Air source vs ground source heat pump: which is right for your home?
People ask this a lot. Usually after a neighbour gets one installed or after opening an energy bill that has gone up again. Both types of heat pump do the same job in broad terms: they take warmth from the environment and use it to heat your home without burning anything. Air source pulls from the air outside. Ground source pulls from the soil below your garden. That difference in where they get their heat shapes everything else about them, including what they cost, how long they take to install, and whether your property can take one at all. For most people reading this in the West Midlands or surrounding areas, the answer is probably an air source heat pump. Not because ground source is bad. It is not. But because it has requirements that most UK properties simply cannot meet. This guide works through those differences properly so you can see which side of the line you fall on. What each system actually does Air source heat pump A fan unit sits outside the building, fixed to a wall or placed on a pad beside the house. It pulls in outside air and passes it across metal fins. Even cold air carries thermal energy. The system extracts that energy, concentrates it using a refrigerant and compressor, and moves it into your home as heat. Inside, it connects to your radiators and a hot water cylinder the same way a boiler would. Day to day, you use it the same way. Set a temperature, the house heats up. Our full guide on how air source heat pumps work covers each stage of the process in detail. Ground source heat pump Instead of the air, a GSHP draws on heat stored underground. A network of pipes, filled with a water and antifreeze mix, gets laid in the garden or drilled vertically into the earth. Below the frost line, ground temperature holds steady at roughly 10 to 13 degrees Celsius all year. That stability is the system’s main advantage over air source. The indoor unit is compact, something like a large fridge freezer, and fits in a utility room. Getting the pipes in the ground is where the time, disruption, and cost all accumulate. The numbers side by side Figures based on UK averages. Your costs depend on property size, insulation level, and whether a borehole is needed. Air source (ASHP) Ground source (GSHP) Installation cost £8,000 to £15,000 £20,000 to £45,000 After £7,500 BUS grant £500 to £7,500 £12,500 to £37,500 Typical COP 2.5 to 4.0 3.0 to 4.5 Space needed Small outdoor area Large garden or borehole Installation time 2 to 3 days 1 to 2 weeks Planning permission Usually not needed Sometimes needed Suitable for flats/terraces Yes Rarely Cold weather efficiency Dips slightly below -10C Stable year-round Lifespan 20 to 25 years 25 years or more Maintenance Annual service Annual service Efficiency: how big is the gap really? Ground source is more efficient. That is not in dispute. The question is how much it matters in practice. Underground temperatures do not change with the seasons. Air temperatures do. On the coldest days of a UK winter, when your heating is working hardest, an air source heat pump is pulling from colder air and has to work harder to produce the same amount of heat. A ground source pump faces no such variation. The numbers from Which? put a typical ASHP at 280 to 380 percent efficiency and a GSHP at 320 to 420 percent. So yes, ground source is better. The gap in real annual bills is usually somewhere between 10 and 20 percent. What softens this considerably is the UK climate. England does not get the kind of sustained deep cold that would push an ASHP into genuinely poor efficiency territory. A few weeks a year, perhaps. The rest of the heating season, the gap between the two systems is fairly small. At -10C, a decent ASHP still produces around 2.5 units of heat per unit of electricity. A gas boiler converts about 90 percent of its fuel at best. The heat pump is still well ahead of the boiler even at its winter low. Cost: where most people make up their mind Air source installations in the UK run from roughly £8,000 to £15,000 before any grant. Ground source starts at around £20,000 and can reach £45,000 or more when a borehole is needed rather than a horizontal ground loop. Both qualify for the £7,500 Boiler Upgrade Scheme grant. After that grant, a smaller ASHP installation can come in below £1,000. A GSHP after the same grant still costs £12,500 at minimum, and often a great deal more. That is a substantial gap. And it does not close quickly through lower running costs. The energy saving from choosing ground source over air source might be a few hundred pounds a year. At that rate, the additional upfront cost takes a very long time to recover. The Energy Saving Trust confirms both pump types receive the same £7,500 grant. The relative affordability of air source only widens once that is factored in. Space: the factor that rules out ground source for most UK homes This is probably the most underappreciated part of the decision. A horizontal ground loop needs roughly 1.5 to 2 times the floor area of the property in clear, unobstructed garden. For a standard three-bedroom semi-detached, that is 200 to 300 square metres. No trees over the pipes. No outbuildings. No services running through. Most UK gardens, particularly in towns and cities, cannot offer that. Boreholes are the alternative when space is tight. The pipes go down vertically rather than spreading horizontally. Problem is, each metre of drilling costs £1,000 to £1,500, heavy equipment has to get to the site, and the process takes days. Boreholes make ground source more accessible but push the cost up further. An air source unit needs a clear metre around it for airflow and a solid wall