If you have read the heat pumps explained guide, you know the basic trick: a heat pump gathers warmth from outside and concentrates it to heat your home, delivering several units of heat for each unit of electricity. The two main types, ground source and air source, do the same job but gather that warmth from different places. That one difference shapes the efficiency, the cost and how much of your garden gets dug up. This guide sets the two side by side honestly so you can see which fits your home.
The short answer. Ground source heat pumps draw warmth from the ground, which stays around 8C to 12C all year, so they run more efficiently and more steadily, especially in a cold snap, and usually edge ahead on running cost. But they cost far more to install and need land for trenches or the budget for boreholes. Air source pumps draw heat from the outside air, are much cheaper and quicker to fit, and perform well in most British winters with only a small efficiency dip on the coldest days. For most homes air source is the practical choice.
Same idea, different source
Both types use the same fridge-in-reverse mechanism covered in heat pumps explained. The difference is where they collect the heat.
An air source heat pump takes warmth from the outside air, using a unit that sits outside the house and looks much like an air conditioner working in reverse. A fan draws air over a heat exchanger, the refrigerant picks up the warmth, and the compressor does the rest.
A ground source heat pump takes warmth from the ground instead, through pipes filled with a water and antifreeze mix. These loops sit either in long horizontal trenches a metre or two down, or in vertical boreholes drilled deep into the ground. The pump draws heat from the loop, the loop slowly recovers it from the surrounding earth.
That last point is the crux of the whole comparison. The ground a metre or two down barely changes temperature between summer and winter, sitting around 8C to 12C all year. The air, by contrast, can be freezing on the very days you most need heat. A heat source that stays warm and steady is easier to extract heat from than one that drops to minus 5C.
Efficiency: ground source wins, but by how much
Because the ground is a warmer, steadier source in winter than cold air, a ground source system runs more efficiently and more consistently. The gap is widest in the depths of a cold snap, exactly when an air source unit has to work hardest against the lowest air temperatures while a ground source loop carries on much as before.
In seasonal terms, a well-installed ground source system typically reaches a SCOP of around 4 to 5, while a good air source unit sits around 3 to 4. The cost of the heat follows from that.
| System | Typical SCOP | Cost per useful kWh of heat |
|---|---|---|
| Air source heat pump | 3.0 to 4.0 | 6.5p to 8.7p |
| Ground source heat pump | 4.0 to 5.0 | 5.2p to 6.5p |
| Gas condensing boiler (for reference) | ~90% | 8.1p |
At 26.11p/kWh electricity and 7.33p/kWh gas, Ofgem cap July to September 2026. Worked examples; check your own tariff.
Air source pumps have improved a great deal and perform well across most British winters. But on the coldest days their efficiency dips while a ground source system holds steady, so over a full year ground source usually edges it on running cost. The annual saving is real but modest, a few pence per unit of heat, which matters when you weigh it against the installation gap below.
Installation cost and disruption: air source wins
This is where the picture flips, and flips hard. The running-cost edge of ground source has to be paid for up front, and the bill is large.
An air source unit is comparatively cheap and quick to install. It needs little more than a spot outside for the unit, a concrete base or wall brackets, and the connections indoors. A typical install is measured in days, which is why the great majority of UK heat pump installations are air source.
A ground source system is a much bigger undertaking. You need either enough land for long horizontal trenches, which means excavating a sizeable area of garden, or the budget for vertical boreholes drilled by a specialist rig. Either way there are substantial groundworks, the cost runs well above air source, often two to three times as much, and the mess is considerable. The buried loop itself lasts decades, but the upfront figure and the disruption are real barriers for most households.
Grants narrow the gap a little. The Boiler Upgrade Scheme offers a grant towards either type in England and Wales, but it does not close the difference in groundworks cost.
There is also the question of space for the loop itself. Horizontal trenches need a garden several times the floor area of the house, dug to a metre or two and then reinstated, which rules out small or paved plots. Boreholes need far less surface area but cost more per metre and require access for a drilling rig. Neither is an option for a typical terraced house, which is another reason air source dominates in dense housing. An air source unit, by contrast, asks only for a clear spot outside with enough airflow and a sensible distance from neighbours' windows for noise.
Which suits your home
The choice usually comes down to land, budget and how long you plan to stay.
- Air source suits most homes. Choose it where space is limited, where the budget is tighter, or where you simply want a simpler, faster job. It is good enough for the large majority of houses, and the small efficiency cost on the coldest days rarely justifies the ground source premium for an average household.
- Ground source comes into its own where you have the land for trenches or the budget for boreholes, where you want the best long-term efficiency and lowest running cost, or where you are building from scratch or deeply renovating anyway, so the groundworks ride along with work you are already doing. On a large or rural property with a long ownership horizon, the better efficiency has time to pay back.
Both reward exactly the same preparation. A well-insulated, draught-proofed home that can be kept warm with gentle, steady heat lets either pump run at a low flow temperature and a high efficiency. A leaky house drags both down.
Do the fabric first, whichever you choose
The lesson from the rest of this site applies with full force here, because the sums involved are large. Whichever pump you fit, do the cheap fabric improvements first so it has less work to do.
Loft insulation, cavity wall insulation or solid wall insulation where it applies, draught-proofing and decent glazing all cut your heat loss. Lower heat loss means the pump can supply the house at a lower flow temperature, which lifts its efficiency, which cuts the running cost, on either type. Improving the fabric can also let you fit a smaller, cheaper pump in the first place, so the saving compounds.
Get a proper heat-loss survey for your specific house before committing to either type or sizing anything. That survey is worth more than the choice of pump.
The verdict
Ground source heat pumps are the more efficient and steadier performers, and they usually win on annual running cost, but they cost far more to install and need land and groundworks. Air source pumps are cheaper, simpler and quicker to fit, and good enough for most homes, at a small efficiency cost on the coldest days.
For the average household the practical choice is air source. Ground source makes most sense with the space, the budget and a long horizon, or as part of a build or major renovation where the groundworks are happening anyway. Either way, the running cost against your current heating is the figure that matters, worked through in heat pump versus gas boiler running cost, and the whole decision sits within the broader home heating picture. Do the fabric first, get a proper survey, and let whichever pump you choose run low and slow.