F Find Heat Pump

Air source heat pumps (ASHP)

Air source heat pumps are the dominant heat pump technology in the UK — roughly 95% of new installs. They pull heat out of outside air (even at sub-zero temperatures) and use it to heat your radiators and hot water. Here's what's good and what isn't, in plain English.

Quick facts

Typical install cost
£10,000 – £14,000 before grant
Net cost after £7,500 BUS
£2,500 – £6,500
Real-world SCOP
2.8 – 4.5 (target 3.5+)
Install time
2 – 5 days on-site
Lifespan
15 – 20 years
UK installers certified
2,212 MCS-certified (1,643 BUS-approved)

How it works

An outdoor unit — about the size of a large washing machine — contains a refrigerant that boils at around −30 °C. Outside air passes over the unit's coil and warms the refrigerant from gas state. A compressor squeezes the gas to raise its temperature, then a heat exchanger transfers that heat into your central heating water. The cooled refrigerant expands back to liquid and the cycle repeats.

For every 1 kWh of electricity the compressor uses, the system delivers 3 – 4 kWh of heat. That's the seasonal coefficient of performance (SCOP) — the number that drives running costs. See how heat pumps work for the full thermodynamic explanation.

Benefits

  • Lower carbon than gas, today. A well-designed ASHP at SCOP 3.5 emits ~80% less CO₂ than a gas boiler, and that gap widens every year as the UK grid decarbonises.
  • Eligible for the £7,500 BUS grant in England and Wales, which cuts net cost to single-digit thousands.
  • No fuel storage. Unlike oil or LPG, you don't need a tank or scheduled deliveries.
  • Lower running costs on the right tariff. Heat-pump-friendly time-of-use tariffs (Octopus Cosy, OVO Heat Pump Plus, EDF) can take effective electricity costs to 15 – 18 p/kWh, beating standard gas running costs.
  • Long lifespan. 15 – 20 years is typical, vs 10 – 15 for a gas boiler.
  • Can run in cooling mode in summer (though most UK installs don't commission cooling — see disadvantages).
  • Quiet enough for permitted development. Modern units run at around 40 – 45 dB at 1 metre, below the MCS PDR noise limit at the neighbour's window.
  • Widely available. Every major manufacturer (Daikin, Mitsubishi, Vaillant, Worcester Bosch, Samsung, LG, NIBE) has UK-rated air source units. Installer choice is broad.

Disadvantages

  • High up-front cost. Even after the £7,500 grant, an ASHP install is £2,500 – £6,500 net, vs ~£3,000 for a like-for-like gas boiler swap.
  • Needs an outdoor location. A wall with at least 1.5 m of clear space in front. Tight terraced sites or apartment blocks without external walls can struggle.
  • Some radiators may need upgrading. Most UK installs need 1 – 3 radiator changes because heat pumps run at lower flow temperatures than gas boilers and need more radiator surface area.
  • Output drops in deep cold. ASHPs work to −15 °C, but output reduces as it gets colder. Sizing must account for your local "design day" temperature.
  • Defrost cycles. In sub-zero damp weather, the outdoor coil ices up. The unit briefly reverses to clear it — visible as a few minutes of mist around the unit. Output briefly stops during this.
  • Slower to recover from cold. Heat pumps work best running steadily at low flow temperatures. They're not designed to blast a house from 12 °C to 21 °C in an hour. You'll learn to use schedules differently from gas.
  • You'll likely need a hot water cylinder. ASHPs don't work as combi boilers; they store heated water. Combi-boiler properties need to add a cylinder, taking up airing-cupboard space.
  • SCOP drops in poorly-insulated homes. The colder the house, the higher the flow temperature needs to be, and the lower the SCOP. Some retrofits need insulation upgrades first.

Best for

  • Most UK detached, semi-detached and terraced homes with an outdoor wall.
  • Properties with reasonable insulation (loft ≥ 270 mm, cavity walls filled, double glazing).
  • Owners on a 10+ year horizon who can use the BUS grant.
  • Off-gas-grid properties currently using oil or LPG — the running-cost case is strongest here.

Less suited to

  • Flats without exterior access or freeholder consent.
  • Listed buildings in conservation areas where outdoor units are restricted.
  • Very leaky uninsulated properties where insulation upgrades aren't possible first.
  • Properties needing immediate emergency heating — heat pump installs need 2 – 6 weeks of design and grant processing.

Sub-types

Air-to-water (the standard UK ASHP)

The outdoor unit heats water that flows to your radiators, underfloor heating and hot water cylinder. This is what "ASHP" means in 99% of UK contexts, and it's the type covered by the Boiler Upgrade Scheme.

Air-to-air

Essentially a heat-pump-style air conditioning system: heated air blown directly into rooms via wall-mounted indoor units. Cheaper to install (£3,000 – £6,000 per zone) but:

  • Doesn't heat hot water — you keep your existing system for that.
  • Not eligible for the BUS grant.
  • Doesn't replace your radiator system; runs alongside.

Useful in small flats or as a supplementary heating/cooling system.

Real-world considerations

Noise

Modern ASHPs run at 40 – 45 dB at 1 metre, dropping to ~30 dB at 4 metres. Quieter than most fridges. The MCS Permitted Development Rights noise limit is 42 dB measured at the nearest neighbour's window. Cheap installers occasionally site units too close to boundaries — push back if the calculation shows borderline numbers.

Planning permission

Most ASHP installs in England fall under Permitted Development as long as the unit is < 0.6 m³, > 1 m from the boundary, and meets the noise limit. Conservation areas, listed buildings, and some London boroughs (Westminster, Camden) have additional rules. Your installer should check and tell you.

Hot water performance

You'll have a stored cylinder at 45 – 48 °C with a weekly Legionella cycle to 60 °C. Two showers in succession is fine; back-to-back baths might catch you out unless the cylinder is sized for it. Combi-boiler users notice the difference.

Electrical supply

A larger ASHP (12 kW+) sometimes needs a consumer-unit upgrade or even a main fuse upgrade from your DNO (free, but takes 2 – 4 weeks). Your installer's survey should flag this.

FAQs

Will an ASHP heat my Victorian terrace?

Yes, but design carefully. Single-glazed sashes, solid walls and chimney draughts increase heat loss, so the heat pump needs to be larger and radiators may need upgrading. Don't accept a quote that ignores building age.

Can I run the ASHP alongside a wood stove?

Yes. Secondary heat sources don't affect BUS eligibility as long as the heat pump is sized to meet full demand on the design day.

Do I have to remove my gas boiler?

For BUS, the ASHP must become your primary heating system. Some installers leave the gas boiler disconnected for safety simplicity; others remove it entirely.

Compare with other types

Ground source heat pumps (GSHP) · Water source heat pumps (WSHP) · Hybrid (heat pump + boiler) · Exhaust air (EAHP)

Find an ASHP installer near you

We list every MCS-certified ASHP installer in the UK by region. Filter by BUS approval to find one who can claim the £7,500 grant on your behalf.

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