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How Do Geothermal Heat Pumps Work?

Updated 8 October 20267 min readGeothermal & Heat Pumps

A geothermal heat pump moves heat between a building and the ground using a refrigerant cycle, rather than burning anything. Buried pipes exchange heat with soil that stays close to the local annual average temperature all year, so the machine works across a small, stable temperature difference — which is exactly the condition under which it delivers several units of heat per unit of electricity.

Key takeaways

  • There are three separate loops: a ground loop, the sealed refrigerant circuit inside the machine, and the distribution loop that heats the building.
  • Shallow ground heat is mostly last summer's sunshine in storage, not heat from the Earth's core — which is why loop design is about the soil around the pipe, not about depth for its own sake.
  • Coefficient of performance is set by the temperature lift. A stable source and a low delivery temperature beat every other efficiency measure combined.
  • A ground loop is sized for the heat the soil can supply over a season, not for the peak hour. Undersize it and the source temperature drifts down year after year.
  • Nothing about the cycle is specific to the ground: an air-source machine runs the same four components against a source that swings with the weather.
On this page
  1. Three loops, one machine
  2. The cycle that does the lifting
  3. Why the shallow ground is such a good source
  4. COP, and why the lift decides it
  5. Loop layouts, and what each one needs
  6. What actually goes wrong

Three loops, one machine

The phrase "geothermal heat pump" describes a system with three distinct circuits, and most confusion about how they work comes from collapsing them into one.

  1. The ground loop. Buried plastic pipe carrying water, usually with antifreeze. It collects heat from the soil in winter and can dump heat into it in summer. Nothing in this loop gets hot or cold in the way the refrigerant does — it typically runs only a few degrees away from the surrounding soil.
  2. The refrigerant circuit. Sealed inside the heat pump. This is where the temperature lift actually happens, using a compressor and a fluid that boils and condenses at useful temperatures.
  3. The distribution loop. Water to underfloor pipes or radiators, or air through ducts, delivering the heat to rooms.
Cutaway of a house with a ground-source heat pump. A buried ground loop carries fluid between the soil and the heat pump indoors. Inside the unit a sealed refrigerant circuit raises the temperature, and a distribution loop carries heat to underfloor pipes. Arrows show heat moving from soil to fluid, from fluid to refrigerant, and from refrigerant to the building.Cutaway of a house with a ground-source heat pump. A buried ground loop carries fluid between the soil and the heat pump indoors. Inside the unit a sealed refrigerant circuit raises the temperature, and a distribution loop carries heat to underfloor pipes. Arrows show heat moving from soil to fluid, from fluid to refrigerant, and from refrigerant to the building.
Three loops in series. Heat is handed from the soil to the ground loop, from the ground loop to the refrigerant, and from the refrigerant to the building.

The cycle that does the lifting

The heat pump itself is a vapour-compression machine: four components, one job each.

Diagram of the vapour-compression cycle: the evaporator absorbs heat from the ground loop and boils the refrigerant, the compressor raises its pressure and temperature using electrical work, the condenser releases heat to the building as the vapour condenses, and the expansion valve drops the pressure before the refrigerant returns to the evaporator.Diagram of the vapour-compression cycle: the evaporator absorbs heat from the ground loop and boils the refrigerant, the compressor raises its pressure and temperature using electrical work, the condenser releases heat to the building as the vapour condenses, and the expansion valve drops the pressure before the refrigerant returns to the evaporator.
The refrigerant boils at low pressure using heat from the ground, is compressed to a higher temperature, gives that heat up indoors, and expands back to start again.

The refrigerant is chosen so that it boils at a temperature *below* the ground loop's, and condenses at a temperature *above* the building's. That is the whole trick: heat always flows from hot to cold, so the machine arranges for the refrigerant to be colder than the soil when collecting and hotter than the room when delivering.

Most of the energy moved is latent heat — the energy absorbed and released by the phase change itself — which is why a modest flow of refrigerant can carry the heat of a whole house.

Why the shallow ground is such a good source

Here is the detail that surprises people: the heat a ground-source system collects is not, for the most part, heat from the Earth's interior. In the first few metres, the soil is warmed by the sun and by the air above it. A ground loop is, in effect, harvesting stored sunshine from the preceding months.

What makes it valuable is not that it is warm but that it is stable. Surface temperature swings through the year, but soil conducts heat slowly, so the swing shrinks with depth and arrives later. A few metres down, the annual variation is small; deeper still, the temperature sits close to the local annual mean air temperature and barely moves.

Chart of soil temperature against depth for four seasons. At the surface the four curves are far apart, ranging from cold in winter to warm in summer. As depth increases the curves converge and cross, and below several metres they all sit close to the local annual average temperature.Chart of soil temperature against depth for four seasons. At the surface the four curves are far apart, ranging from cold in winter to warm in summer. As depth increases the curves converge and cross, and below several metres they all sit close to the local annual average temperature.
Illustrative. The seasonal swing fades with depth and lags behind the surface. That stability, not warmth, is what a ground loop is buying.

An air-source machine faces its largest temperature lift on the coldest hour of the year — exactly when the building needs the most heat. A ground-source machine faces roughly the same lift in January as in April. That difference in *timing*, not in average conditions, is the core of the case for digging, and it is drawn out in ground-source versus air-source in extreme cold.

COP, and why the lift decides it

Coefficient of performance is heat delivered divided by electrical work supplied. A machine drawing 1 kW and delivering 4 kW has a COP of 4; the other 3 kW came out of the ground. That multiplication is also why a heat pump is one of the most effective places to send surplus solar electricity: each kilowatt-hour diverted to it delivers several of heat.

For an ideal reversible cycle, the ceiling is Th / (Th − Tc), with both temperatures in kelvin. Like the Betz limit for a wind rotor, it is a bound set by physics that no engineering can beat, only approach. Real machines reach a fraction of it, because compressors, heat exchangers and pressure drops all have losses — but the *shape* of the relationship is what matters for design.

Ideal COP ceiling for typical ground-source conditions
Ground loop temperatureDelivery temperatureLiftIdeal (Carnot) COP
5 °C (41 °F)35 °C (95 °F)30 Kabout 10.3
5 °C (41 °F)45 °C (113 °F)40 Kabout 8.0
5 °C (41 °F)55 °C (131 °F)50 Kabout 6.6
0 °C (32 °F)35 °C (95 °F)35 Kabout 8.8
0 °C (32 °F)55 °C (131 °F)55 Kabout 6.0

Carnot ceilings for a reversible cycle, calculated from the stated temperatures. Real machines deliver a fraction of these values; the pattern, not the number, is the point: every degree of lift removed is worth more than any other single improvement.

Read down the first column and the pattern is obvious: the delivery temperature moves the answer more than the source does. This is why underfloor heating and heat pumps are so often mentioned in the same breath — a large emitter surface delivers the same heat to a room at a much lower water temperature, and a lower water temperature is a smaller lift.

Chart of coefficient of performance against temperature lift, showing the Carnot ceiling falling steeply as lift increases and a shaded band beneath it representing what real machines achieve, with the ground-source and air-source operating regions marked.Chart of coefficient of performance against temperature lift, showing the Carnot ceiling falling steeply as lift increases and a shaded band beneath it representing what real machines achieve, with the ground-source and air-source operating regions marked.
Illustrative. Real machines track under the Carnot ceiling at a fraction of it, but they follow the same steep curve — which is why lift is the number worth attacking.

Loop layouts, and what each one needs

The ground loop has to move a season's worth of heat through soil, so it is sized by ground conditions and load, not by the size of the machine alone.

  • Horizontal trenches are the cheapest to excavate where land allows. They sit in the layer that still swings seasonally, so they need length and benefit from moist soil.
  • Slinky or coiled trenches pack more pipe into a shorter trench, trading pipe for excavation. Overlapping coils interfere thermally with each other, so the packing is not free.
  • Vertical boreholes reach the stable zone and need little land area, at the price of drilling. They also draw from a much larger volume of ground per metre of pipe.
  • Open-loop and pond systems exchange heat with groundwater or a body of water directly, which is thermally excellent and comes with its own water-handling requirements — compared properly in closed-loop versus open-loop systems.
Four ground loop layouts drawn in section: straight horizontal pipes in trenches, a coiled slinky trench, vertical boreholes reaching deeper stable ground, and a loop submerged in a pond, each labelled with the land or depth it requires.Four ground loop layouts drawn in section: straight horizontal pipes in trenches, a coiled slinky trench, vertical boreholes reaching deeper stable ground, and a loop submerged in a pond, each labelled with the land or depth it requires.
The same machine runs with any of these. What changes is how much ground the loop can draw from, and how quickly that ground recovers between seasons.

What actually goes wrong

Ground loops are the part people worry about and rarely the part that fails — they are sealed plastic pipe with nothing moving inside. The recurring problems are thermal and hydraulic.

Undersized loop. If the loop extracts more heat each winter than the surrounding ground recovers each summer, the source temperature trends downward season after season. The symptom is a system that performed well in its first winter and worse in its third, with no component fault to find.

Chart of average ground loop temperature over five heating seasons. A correctly sized loop returns to the same starting temperature each autumn, while an undersized loop starts each season colder than the last as the surrounding soil fails to recover.Chart of average ground loop temperature over five heating seasons. A correctly sized loop returns to the same starting temperature each autumn, while an undersized loop starts each season colder than the last as the surrounding soil fails to recover.
Illustrative. A correctly sized loop is a seasonal store that refills; an undersized one is a slow withdrawal from the ground's heat balance.

Flow and air. Too little flow through the heat exchanger, or trapped air in the loop, starves the evaporator and drops capacity in a way that looks like a refrigerant fault.

Delivery temperature creep. A system commissioned for low-temperature emitters and later asked to feed hot radiators gives back exactly what the COP table predicts.

Short cycling. An oversized machine in a well-insulated building switches on and off repeatedly instead of running long and low, which wastes energy and wears the compressor, and each start is a motor-starting surge. If the cycle is running but the building is not warming as expected, the fault-finding path in why a heat pump freezes up covers the air-side version of the same diagnostic logic.

Frequently asked questions

Is geothermal heat the same as the heat from volcanoes?

No. Deep geothermal taps heat from the Earth's interior, which is why it needs depth. A ground-source heat pump works in the first few metres of soil, where the heat is stored solar energy from the preceding months. The name is shared; the resource is not.

Does a ground loop freeze the soil around it?

A correctly sized loop cools the soil during the heating season and lets it recover before the next one. A loop that is too short for the load drives the soil steadily colder, so the source temperature — and with it the COP — falls year on year. Ice around the pipe is a symptom of undersizing, not a normal state.

Can a geothermal heat pump also cool a building?

Yes, and the ground is well suited to it: running the cycle in reverse dumps heat into soil that is cooler than summer air. In buildings with both heating and cooling seasons, that also helps rebalance the ground thermally.

Why do installers care so much about the delivery temperature?

Because COP depends on the gap between source and delivery temperatures. Feeding underfloor loops at a low temperature instead of small radiators at a high one can change the running efficiency of the same machine substantially. Emitter sizing is part of the heat pump design, not a detail after it.

What maintenance does a ground loop need?

Very little: it is a sealed circuit of plastic pipe with no moving parts. What needs attention is the circulation pump, the antifreeze concentration, and the flow rate through the heat exchanger. Most performance complaints trace back to flow or control settings rather than the buried pipe.

Sources

Named organisations whose published material underpins this article. Where no link is given, the source is named rather than linked.

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Research, drafting and review

Articles are drafted from primary engineering and physics references with AI-assisted tools, then reviewed and fact-checked line by line by a human editor before publication. We publish explanations, not recommendations: no products, no pricing, no country-specific rules, and no invented author personas.

Last reviewed 8 October 2026. How we research and review