— reviewed against the regs as they stand.
Quick answer: an air source heat pump (ASHP) heats a home by extracting heat from outside air — working even below freezing — and upgrading it with a compressor. A modern unit is rated at 3.5–4.5 units of heat per unit of electricity in the lab and returns a median of about 2.8 in monitored UK homes, against roughly 0.9 for a gas boiler’s best. Grant of £7,500 (£9,000 off-grid from oil or LPG), no planning permission on most houses since May 2025, and the default heating in new-build SAP and under the Future Homes Standard.
What ASHP means
Air source heat pump. It is one of two families of heat pump you will meet — the other, the ground source heat pump or GSHP, takes its heat from the ground through buried pipes instead. An ASHP takes it from the air, which is why it is a box with a fan on an outside wall rather than a trench in the garden, and why it costs a fraction of a GSHP to install.
There are two types. Air-to-water heat pumps — the great majority of UK installations — heat the water in a wet central heating system: radiators or underfloor heating, plus a hot water cylinder. Air-to-air heat pumps blow warm air straight into the room through indoor units and generally do not provide hot water; they are what a reversible air-conditioning unit is. Everything on this page about SAP, grants and heating design is about air-to-water unless it says otherwise.
How it works
It runs a fridge in reverse, in four steps. A refrigerant at a very low temperature passes through the outdoor unit and absorbs heat from the air, evaporating as it does. A compressor squeezes the gas, and compressing a gas raises its temperature sharply. The hot gas then gives its heat up to the water in your heating circuit and condenses back to a liquid. Finally it passes through an expansion valve, drops in pressure and temperature, and goes round again.
The trick is that the pump moves heat rather than making it. The electricity runs the compressor and the fan; the heat itself came free from the air. That is why one unit of electricity can deliver three or more units of heat, and why the physics still works when the air is below freezing — there is heat in air at any temperature above about minus 273°C, and modern units run down to about minus 25°C. The coldest UK design temperatures are only around minus 2 to minus 5°C.
Efficiency — the three numbers, and the one that matters
You will see three ratios, and they are not the same thing.
COP — coefficient of performance — is an instantaneous figure at one set of test conditions. SCOP — seasonal COP — is a laboratory figure representative of a heating season, calculated to BS EN 14825 and quoted on the product label; current units are typically in the 3.5–4.5 range, higher at low flow temperatures. SPF — seasonal performance factor — is what the system actually did in a real house over a real year, and it is the figure that decides your bills.
The largest UK field trial, the Electrification of Heat project, monitored 742 heat pumps for nearly two years. The median SPF for air source was 2.80, with the middle half of installations between 2.53 and 3.09, falling to about 2.44 on the coldest days. That gap between a label of 4 and a home at 2.8 is mostly design and installation: flow temperature, emitter sizing, controls and commissioning. In a well-insulated, airtight new build with underfloor heating or correctly sized radiators, the real figure sits at the top of that range.
Running costs against gas
Whether an ASHP is cheaper to run than a gas boiler comes down to one ratio: the price of a unit of electricity divided by the price of a unit of gas, set against the heat pump’s SPF and the boiler’s efficiency.
For July to September 2026 the Ofgem price cap puts electricity at 26.11p per kWh and gas at 7.33p — a ratio of 3.56. Against a condensing boiler at 85% efficiency, a heat pump needs an SPF of about 3.0 to break even. From October the cap moves to 26.32p and 7.97p, a ratio of 3.30, and break-even drops to about 2.8 — right on the monitored median. On a flat standard tariff, then, a typical installation is roughly a draw with gas and a good one is ahead. A heat pump time-of-use tariff, with cheap overnight and off-peak periods, is what moves the sums decisively — and a new build designed around the pump from the start beats the median comfortably.
The grant
The Boiler Upgrade Scheme in England and Wales pays £7,500 towards an air-to-water heat pump, and since 21 July 2026 £9,000 for homes off the gas grid that are replacing oil or LPG heating — that uplift runs to 31 March 2027. Air-to-air heat pumps qualify for £2,500. The scheme has been extended to 2030, the requirement to have an EPC was removed in 2026, and the earlier insulation conditions went in 2024. The installer must be MCS certified and commission within 120 days of the application. One grant per property.
In Scotland, Home Energy Scotland offers a £7,500 grant plus a £7,500 interest-free loan, with an additional £1,500 on each for rural and island homes — up to £18,000 in all — for a heat pump that provides all the home’s heating and hot water.
The grant is for existing homes. In a new build there is no grant, and the heat pump is simply what the SAP expects — see below.
Planning and noise
For most houses in England an ASHP is permitted development — no planning application — and the rules were loosened on 29 May 2025. The old requirement to keep the unit at least one metre from the boundary has been removed. The size limit for the outdoor unit rose from 0.6 to 1.5 cubic metres for a house (0.6 for a block of flats). A detached house may have two units; any other house or a block of flats, one. The unit may not be used solely for cooling.
What stayed: the unit must comply with the MCS planning standard, MCS 020, and permitted development still does not cover listed buildings, pitched roofs, flat roofs within a metre of the edge, or — in conservation areas and World Heritage Sites — walls or roofs facing a highway.
MCS 020 is where noise comes in. Since September 2025 the test is a calculated sound level of no more than 37 dB(A) one metre from the centre of a neighbour’s nearest habitable-room window, worked out from the unit’s sound power, the distance and any barrier between them. (You may see 42 dB quoted; that was the old way of expressing the same limit and is superseded.) The unit itself typically runs at 40–60 dB up close; distance and siting do the rest, and on most plots a modern unit passes comfortably.
Designing it properly
The difference between a heat pump that returns 2.5 and one that returns 3.5 is nearly all design. The MCS design standard requires a room-by-room heat loss calculation to BS EN 12831-1 — not a rule of thumb from the boiler that was there before — at design temperatures for your location, and the pump has to meet 100% of that heat load without help from its backup electric heater.
Flow temperature is the lever: the standard steers designers to 55°C or below, and the lower the better for efficiency, which is why radiators are often upsized and why underfloor heating suits heat pumps so well. A hot water cylinder is needed — the standard sizes it at 45 litres per person, counting bedrooms plus one — and the pump must be able to reach 55°C to heat it, with a legionella regime built in. Cylinders need a cupboard; plan for it.
In the SAP calculation
This is where an ASHP earns its place in a new build. SAP 10’s carbon factor for electricity is 0.136 kgCO₂ per kWh against 0.210 for mains gas, and its primary energy factors are 1.50 and 1.13. Divide either by an SPF of 3 and a heat pump beats a gas boiler on both carbon and primary energy by a wide margin — often enough to carry the SAP pass on its own, and the usual answer to a planning renewables condition too.
The product has to be in the PCDB, the Product Characteristics Database that SAP draws its performance data from; a unit that is not listed gets conservative defaults and a worse result than it deserves, and an MCS certificate does not guarantee a PCDB entry. Tell us the make and model.
Under the Future Homes Standard, in force from 24 March 2027, the notional dwelling that new homes are measured against is heated by a heat pump — so the heat pump stops being the ambitious option and becomes the baseline. Gas boilers are not banned by name, but the government expects them to be effectively unviable in new homes.
ASHP or GSHP?
A ground source heat pump runs a little more efficiently in deep winter, because the ground stays warmer than the air, and it makes no noise outside. It also costs several times as much to install and needs the land for a ground loop or the budget for a borehole. For most new builds the ASHP is the pragmatic choice — and the one the SAP will have assumed.
Common questions
What does ASHP stand for?
Air source heat pump. It is a heating appliance that sits outside the building and extracts heat from the outdoor air, concentrates it with a refrigerant compressor, and delivers it indoors — to radiators, underfloor heating and a hot water cylinder in the air-to-water type, or straight into the room as warm air in the air-to-air type. The abbreviation turns up on drawings, SAP reports and planning documents; GSHP is the ground-source equivalent.
How does an air source heat pump work?
It runs a fridge in reverse. A refrigerant at very low temperature passes through the outdoor unit and absorbs heat from the air, evaporating as it does so. A compressor squeezes the gas, which raises its temperature sharply. That hot gas gives up its heat to the water in your heating system and condenses back to a liquid, then passes through an expansion valve, drops back to a low temperature and pressure, and goes round again. Because it moves heat rather than making it, it delivers more heat energy than the electricity it uses.
Is an ASHP cheaper to run than a gas boiler?
On the standard price cap it is roughly a draw. For July to September 2026 electricity is 26.11p per kWh and gas 7.33p — a ratio of 3.56 — so against a gas boiler running at 85% efficiency a heat pump needs a seasonal performance factor of about 3.0 to break even, and the median measured in UK homes is 2.8. From October the ratio narrows to 3.3 and break-even drops to about 2.8. A heat pump electricity tariff with cheap off-peak periods is what moves it clearly ahead; a well-designed system in a well-insulated new build beats the median.
What is the grant for an air source heat pump in 2026?
The Boiler Upgrade Scheme in England and Wales pays £7,500 towards an air-to-water heat pump, rising to £9,000 for homes off the gas grid that are replacing oil or LPG heating — that uplift runs until 31 March 2027. Air-to-air heat pumps get £2,500. The scheme now runs to 2030, no longer needs an EPC, and the installer must be MCS certified. In Scotland, Home Energy Scotland offers a £7,500 grant plus a £7,500 interest-free loan, with an extra £1,500 on each for rural and island homes.
Do I need planning permission for an ASHP?
Usually not, in England. Since 29 May 2025 the rule that kept the unit a metre from your boundary has gone, the size limit has risen to 1.5 cubic metres for a house, and a detached house may have two units. The unit must comply with the MCS planning standard, which sets a noise limit of 37 dB(A) at a neighbour's nearest habitable-room window, and it must not be used solely for cooling. Permitted development still does not cover listed buildings, pitched roofs, or highway-facing walls in conservation areas.
How noisy is an air source heat pump?
The unit itself typically produces between 40 and 60 decibels when running — about the level of a quiet conversation to a normal one. The planning test is different: MCS 020 requires the calculated sound level one metre from a neighbour's nearest habitable-room window to be no more than 37 dB(A), taking distance and any barrier into account. Siting the unit sensibly is what passes it; most modern units on a typical plot do.
Do air source heat pumps work in winter?
Yes. Modern units operate down to about minus 25°C, and the coldest UK design temperatures are only around minus 2 to minus 5°C. Efficiency does drop as it gets colder — the monitored median on the very coldest days is about 2.4 rather than 2.8 — but that is still more than twice the heat per unit of electricity that a direct electric heater gives. What matters is that the system was sized to the house's actual heat loss and the radiators to the flow temperature, so the pump does not lean on its backup heater.
What SCOP should an ASHP have?
Manufacturers quote a seasonal coefficient of performance from a laboratory test, and current units are typically in the range of 3.5 to 4.5 depending on flow temperature. Monitored UK installations return a lower real-world figure, called the seasonal performance factor: a median of 2.8, with the best quarter above 3.1. The gap is mostly design and installation, not the box — low flow temperatures, correctly sized emitters and good controls close it.