Types of Heat Pump Explained: Air, Ground and Water Source Compared

“Heat pump” gets talked about as if it’s one product, but in the UK it covers several genuinely different technologies, and they don’t all do the same job. Air-to-water is the one most people mean: it replaces a gas boiler and runs your existing radiators or underfloor heating. Air-to-air is a different system entirely, closer to reversible air conditioning than a boiler swap. Ground and water source heat pumps do the same underlying job as air-to-water, just pulling their heat from the earth or a body of water instead of the outside air. Which one applies to you also decides how much government grant you’re entitled to, and the amounts aren’t the same across all four.

The main types at a glance

Type	What it heats	Typical efficiency (SPF)	Installed cost guide	Boiler Upgrade Scheme grant
Air-to-water	Radiators, underfloor heating and hot water	2.8 to 3.5	£8,000 to £15,000	£7,500
Air-to-air	Room air directly, no hot water	3.0 to 4.0	£3,000 to £8,000	£2,500
Ground source	Radiators, underfloor heating and hot water	4.0 to 5.0	£18,000 to £35,000	£7,500
Water source	Same as ground source, site-dependent	Similar to ground source	Highly site-specific	£7,500

Grants are the Boiler Upgrade Scheme amount for England and Wales, paid directly to your MCS-certified installer rather than to you. Get matched with an installer to find out what your own property qualifies for.

Air-to-water: the one most people actually mean

An air-to-water heat pump pulls heat out of the outside air, even when it’s cold, and transfers it into a wet central heating system, the same radiators, underfloor pipework and hot water cylinder a gas boiler would run. It’s the most direct like-for-like replacement for a gas, oil or LPG boiler, which is why it’s also the version the Boiler Upgrade Scheme’s full £7,500 grant is built around. See our heat pump cost and savings guide for a fuller breakdown of running costs.

Air-to-air: not a boiler replacement

An air-to-air system doesn’t touch your radiators at all. It heats air directly and blows it into rooms through wall or ceiling-mounted units, more or less the same setup as an air conditioner, because in most cases it genuinely is one, run in reverse. That makes it a real option for cooling in summer as well as heating in winter, something none of the other types on this list offer without extra kit.

The trade-off is that it can’t provide domestic hot water, so you’d still need an immersion heater or another way of heating water separately. Because it isn’t replacing a full wet heating system, its Boiler Upgrade Scheme grant is smaller too: £2,500 rather than £7,500. It suits homes without existing radiators, like some flats or extensions, more than it suits a straightforward gas boiler swap.

Ground source: better efficiency, bigger project

Ground source heat pumps pull heat from the earth rather than the air, which stays at a more stable temperature through the year, and that shows up directly in performance: seasonal efficiency of 4.0 to 5.0 against 2.8 to 3.5 for a typical air-to-water system. The catch is what it takes to get there. Heat is collected through either a horizontal loop buried in trenches across your garden, needing roughly two to three times your home’s floor area of open ground, or one to three vertical boreholes drilled 100 metres or more down where space is tight. Either way, you’re looking at a proper groundworks project running two to four weeks, not the three to five days a typical air source install takes.

That efficiency gain is real, but it’s worth being honest about the numbers: a ground source system typically costs £9,000 to £15,000 more to install than air source, for something in the region of £100 to £200 a year in extra running-cost savings. On payback terms alone it rarely stacks up against air-to-water. It makes more sense where you already have the land, you’re building new, or the quieter, more discreet setup and higher efficiency matter to you beyond the pure financial case.

Water source: the rare one

A water source heat pump works the same way as ground source, drawing heat from a river, lake or shared ground loop instead of soil, but it only applies if you happen to have a suitable water source close enough to your property. It’s genuinely uncommon in UK residential installs for that reason, and where it is viable, environmental permitting for the water abstraction adds another step most other heat pump installs don’t need. It qualifies for the same £7,500 Boiler Upgrade Scheme grant as ground source.

Exhaust air heat pumps: built for flats, not retrofits

Exhaust air heat pumps recover heat from stale air already being extracted by a home’s ventilation system and use it to preheat incoming air or hot water. They’re mostly seen in new-build flats and well-sealed developments rather than offered as a retrofit for an existing house. Worth knowing they exist and that they can qualify for the Boiler Upgrade Scheme grant, but new-build properties generally aren’t eligible for the scheme at all, since it requires replacing an existing fossil fuel heating system, so this one’s unlikely to be relevant unless you’re in an older property with an unusual ventilation setup.

Hybrid heat pumps: not grant-eligible

A hybrid system pairs a heat pump with a gas boiler that kicks in on the coldest days, when the heat pump alone would run less efficiently. It’s a genuine product some installers offer, but it isn’t eligible for the Boiler Upgrade Scheme grant under any of the categories above, so it’s worth factoring that into the comparison if a hybrid quote comes up against a full heat pump one.

Do you need planning permission for a heat pump?

Most domestic air source heat pumps in England still qualify as permitted development, meaning no planning application, but only if the installation meets a specific noise standard. Since 28 May 2026, that standard is MCS 020 a), and it’s now the only accepted route: the older option, where an installer could rely on manufacturer noise data or a separate third-party acoustic assessment instead, was closed on that date. The noise limit itself also tightened, from 42 dB(A) down to 37.0 dB(A), measured at your nearest neighbour’s habitable room window. If your installer can’t produce an MCS 020 a) assessment showing your specific unit and its position on your property meet that limit, you’re into a full planning application rather than a straightforward install. Worth asking about before you sign, not after the unit’s on the wall. This applies to air source systems (air-to-water and air-to-air); ground and water source don’t have the same outdoor noise consideration.

Which one actually suits your home

For most UK homes replacing a gas, oil or LPG boiler, air-to-water is the default and the one worth getting quoted first. Air-to-air is worth a look if you don’t have a wet central heating system to connect to, or if summer cooling is genuinely useful to you. Ground and water source are worth the extra cost mainly if you have the land, are building new, or want the higher efficiency for its own sake rather than for a faster payback. Get matched with vetted installers who can tell you which of these actually fits your property before you commit to anything.

Frequently asked questions