Is an air conditioner a different thing from a heat pump, or the same thing under another name? It sounds like vocabulary. It is not. The answer decides how a system is taxed, whether it needs planning permission, and whether any government scheme can reach it.
The same machine, two names
Specified as air conditioning or as an air-to-air heat pump, the equipment is in most cases identical: an outdoor unit, one or more indoor units, a sealed refrigerant circuit joining them, and a reversing valve that lets the circuit run either way. HM Revenue & Customs says as much in its internal manual: “HMRC’s understanding is that most air conditioning units are air source heat pumps” (VAT Energy-Saving Materials and Grant-Funded Heating manual, VENSAV3080). The wording is deliberately careful — most, not all, and applied on the facts of each case — but it is the tax authority saying it, not the trade.
What separates the two names is not the hardware but how a system is designed, sized and used. A single indoor unit fitted to take the edge off one hot bedroom is comfort cooling. A multi-room system sized to heat a whole house is, in the eyes of the rules, an air-to-air heat pump. The box on the wall may be identical; the legal treatment is not.
How the machine actually works
The reason one appliance can do both jobs is that it never makes heat in the first place. It moves it. The statutory definition used by the Boiler Upgrade Scheme regulations, as amended by SI 2026/390, puts it in almost those words: an air-to-air heat pump is plant which generates heat using a thermodynamic cycle by transferring energy stored in the form of heat in the air, and uses that energy to heat air for the purposes of space heating.
Mechanically it is a closed loop: a low-boiling-point refrigerant absorbs heat as it evaporates in one exchanger, a compressor raises its pressure and temperature, and it gives that heat back — plus the energy the compressor added — as it condenses in the other. A reversing valve swaps which exchanger does which, and the machine that was cooling a room is heating it instead. HMRC’s VAT guidance describes exactly that: fixed air source heat pumps can be reversed so that they draw heat from inside a building, providing cooling in summer as well as heating in colder periods.
Because the electricity moves heat rather than creating it, the heat delivered is several times the electricity consumed. That ratio is the coefficient of performance, or COP — heat out divided by electrical energy in, at one fixed set of conditions. A plug-in electric heater is stuck at a COP of one; a heat pump running at a COP of four delivers four times as much heat.
Labels carry seasonal versions of the same idea, calculated across a whole reference season rather than at a single test point:
- SEER — seasonal energy efficiency ratio, the cooling figure.
- SCOP — seasonal coefficient of performance, the heating figure.
- EER and COP — the instantaneous cooling and heating figures, measured at a single fixed test condition rather than across a season. A published SEER is typically around double the same unit’s EER, because the machine almost never runs flat out.
There is a legal floor under all of this. Under assimilated Regulation (EU) No 206/2012, an air conditioner other than a single-duct or double-duct unit, rated under 6 kW and using a refrigerant with a global warming potential above 150, cannot be placed on the Great Britain market unless it reaches SEER 4.60 in cooling and SCOP 3.80 in heating over the average heating season; the energy-label bands in assimilated Commission Delegated Regulation (EU) No 626/2011 start their top A+++ class at SEER 8.50 and, for the average heating season, SCOP 5.10. Approved Document L — the Government’s guidance on meeting the Building Regulations in England — says an air conditioner in cooling mode should reach a seasonal energy efficiency ratio of at least 4.0; its 2026 edition raises that to 4.6 for new dwellings and keeps 4.0 for existing ones, taking effect on 24 March 2027 (24 September 2027 where the work is in connection with higher-risk building work), calculated according to BS EN 14825.
Why heat pump usually means something else in Britain
In ordinary British usage, heat pump nearly always means air-to-water. That machine runs the same refrigeration cycle but hands its heat to water rather than air: a flow and return to radiators or underfloor pipework, and usually a hot water cylinder. It is the technology that replaces a gas boiler, it is the technology the Boiler Upgrade Scheme was built around, and it is already inside MCS installer certification in a way air-to-air is not.
Air-to-air is the same principle without the water. Heat goes straight into the room air through a fan coil, which makes the installation far less invasive — no wet system, no cylinder, no lifted floors — and also means most air-to-air systems make no domestic hot water at all. That omission is why the two technologies are treated so differently by government schemes.
Their headline efficiencies are not comparable either: air-to-air SEER and SCOP come from a laboratory test regime, and whole-system performance in occupied homes is a different quantity. The DESNZ-funded Electrification of Heat Demonstration Project, led by Energy Systems Catapult, measured a median in-situ whole-system efficiency (SPFH4) of 2.78 across a performance sample of 428 air source heat pumps — a real-world figure, on a different technology, doing a job that includes hot water. A laboratory SCOP is a different measurement.
Consequence one: how it is taxed
The zero rate of VAT on installing energy-saving materials runs from 1 May 2023 to 31 March 2027, after which it reverts to the 5% reduced rate. Air source heat pumps qualify, but only on conditions. VAT Notice 708/6 states that only air source heat pumps which are permanently fixed and are not portable or moveable qualify, and HMRC’s manual requires the unit to provide space heating, water heating or both.
A fixed, reversible system that heats as well as cools is capable of qualifying; a cooling-only system is not an energy-saving material at all, and nor is a portable plug-in unit. The heating capability, not the cooling, carries the relief. Two further limits catch people out: the zero rate covers supply and installation by the same person, so equipment supplied alone is standard-rated, and it does not extend to servicing, maintenance or repair. The guide to VAT on air conditioning in the UK goes further into it.
Consequence two: whether planning permission is needed
In England, air source heat pumps on domestic premises sit in Class G of Part 14 of Schedule 2 to the Town and Country Planning (General Permitted Development) (England) Order 2015, substantially rewritten by SI 2025/560 with effect from 29 May 2025. There is no equivalent class called air conditioning: the right is drawn around the heat pump, and a condition added in May 2025 states plainly that the air source heat pump must not be solely used for the purpose of cooling.
A reversible system that genuinely heats is capable of sitting inside permitted development; a cooling-only installation is outside it and will normally need a planning application. The other conditions still apply, and several bite hard on Yorkshire housing stock. The outdoor unit must not exceed 1.5 cubic metres including its housing on a house, or 0.6 cubic metres on a block of flats. Only one unit is permitted on or within the curtilage of a house that is not detached, or of a block of flats, and two on a detached house, counting anything already there. Units may not go on a pitched roof, and must sit at least a metre in from a flat roof edge. Even outside a conservation area, a unit cannot be fixed to a highway-fronting wall above ground-floor storey level — which takes the front elevation off the table on many Victorian terraces and first-floor flats. Listed buildings have no Class G rights at all. The right is also conditional on the installation meeting the sound calculation in the MCS planning standards named in the Order; without it, the permitted-development route is not available.
Consequence three: whether any grant can apply
Air-to-air heat pumps were brought into the Boiler Upgrade Scheme on 28 April 2026, for residential properties in England and Wales. The scheme funds heating: the statutory definition of the eligible plant is confined to heating air for the purposes of space heating, and there is no government grant for cooling as such.
The conditions follow. The system being replaced must have been a fossil-fuel system — gas, oil, LPG or coal — or an electric heating system that is not already a heat pump, such as storage heaters, panel radiators or an electric boiler. The new system must fully replace it and be sized to meet 100% of the property’s space heating demand on its own, so a single indoor unit fitted for summer comfort cannot meet the test at any efficiency. Hot water may come from a non-fossil supplementary appliance such as an immersion heater or a hot water heat pump.
Two points are widely got wrong. Any grant-funded installation must be MCS certified, and MCS has not yet opened air-to-air certification to installers — the technology is still being brought into the scheme. In the other direction, the scheme rules expressly list air-to-air heat pumps among the non-fossil supplementary heating appliances permitted alongside a grant-funded heat pump, so an air-to-air system sitting beside an air-to-water one is a configuration the rules anticipate rather than penalise.
One machine, four sets of rules
| Regime | Cooling-only unit | Fixed reversible system (heats and cools) |
|---|---|---|
| VAT (energy-saving materials relief) | Not a qualifying material; standard-rated, as are portables. | Capable of qualifying for the zero rate on supply and installation together, to 31 March 2027, then 5%. |
| Permitted development (England, Class G) | Excluded by the not-solely-for-cooling condition; a planning application is normally needed. | Capable of falling inside Class G, subject to volume, siting, listed-building and sound conditions. |
| Boiler Upgrade Scheme (England and Wales) | No grant exists for cooling. | Only where designed as the home’s heating: full replacement, 100% of space heating alone, MCS-certified. |
| MCS certification | Cooling-only systems are excluded. | Air-to-air is being incorporated; certification is not yet open to installers. |
What this means when choosing a system
Approved Document L says reversible heat pump systems — those providing both cooling and heating — should be designed such that they are optimised for heating. The MCS heat pump design standard, amended to cover air-to-air, sizes those systems on the building’s heat loss calculated to BS EN 12831-1:2017 — the winter number, not the summer one. Which number governs a particular property — the winter heat loss or the summer heat gain — is settled by calculation and survey, not by a rule of thumb. Sizing is covered in the guide to what size air conditioning a room needs.
The installation question is layered too. Fitting a fixed system in a home is building work under the Building Regulations 2010, because a fixed air conditioning system counts as a controlled service or fitting, and the refrigerant circuit is separately regulated work requiring F-gas certification — a different qualification from an electrical one. The guide to who can legally install air conditioning sets out how those pieces fit together, and the explainer on the types of air conditioning covers the physical formats.
Nothing in these regimes turns on what a system is called; every one turns on whether it can heat. A machine that only cools is treated as an appliance. The identical machine, fitted so that it heats as well, is treated as a heat pump — and that is where the tax relief, the planning right and the grant scope all live.