loop fluid itself is treated with antifreeze (propylene glycol or methanol, typically 20–30% concentration) to prevent freezing within the pipes in cold climates. The heat pump equipment operates normally in outdoor temperatures down to −20°F or lower; it is a sealed indoor unit and is not exposed to outdoor conditions.
What temperature can geothermal heat to?
Residential ground-source heat pumps typically deliver supply air temperatures of 90–105°F in forced-air systems and water temperatures of 95–120°F for radiant floor systems. These temperatures are somewhat lower than a gas furnace’s 120–140°F supply air, which means geothermal systems are often sized slightly larger than gas furnaces for the same home, and work particularly well with low-temperature radiant floor heating. High-efficiency geothermal units with two-stage or variable-speed compressors can push water temperatures to 120–130°F if needed for older high-temp distribution systems, though efficiency decreases at higher output temperatures.
Sources
- ASHRAE Standard 169-2020: Climatic Data for Building Design Standards — ashrae.org
- IECC 2021 Climate Zone Map, Building America Solution Center — basc.pnnl.gov
- DOE Energy Saver: Geothermal Heat Pumps — energy.gov
- DOE Building America Climate Region Guide v7.3 — energy.gov
- ENERGY STAR: Ground Source Heat Pumps — energystar.gov
- USDA NRCS Soil Survey Geographic Database (SSURGO) — nrcs.usda.gov
- NOAA Climate Normals 1991–2020 — ncei.noaa.gov
- BuildItSolar: Ground Temperatures as a Function of Location, Season, and Depth — builditsolar.com
- ISLANDSSTOFA: 90% of house heating in Iceland is geothermal — islandsstofa.is
- Minnesota Geothermal Heat Pump Association: How Does It Work — mnghpa.com