The short answer
A wall-mounted split system pairs an indoor unit fixed high on a wall with an outdoor unit outside, joined by refrigerant pipes. It is a common choice for fixed air conditioning in UK homes, and most models heat as well as cool.
On this page
The name describes the design. The compressor and the other noisy, heavy parts sit in a box outside, and the room gets only a slim unit holding a fan, a heat exchanger, filters and the electronics. Keeping the two apart is what lets a split system stay quiet indoors while still cooling a room properly on a hot day.
How it works
The indoor unit draws in room air, passes it over a cold coil and blows it back out cooler and drier. The heat it picks up travels through refrigerant pipes to the outdoor unit, which releases it outside. To heat, the process runs in reverse: the outdoor unit gathers heat from the outside air, even on a cold day, and the indoor unit releases it into the room.
Three things run from the indoor unit to the outside, usually bundled together:
- a pair of insulated copper pipes carrying refrigerant to and from the outdoor unit
- an electrical cable linking the two units
- a condensate drain that carries away the water taken out of the air
These usually leave the room through one hole in the wall a few centimetres across, either straight behind the indoor unit or at the end of a run of plastic trunking. Many small systems come pre-charged with refrigerant for a set pipe length, often between 5 and 15 metres depending on the model. Longer runs need refrigerant added on site, and every model has maximum length and height limits.
Common sizes
These are common sizes for homes. The BTU figures are rounded, as they usually are on labels.
| Cooling capacity | About (BTU per hour) | Often used for (roughly) |
|---|---|---|
| 2.0 kW | 7,000 | A small bedroom or study |
| 2.5 kW | 9,000 | Most bedrooms |
| 3.5 kW | 12,000 | A large bedroom or living room |
| 5.0 kW | 18,000 | A large living room or open-plan room |
| 7.0 kW | 24,000 | A very large open-plan space |
The last column is only a rough guide. Ceiling height, insulation, glazing, which way the room faces and the heat from people and equipment all matter, and a bigger unit isn't a better one. See what size air conditioner you need.
Where the units go
The indoor unit sits high on a wall, with the clear space around it that the manufacturer specifies. Putting it on or near an outside wall keeps the pipe run short, and it should spread air across the room rather than blow straight at a bed, sofa or desk. Where to put the indoor unit goes into the details.
The outdoor unit goes on a wall bracket or a ground stand, with room for air to flow around it. It also needs somewhere for water to drain, because it produces water when heating, especially while defrosting.
In England, a split system can usually be fitted without planning permission if it meets the permitted development conditions. These include no more than one heat pump on a house that isn't detached (two on a detached house), not using the system solely for cooling, and keeping noise from the outdoor unit to no more than 37 dB(A) at 1 metre from the nearest window or door of a habitable room in a neighbouring home. Scotland, Wales and Northern Ireland have their own rules, and the planning permission guide covers the full list.
What it does well
- Quiet in the room. With the compressor outside, quiet indoor units are often around 19–26 dB(A) on their lowest fan speed.
- Efficient heating as well as cooling. In mild weather, small air-to-air systems commonly reach a COP of 3–5, meaning 3 to 5 units of heat for each unit of electricity. Efficiency falls as it gets colder.
- No floor space used, and the windows can stay shut while it runs.
- Modest building work. There's usually one hole through the wall, and a single system is often fitted in a day.
- Plenty of choice in sizes, finishes and features.
The drawbacks
- It's on show. The indoor unit is visible, and outside there's the outdoor unit and usually some pipework.
- It needs a suitable place for the outdoor unit, within the planning rules and the noise limit. Some homes, particularly flats, have few options.
- It isn't DIY. The refrigerant circuit is connected on site, so the work needs an engineer with the right F-gas qualification, and the business doing it needs a company F-gas certificate.
- It usually needs its own electrical circuit from the consumer unit. In England and Wales, a new circuit is notifiable under Part P of the Building Regulations.
- One unit serves one room. Several rooms mean several systems, or a multi-split running them all from one outdoor unit.
- It needs regular care. Filters usually need cleaning every few weeks while it's in regular use, and manufacturers typically recommend a service, often yearly.
When another type may suit better
- No suitable high wall, such as in a loft room with sloping ceilings: a floor-standing console unit.
- A large open-plan space with room above the ceiling: a ceiling cassette.
- Almost nothing visible in the room, and space for ductwork: a ducted system.
- No way to fit an outdoor unit: a through-the-wall unit or a portable, both of which are noisier in the room and usually less efficient.
Related guides
- What the indoor and outdoor units doThe indoor unit pulls room air through a filter and across a coil that cools or warms it. The outdoor unit holds the compressor that pumps the refrigerant, plus a second coil and fan that release heat outside when cooling and collect it when heating.
- Multi-split systemsA multi-split system runs two or more indoor units, usually in different rooms, from one outdoor unit. Each room has its own controls, and your home needs only one outdoor unit rather than one for each room.
- Where to put the indoor unitA wall-mounted indoor unit usually goes high on a wall with clear space around it, where its airflow can reach the whole room without blowing straight at people. The best spot also depends on the route for the pipes and where the condensate can drain.
- Where the outdoor unit can goAn outdoor unit needs a firm fixing, free airflow, service access and somewhere for defrost water to drain. On a house in England it can usually go on a wall, the ground or a flat roof without planning permission if set conditions are met.