Air conditioning is not one product. Five fixed system types are sold into UK homes and small commercial premises, and they are not interchangeable: they differ in where the equipment goes, what building work is involved, what planning allows, and whether they heat as well as cool.
Air conditioning and air-to-air heat pumps are the same machine at hardware level
At hardware level, a modern air conditioning system and an air-to-air heat pump are usually the same equipment. HMRC puts it this way: "HMRC's understanding is that most air conditioning units are air source heat pumps." What differs is how a system is designed and used. Approved Document L, the Government's guidance supporting the Building Regulations in England, says reversible heat pump systems should be designed such that they are optimised for heating. Permitted development in England for domestic air source heat pumps carries a condition that the unit is not solely used for cooling. And the VAT zero rate for energy-saving materials reaches a heat pump only where it is permanently fixed and capable of heating. A cooling-only machine is legally a different animal — a distinction drawn out in the guide to air conditioning versus air-to-air heat pumps.
How capacity is described: kW, BTU/hr and the ton of ice
Capacity is quoted two ways. One kilowatt is 3,412 BTU per hour on the International Table BTU: 2.5 kW is 8,530 BTU/hr, 3.5 kW is 11,942, 5.0 kW is 17,061 and 7.1 kW is 24,226. The familiar retail labels are not rounded metric sizes — the derivation runs the other way. 12,000 BTU/hr is definitionally one ton of refrigeration, the rate that melts a short ton of ice in a day; 9,000, 18,000 and 24,000 are three-quarters, one and a half, and two tons. The capacity bands, dimensions and installation timings in the sections that follow are market convention rather than specification: Approved Document L says a comfort cooling system should be specified from a heat gain calculation for the dwelling, using CIBSE Guide A with the manufacturer's guidance, and should not usually exceed 120% of the design cooling load. Floor-area rules of thumb are not a recognised sizing method, as the guide on what size air conditioning a room needs explains.
The single wall-mounted split
The default domestic system: one outdoor condensing unit, one indoor head high on an internal wall, joined by insulated refrigerant pipes, a control cable and a condensate drain. Capacities run from about 2.0 kW to 7.1 kW, roughly 7,000 to 24,000 BTU/hr. The market steps are 2.0 and 2.5 kW, then 3.5, 5.0 and 7.1 kW. Which step a room needs comes from a calculated heat gain, not from its floor area: two rooms of identical area can need different sizes depending on glazing, aspect and build-up. It is the least invasive to fit: one core through the external wall, clearance above the head to the manufacturer's figure, a condensate route to a gully, soakaway or pump, and its own circuit. On trade convention, a single split is usually a one-day job. It is the wrong choice when more than one room is the problem, since extra splits multiply outdoor units and planning limits how many a curtilage may hold; when there is no usable high wall; or when the outdoor unit has nowhere lawful to go.
The multi-split
One outdoor unit serving several individually controlled indoor heads: on market convention, a 4–12 kW outdoor unit feeding two to five indoor units of 2.0–5.0 kW each, with styles usually mixable. One practical reason multi-splits are used in England is planning. The Town and Country Planning (General Permitted Development) (England) Order 2015 allows no more than one air source heat pump on, or within the curtilage of, a house that is not detached or a block of flats, and no more than two on a detached house; the count is cumulative, so an existing air-to-water heat pump already uses part of the allowance. The outdoor unit is also capped by volume at 1.5 cubic metres including housing, or 0.6 on a block of flats. Those numbers are limits inside the right, not the whole of it. The Order provides that development is not permitted under Class G unless the air source heat pump complies with the MCS Planning Standards the Order names — a sound calculation assessed at neighbouring properties' habitable-room windows — and since 29 May 2026 that calculation has been the only recognised way to show the noise condition is met. Without it the permitted-development route is not available, so a unit inside every size and siting limit can still need a planning application on sound alone. The Order does not spell out how a multi-split is counted, so on a tight allowance that is a question for the local planning authority, not an assumption. It is the wrong choice when only one room genuinely overheats, or where continuity matters: one outdoor unit is a single point of failure, taking every room offline at once.
The floor-standing or low-wall console
A console sits at skirting or low-wall level in roughly the footprint a radiator occupies, and on market convention runs 2.0–5.0 kW — about 7,000 to 17,000 BTU/hr. Delivering warm air at low level suits a system that will do serious heating work, and a console can take a position a radiator already occupies, keeping the pipe drop short and the wall core low — frequently the least disruptive route in a solid-wall property. About a day to fit, though a condensate pump is needed more often than with a high-level head. It is the wrong choice where the floor and low wall are committed to furniture, or where a large open-plan space needs air thrown across it.
The ceiling cassette
A cassette sits within the ceiling showing only a flat grille, blowing in two, three or four directions. On market convention, compact units sized for a standard 600 × 600 mm suspended ceiling grid run about 2.0–5.6 kW; full-size four-way cassettes reach the low teens of kW. The constraint is the void: typically 250–350 mm, measured rather than assumed, since a plasterboard ceiling under a joisted floor rarely offers it. The unit is supported from the structure above and a condensate pump is effectively mandatory. It is predominantly a light-commercial format, and a legal duty comes with that scale: under the Energy Performance of Buildings (England and Wales) Regulations 2012, where an air conditioning system has an effective rated output of more than 12 kW the person responsible for it must ensure the accessible parts are inspected by an energy assessor at regular intervals not exceeding five years. Separate units count as one system only where one person can control the temperature of more than one of them. It is the wrong choice wherever there is no real void, which rules out most houses and almost every converted loft.
The ducted or concealed system
A concealed indoor unit hidden in a void, bulkhead or cupboard, distributing air through ducts to discreet grilles in several rooms. On market convention, slim-duct units run about 2.0–5.0 kW; larger concealed units reach about 14 kW. Visually the least intrusive and the most demanding to install: it needs a void or purpose-built bulkhead, planned duct routes, grille positions and access hatches designed in before the ceilings close. A multi-day job, best specified at build or refurbishment stage — not least because Approved Document L expects independent timing and temperature control on each zone and each terminal unit. It is the wrong choice for retrofit into a finished house with no void, and wherever room-by-room control is the point but a single-zone duct gets designed.
Portable and monobloc units, and why they sit outside the fixed-system rules
A portable is a box on castors venting hot air through a hose out of a partly open window, so it works against itself: the window that lets the hose out lets warm air back in. A portable is a plug-in appliance rather than a fixed building service, which is why it sits outside the rules that govern fixed systems: no refrigerant circuit is made on site, and the minimum seasonal efficiency standard in Approved Document L applies to fixed cooling, not to plug-in appliances.
Two rules make the split explicit. On VAT, the zero rate for energy-saving materials running from 1 May 2023 to 31 March 2027, after which these installations revert to the 5% reduced rate, reaches a heat pump only where it is permanently fixed: VAT Notice 708/6 states that only air source heat pumps that are permanently fixed and are not portable or moveable qualify, so portable and cooling-only equipment is standard-rated, as the guide to VAT on air conditioning explains. On refrigerants, GB F-gas rules are stricter for movable equipment: hydrofluorocarbons above a global warming potential of 150 are already banned in new movable air conditioning equipment, with no exceptions listed, while the equivalent ban for new fixed single splits holding under 3 kg of refrigerant bites above 750 and carries four listed exceptions.
None of which makes a portable useless: for a rented flat, or a fortnight of unusual heat, it is a reasonable stopgap, fairly compared against a fan rather than a split.
The five fixed types compared
| System type | Typical capacity | Typically suits | Heats too | Install & planning notes |
|---|---|---|---|---|
| Single wall split | 2.0–7.1 kW (7,000–24,000 BTU/hr) | One room | Usually; cooling-only versions differ in law | One core; its own circuit; about a day |
| Multi-split | Outdoor 4–12 kW, 2–5 heads of 2.0–5.0 kW | Two to five rooms | Yes; indoor styles usually mixable | 2–4 days; a single point of failure |
| Floor-standing / console | 2.0–5.0 kW (7,000–17,000 BTU/hr) | Rooms with no usable high wall | Yes | Least disruptive route in solid-wall property; condensate pump often needed |
| Ceiling cassette | 600 × 600 mm 2.0–5.6 kW; four-way into the low teens of kW | Open-plan with a genuine void; mostly light-commercial | Yes | Measured void 250–350 mm; supported from the structure |
| Ducted / concealed | Slim-duct 2.0–5.0 kW; larger concealed to about 14 kW | Whole floors and new-builds | Yes | Void or bulkhead, duct routes, grilles, access hatches; multi-day |
Matching the type to Yorkshire housing stock
- Victorian and Edwardian terraces. Where the walls are solid, the harder constraint is usually outside: permitted development bars an outdoor unit from a wall fronting a highway above ground-floor storey level, even outside a conservation area, and listed buildings have no such rights at all.
- Modern new-builds. Approved Document O covers overheating in new residential buildings in England only, expecting all practicable passive means of limiting solar gain and removing excess heat to be used first, and mechanical cooling only where the requirement cannot be met using openings; it does not reach a home that already exists and already overheats. Approved Document L says an air conditioner working in cooling mode should have a seasonal energy efficiency ratio of at least 4.0. Under the 2026 edition, which takes effect on 24 March 2027, that becomes at least 4.6 for new dwellings and stays at 4.0 for existing ones, calculated according to BS EN 14825.
- Loft conversions. Often the worst room in the house and the hardest to serve: rarely a ceiling void for a cassette, low eaves walls limiting where a head can go, and no permitted development for an outdoor unit on a pitched roof.
What decides the choice
Type follows the calculation, not preference: the heat gain the room actually has, where an outdoor unit can lawfully go, and what the consumer unit and supply can carry. A fixed system is separately a fixed building service under the Building Regulations in England, so it needs its own building control route — a different competent-person registration from an installer's electrical one under Part P, and different again from refrigerant certification, as the guide to who can legally install air conditioning sets out. Where a property already carries solar, battery storage and an EV charge point, adding cooling becomes a whole-board maximum-demand question rather than a spare-way question. What the system then costs to run is a separate calculation, set out in the guide to air conditioning running costs.