Ground source and air source heat pumps both do the same fundamental job — move heat from the environment into your home rather than generating it by burning fuel. The difference is where that heat comes from, and that difference has profound implications for installation cost, efficiency, and which properties can realistically accommodate each technology.
The Core Difference
An air source heat pump (ASHP) extracts heat from outdoor air. Even at -5°C, there is usable heat energy in the air. The unit sits outside, typically on a wall mount or ground plinth, and works year-round with no ground disturbance.
A ground source heat pump (GSHP) extracts heat from the ground, which maintains a stable temperature of 8–12°C year-round regardless of air temperature. The heat extraction happens through a closed-loop system — either horizontal ground loops buried at 1.2–1.5 m depth, or vertical boreholes drilled 80–150 m deep.
Efficiency: SCOP and Why It Matters
The measure of heat pump efficiency is the Seasonal Coefficient of Performance (SCOP) — the ratio of heat output to electrical energy consumed, averaged over a heating season.
| Technology | Typical SCOP | Heat out per 1 kWh electricity |
|---|---|---|
| Air source heat pump | 2.5–3.5 | 2.5–3.5 kWh |
| Ground source (horizontal loop) | 3.2–4.0 | 3.2–4.0 kWh |
| Ground source (borehole) | 3.5–4.5 | 3.5–4.5 kWh |
| Modern gas boiler | ~0.90 | 0.9 kWh |
A higher SCOP means lower running costs for the same heat output. At a typical electricity tariff of 24–28 p/kWh (2026), a GSHP with a SCOP of 4.0 delivers heat at an effective cost of 6–7 p/kWh. An ASHP with a SCOP of 3.0 delivers heat at 8–9 p/kWh. A gas boiler at current gas prices (around 7 p/kWh at the unit rate) is still competitive — which is why heat pump economics depend heavily on the electricity-to-gas price ratio.
Installation Costs
| System | Installed cost (inc. VAT, 2026) | After £7,500 BUS grant |
|---|---|---|
| ASHP (3-bed semi) | £9,000–£14,000 | £1,500–£6,500 |
| ASHP (4-bed detached) | £12,000–£18,000 | £4,500–£10,500 |
| GSHP (horizontal loop, 3-bed) | £15,000–£22,000 | £7,500–£14,500 |
| GSHP (horizontal loop, 4-bed) | £18,000–£28,000 | £10,500–£20,500 |
| GSHP (borehole, any size) | £20,000–£40,000 | £12,500–£32,500 |
The GSHP premium over an equivalent ASHP is typically £6,000–£15,000 for horizontal loops and £12,000–£25,000 for boreholes, after accounting for the same BUS grant amount. This premium must be recovered through lower running costs to justify the choice.
Borehole drilling alone (for a typical 3-bed property requiring 2 × 100 m boreholes) costs £8,000–£16,000 before any heat pump equipment.
Ground Requirements
| System type | Land requirement | Soil suitability |
|---|---|---|
| Horizontal loop (standard) | ~300–500 m² per 10 kW | Clay soils best; poor in dry sand |
| Horizontal Slinky loop | ~150–300 m² per 10 kW | As above, more compact |
| Vertical borehole | No surface area (just borehole access point) | Suitable for most sites; geological survey needed |
| ASHP | 1–3 m² for outdoor unit | No ground works |
Horizontal loops require a large, unobstructed garden area that can be excavated and reinstated. A 3-bed property typically needs 10–15 kW of heat pump capacity, requiring 400–700 m² of undisturbed loop field. Most urban and suburban gardens cannot accommodate this.
Boreholes solve the space problem but add significant cost and require specialist drilling equipment — the rig needs site access, typically vehicle access of at least 2.5 m width and 4.5 m height clearance.
The BUS Grant: Does It Apply to Both?
Yes. The Boiler Upgrade Scheme (BUS) grant of £7,500 applies to both ASHPs and GSHPs installed in existing properties in England and Wales. The installer must be MCS-accredited, and the property must have a valid EPC with loft and cavity wall insulation recommendations completed (or confirmed as not applicable).
From April 2026, the BUS scheme has been extended to cover properties that previously missed out due to minor EPC issues — check the current DESNZ guidance for the latest eligibility rules.
Which Is Right for Your Property?
Choose an ASHP if:
- You have a standard suburban or urban property.
- Your garden is small or fully landscaped.
- You want minimum ground disturbance.
- Budget is a primary concern — the ASHP payback period is shorter despite lower SCOP.
Choose a GSHP if:
- You have over 0.15–0.25 hectares of unobstructed garden suitable for ground loops.
- You can accommodate borehole drilling access.
- You want the highest possible system efficiency (important for large, older, or poorly insulated properties where heating loads are high).
- The property is in a rural area with low ground disturbance costs.
- You are building new and can integrate ground loops at the groundworks stage cheaply.
Running Cost Comparison: Worked Example
Assumptions: 3-bed semi-detached, 80 m² of heated floor area, heating demand 12,000 kWh/year, electricity at 26 p/kWh.
| System | SCOP | Annual electricity use | Annual heating cost |
|---|---|---|---|
| ASHP | 3.0 | 4,000 kWh | £1,040 |
| GSHP (horizontal) | 3.8 | 3,158 kWh | £821 |
| GSHP (borehole) | 4.2 | 2,857 kWh | £742 |
| Gas boiler (85% efficiency) | 0.85 | N/A (gas) | £988 (at 7p/kWh gas) |
The annual saving of a GSHP over an ASHP is £220–£300 per year in this example. At an installation premium of £10,000, the additional payback period is 33–45 years — significantly longer than the system’s warranty period. This calculation explains why ASHPs dominate new heat pump installations in the UK, with GSHPs reserved for properties where their advantages are clear.
Finding a Qualified Installer
All heat pump installations eligible for the BUS grant must be carried out by MCS-accredited contractors. The MCS (Microgeneration Certification Scheme) directory at mcscertified.com lists accredited installers. Get at least three quotes — heat pump pricing varies considerably, and the cheapest is rarely the best value in a high-stakes installation.
