Ground source heat pumps run about 10–20% more efficiently than air source and hold that efficiency all winter, because ground below 1.5 m stays at 8–12 °C year-round. UK field monitoring measured an average seasonal performance factor of 2.81 across 428 air source units, and a median ground source COP of 3.10 at −5 °C against 2.30 for air source. Air source wins on installed cost — typically a half to a third of a ground source job — as well as space and replacement simplicity, which makes it the better-value choice for most retrofit homes.
How much more efficient is a ground source heat pump?
Ground source runs roughly 10–20% more efficiently across a season, and it holds that efficiency when the weather is at its worst. The UK Electrification of Heat demonstration project measured an average seasonal performance factor of 2.81 across 428 monitored air source units, and its ground source systems held a median COP of 3.10 at −5 °C when the air source units had fallen to 2.30.
The reason is source temperature. Air at −3 °C forces the compressor to lift heat 48 °C to produce 45 °C water. Ground below about 1.5 m sits at 8–12 °C all year across most of northern and central Europe, so the same output is only a 35 °C lift. Ground source also never runs a defrost cycle, removing a loss that costs air source units single-digit percentages of seasonal COP in damp winters. Both technologies still depend far more on your emitters than on the source: a ground source system pushed to 55 °C will lose to an air source system running at 35 °C.
What does the ground loop cost in space and money?
The collector is the whole difference between the two technologies.
Horizontal loops are buried 1.2–1.5 m deep and need roughly 10 m of trench per kW of heat pump, or about 30–50 m² of undisturbed ground per kW in damp cohesive soil. An 8 kW system therefore wants 250–400 m² of garden you are willing to excavate and then never build on, pave, or plant trees in.
Vertical boreholes are drilled 80–200 m deep. Specific extraction rates run from about 25 W per metre in dry sand and gravel to 70–80 W/m in water-bearing gravels and hard rock, with moist clay and loam typically 35–50 W/m. An 8 kW heat pump drawing roughly 6 kW from the ground therefore needs anywhere from 75 m to 240 m of borehole depending purely on geology. Boreholes consume almost no surface area but bring a drilling rig, a ground investigation and, in most countries, a groundwater permit.
| Air source | Ground source (horizontal) | Ground source (borehole) | |
|---|---|---|---|
| Typical measured SPF | 2.8 – 3.2 | 3.2 – 4.0 | 3.2 – 4.2 |
| Outdoor space | 1 – 2 m² beside a wall | 30 – 50 m² per kW | ~5 m² per borehole |
| Ground works | None | Excavation 1.2 – 1.5 m deep | Drilling 80 – 200 m |
| Cost vs air source | Baseline | ~1.5 – 2× | ~2 – 3× |
| Defrost losses | Yes | None | None |
| Outdoor noise | Fan and compressor | None | None |
| Consent risk | Siting and noise rules | Excavation, roots, services | Groundwater / drilling permit |
Which one suits my property?
Choose ground source when you have the land or the drilling budget, expect to stay long enough for the running-cost gap to repay the ground works, want zero outdoor noise, or need summer cooling — a ground loop provides near-free passive cooling that an air source unit has to run its compressor for.
Choose air source when the garden is small, shared or already landscaped; when the project must fit a normal retrofit budget and timescale; or when the building is a flat or mid-terrace. Air source is also far easier to replace at end of life: the unit is a bolted box on a wall, not a field of buried pipe.
Does ground source still win once you count the whole job?
Often not on money alone. The efficiency gain is real but modest — perhaps 15% of your heating electricity — while ground works can double or triple the installed cost. Most European grant schemes pay the same headline amount for either technology, which quietly favours the cheaper one; see which European countries offer heat pump grants in 2026?. Ground source earns its keep on large or long-lived buildings, on shared arrays serving several dwellings, and wherever the loop can double as a cooling sink.
What decides the outcome in both cases is identical: the heat load calculation, the emitter sizing and the flow temperature the system is designed around. Get those right and an air source unit beats a badly designed ground source one comfortably. Solimpeks builds its air-to-water heat pumps around that priority, including Varm Up EVI models rated for operation down to −30 °C ambient so the air source option stays viable in genuinely cold climates.
Which lasts longer and needs less maintenance?
The ground array is the longest-lived component in any heating system — polyethylene loops are normally specified for 50 years or more, so a ground source home eventually replaces only the indoor unit after 20–25 years. Air source units live 15–20 years outdoors, with the heat exchanger, fan and defrost circuit all exposed to weather, salt and leaves. Annual servicing is otherwise comparable: refrigerant and flow checks, strainer cleaning, and a control review.
Frequently asked questions
Is a ground source heat pump worth the extra cost?
It is worth it when the property is large, long-lived, or already needs groundworks, and when cooling is also required. For a typical retrofit house the 10–20% efficiency gain rarely repays ground works that cost two to three times an air source installation within the equipment's own lifetime.
How deep does a ground source borehole need to be?
Between 80 m and 200 m for a domestic system, sized on geology rather than on house size. Extraction rates range from about 25 W/m in dry sand to 70–80 W/m in water-bearing rock, so an 8 kW heat pump may need 75 m in good ground or 240 m in poor ground.
Can a ground source heat pump work in a small garden?
Only with boreholes. Horizontal collectors need roughly 30–50 m² of ground per kW, which rules out most urban plots, while a borehole occupies a few square metres of surface. Boreholes cost more and usually require a permit.
Do ground source heat pumps need planning permission?
In most European countries the heat pump itself does not, because there is no outdoor unit to see or hear. The ground works are the regulated part: drilling normally requires notification to the groundwater or geological authority, and excavation must avoid protected trees and buried services.