Side-by-side comparison
| Factor | Air source (air-to-water) | Ground source |
|---|---|---|
| Official median installed price, 2026 Q3 (including grant) | £12,826 | £34,630 |
| Energy Saving Trust typical cost | Around £12,000 | Around £15,000 with trenches; up to about £57,000 with a borehole |
| Grant | £7,500 (£9,000 if off-gas oil or LPG, applications to 31 March 2027) | £7,500 (£9,000 if off-gas oil or LPG) |
| Voucher validity | 3 months | 6 months |
| Space needed | Wall or ground spot outside for a unit; cylinder cupboard inside | Land for trenches or a borehole; indoor unit often combined with the cylinder |
| Efficiency | Good; drops as it gets colder | Tends to be higher, because ground is steadier than air |
| Share of grant redemptions | 98% | Small share; 82% of ground source redemptions were off the gas grid |
Cost and the grant
The grant is the same, so the gap is almost entirely installation. The Energy Saving Trust gives around £12,000 for an air source installation and around £15,000 for a ground source system with a trench ground loop, with a borehole system costing considerably more (its in-depth guide says up to about £57,000). On the official medians that leaves roughly £5,326 to pay for air source and £27,130 for ground source after a £7,500 grant. Both fall by 1,500 pounds where the £9,000 off-gas band applies. See air source costs and ground source costs for the breakdown, and grant amounts for every band.
Space, ground and disruption
Air source needs somewhere outside, on a wall or the ground, with room around it for airflow. The Energy Saving Trust says it suits most homes, and compact models exist for small well-insulated flats. Ground source needs a suitable area of land: for a newbuild three-bedroom house the Trust expects two trenches of 30-40 metres, with bigger loops for larger or poorer-insulated homes, and ground that is diggable, free of trees and reachable by machinery. If space is tight, a borehole of between 75 and 200 metres deep may be possible, usually at greater cost and with a specialist ground survey.
Disruption differs too. The Energy Saving Trust says installation of a standard air source system typically takes around 3 days. In the Energy Systems Catapult trial ground source installations took longer because of the ground works. Neighbours may matter for air source: the noise rule is in the noise guide, and the planning guide explains that ground source is permitted development without the Part 14 noise limits.
Efficiency and running cost
Ground temperature is steadier than air temperature through the winter, which is the main reason ground source systems tend to be more efficient. In the Catapult trial the ground source median COP at -5°C was 3.1 against 2.3 for air source, though the ground source sample was small. Higher efficiency lowers the electricity bill, but whether it repays the extra installation cost depends on the heat demand and your tariff, so ask installers to price both and show the running-cost difference. The efficiency guide shows how to translate ratios into pounds.
Which is cheaper to run, air source or ground source?
Ground source, usually, because the ground stays warmer than the air in winter and the heat pump works less hard. The government-funded Electrification of Heat trial gives one measured comparison: at minus 5°C outside, the median ground source system delivered 3.1 units of heat per unit of electricity, against 2.3 for air source (December 2024; the ground source sample was small). At the Ofgem cap for 1 October to 31 December 2026 (26.32p per kWh, GB average, Direct Debit) that is about 8.5p against 11.4p per kWh of heat on a cold day.
The catch is the upfront gap. The DESNZ medians for 2026 Q3 are £12,826 for air source and £34,630 for ground source, both including the grant value, and the ground source figure moves between quarters with system size (a median 15 kW in 2026 Q3, against 8 kW for air source). The grant is the same £7,500 for both, so the running-cost saving has to repay a much larger balance. The rules for the ground source side are on the ground source heat pump grant page.
Which should you choose?
- Choose air source if budget matters most, you have an outside wall or ground spot for a unit, and you want the quickest install.
- Consider ground source if you have a large garden or land, a high heat demand, a rural property or a new build with landscaping still to come. 84% of ground source redemptions were rural.
- Ask for both quoted if you are unsure, so the efficiency gain and the extra cost can be compared in your numbers.
Tell installers which you prefer and ask them to justify the recommendation with a heat-loss calculation. The ground source grant page covers shared loops and boreholes in more detail, and the eligibility checker tells you which grant band fits first.