Side by side
| Air source | Ground source | |
|---|---|---|
| Heat source | Outside air | The ground, through a buried loop |
| Source temperature | Follows the weather | Steadier through the year |
| Efficiency in cold spells | Falls as the air gets colder | Holds up better |
| Outdoor equipment | A fan unit on the ground or a wall | Nothing visible once the ground is reinstated |
| Groundworks | A base or bracket | Trenches or boreholes |
| Installation cost | Lower | Higher, mainly the ground loop |
| Noise outside | A steady fan and compressor sound | Little or none |
| Defrosting | Needed in cold, damp weather | Not needed |
Efficiency: why the source temperature matters
A heat pump's efficiency depends on how far it has to lift heat: from the temperature of its source to the temperature of the water going to the radiators. Air is coldest on exactly the days a home needs most heat, so an air source heat pump does its hardest work when its source is least helpful. The ground a metre or more down warms and cools far more slowly than the air, so a ground source system starts its lift from a higher, steadier point in midwinter.
Over a season, that usually gives ground source the edge, but not by a fixed amount. A well-designed air source system running at a low flow temperature can outperform a poorly designed ground source one. Field data puts the scale of air source performance in context: the Electrification of Heat trial, which installed 742 heat pumps in British homes, recorded an interim median seasonal figure of 2.80 for air source units. The government's general rule of thumb for heat pumps is around 3 units of heat per unit of electricity.
Space and disruption
Air source
An air source heat pump needs a spot outside for the unit, with space around it for air to flow, and a route for pipes into the house. Inside, it needs a hot water cylinder and the controls. Fitting usually takes days, not weeks, and the garden is barely touched.
Ground source
A ground source system needs a ground loop. Horizontal trenches need a reasonable area of open ground that can be dug up and reinstated; vertical boreholes need only a small area but a drilling rig has to reach it. The heat pump unit lives indoors, so it needs a plant space as well as a cylinder. The disruption is concentrated at the start, and afterwards nothing is visible outside.
Noise
An air source unit has a fan and, usually, the compressor outside, so it makes a steady sound when it runs, louder when it is working hard on cold days. Planning rules in England require it to meet a sound standard at the nearest neighbour for it to be installed without an application. A ground source heat pump's compressor is indoors and there is no outdoor fan, so it is quieter outside; the unit indoors makes a sound similar to a large fridge or washing machine. See the noise guide.
Maintenance and lifespan
Both need an annual check of the system and the controls. An air source unit's coil and fan sit outdoors, so keeping leaves and snow clear of the unit matters. A ground loop, once buried, has no moving parts and usually needs no attention beyond the pressure checks a service covers.
Which suits which home
- Most houses and bungalows with a garden or yard: air source is the common choice, because it is simpler and cheaper to fit.
- Homes with plenty of land, or new builds where the ground is being dug anyway: ground source becomes attractive, as the trenching cost is easier to absorb.
- Homes with no outdoor space for a unit but room for a borehole: ground source with a borehole may be the only option.
- Very well insulated homes with low heat demand: the efficiency gap matters less, because the total heat needed is small.
Grants treat the two alike in most cases; the conditions are on GOV.UK. The real decider is the design for the specific home, so ask for the expected seasonal efficiency of each option in writing if you are comparing them.
Questions to put to a designer
If you are weighing the two for a particular home, the useful comparison is two designs, not two brochures. For each, ask for the heat-loss figure the design is based on, the size of heat pump it chooses, the flow temperature it plans to run at, and the seasonal efficiency it expects at that temperature. For ground source, ask how the loop has been sized, whether trenches or boreholes are proposed and what reinstating the ground involves. For air source, ask where the unit will sit, how the sound calculation came out at the nearest neighbour, and what clearance it needs for air flow and servicing.
Then set the two expected efficiencies against your electricity price. The difference between them, divided into the extra cost of the ground loop, tells you roughly how long the ground source premium takes to pay back on running costs alone. If the expected efficiencies are close, the simpler system usually makes more sense; if the ground source design is markedly better on a cold-weather home, the case for it strengthens.
Both types run on the cycle described in how heat pumps work, and their efficiency is compared using the measures in COP, SCOP and SPF. The running costs estimator shows what a difference in efficiency can mean for a year's bills.