How Do Geothermal Heat Pumps Work?

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.

The cycle that does the lifting

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

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.

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.

For an ideal reversible cycle, the ceiling is Th / (Th − Tc), with both temperatures in kelvin. 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.

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.

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.

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.

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. 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.