The wall-mounted split is the most common air conditioner in UK homes and small offices. Where it goes, how it is fitted and how it is set up decide how well it performs for the next decade.
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A wall-mounted air conditioning unit is the indoor half of a split system. It's a slim cabinet fixed high on a wall, connected by refrigerant pipework to an outdoor unit outside. Most people in the UK picture exactly this when they think of home air conditioning, and with good reason. It is compact, quiet and efficient, and comparatively quick to install. Because it is a reversible heat pump, it heats as well as cools.
Not every wall is a good wall, though. Put a unit in the wrong spot and it can blow cold air straight onto a bed or struggle to read the room temperature. You might also end up with a long run of visible trunking. This article covers what a wall-mounted unit is and where it should go, then walks through a professional installation and what makes the system cheap to run.
In the UK, "wall-mounted air conditioning" almost always means a high-wall split system. The indoor unit contains a heat exchanger coil, a quiet cross-flow fan, a filter and the electronics that read room temperature. The outdoor unit holds the compressor and a second coil. Between them run two insulated copper pipes carrying refrigerant (typically R32 on modern systems), along with a power and communication cable and a condensate drain.
It isn't the same as the through-the-wall or window units common in other countries. Those pack everything into one box and are rarely fitted in British homes. Nor is it a portable air conditioner, which stands on the floor and vents hot air through a hose out of a window. Compared with both, a properly installed split is far quieter indoors and considerably more efficient.
Wall-mounted units come in two configurations:
We fit more wall-mounted splits than any other type of indoor unit. The reasons are practical, not fashionable.
Ideally the unit goes high on a wall with clear space around it. From there, air should be able to travel across the room without hitting anyone directly. It also helps to be as close as practical to the route the pipework will take outside. Real rooms nearly always involve some compromise, so these are the rules we weigh up on survey.
Indoor units are mounted high, typically with a gap of at least 150mm or so below the ceiling (the exact figure depends on the model) so air can be drawn in through the top grille. Side clearances matter as well. Jam a unit into a corner or under a bulkhead and it will recirculate its own air and short-cycle.
Cool air sinks and warm air rises, so a unit high on a wall naturally throws conditioned air across the room. Where we can, we aim the airflow along the length of the room and away from where people sit or sleep. In a bedroom that usually means not mounting the unit directly above the pillow end of the bed, or facing it. Above a door often works well. So does the wall opposite a desk, as opposed to the one behind it.
An external wall gives the shortest, neatest pipe run. The pipework can exit through a single core hole (around 65mm) directly behind the unit and stay completely hidden from inside. An internal wall is possible, but you'll have a visible pipe route finished in trunking. It might run along a wall, through a cupboard, up into a loft or down through a floor void.
The outdoor unit can be fixed to the wall on brackets or placed on a ground stand. Ideally it sits close to the indoor unit with unobstructed airflow front and back. It should be reachable for servicing without scaffolding, and it shouldn't be somewhere it will annoy anyone. Anti-vibration mounts are standard on our installs. We also steer clear of tight corners that reflect sound, and keep units away from neighbours' bedroom windows where we can.
In flats, terraces and period properties, the outdoor unit often decides the whole design. Planning permission, listed building consent or conservation area rules can apply to external units, and leaseholders usually need written consent from the freeholder. Where needed, we check with the local planning authority before installation.
Two engineers usually install a single wall-mounted split in one day. The sequence goes like this.
Dust sheets go down, the mounting plate is fixed level in the agreed position, and the pipe exit point is marked behind it.
A core hole is drilled through the wall with a slight fall to the outside, and the pipework, cable and drain are prepared and insulated.
The indoor unit hangs on its plate, the outdoor unit goes on brackets or a stand, and the pipework is flared or brazed to both.
A nitrogen pressure test and vacuum prove the circuit, refrigerant is released, and the system is run in cooling and heating.
The system needs its own electrical supply from your consumer unit, with suitable protection and an isolator near the outdoor unit. Our air conditioning installation team wires this to current regulations. The condensate drain has to fall continuously to a suitable outlet, such as an external gully or a waste pipe. On an internal wall, gravity drainage is sometimes impossible, so we conceal a small condensate pump in trunking or a void.
On commissioning we measure the air temperature leaving the unit and pour water through the tray to confirm the drain clears properly. We check both units for vibration and noise, then set up the controller or app. The checklist is the same on every UK Aircon job, from a box room in Gateshead to a big open-plan lounge. Before we go, we'll show you how to remove and clean the filters. That's the one regular job that falls to you.
Very quiet on low fan speed. Manufacturers publish sound pressure levels in dB(A) for each fan speed. The quietest domestic indoor units are rated in the low 20s dB(A) on their lowest setting, which is comparable to a quiet room at night. Most also have a night or sleep mode that limits fan speed and nudges the set temperature overnight.
Standard indoor units are white and compact. Several manufacturers offer premium designs in matt finishes, silver, black or textured fronts that sit more comfortably in a modern living room. Pipework that exits directly behind the unit keeps the wall clean.
Every unit comes with a handheld remote. Many can also take a wired wall controller or connect to Wi-Fi for app control and scheduling, so you can switch the system on before you get home.
Three things work together to make modern wall-mounted splits efficient.
Inverter compressors. The compressor doesn't just switch fully on and off. It varies its speed to match the load, and once the room reaches temperature it ticks over at low output. You use less energy and the temperature stays steadier.
Heat pump heating. In heating mode, a split moves heat from the outside air into the room rather than generating it. That's why its heating efficiency, shown as SCOP on the energy label, is several times higher than a plug-in electric heater's.
Zoning. You only condition the room you are using. A bedroom unit running for a few hours in the evening uses a fraction of the energy it would take to cool a whole house. For the maths behind running costs, see how much air conditioning costs to run.
Sizing is the biggest efficiency factor you control. A unit chosen for the actual heat load of the room runs smoothly, while one that is too big or too small wastes energy. Our guide on what size air conditioning unit you need goes through the factors.
| Indoor unit | Best suited to | Needs | Visibility |
|---|---|---|---|
| Wall-mounted | Bedrooms, studies, living rooms, small offices | Clear wall space high up | Visible on the wall |
| Ceiling cassette | Open-plan spaces, offices, shops | Ceiling void | Flush grille in ceiling |
| Floor console | Loft rooms with sloping ceilings, under windows | Low wall space | Visible at floor level |
| Ducted | Whole floors, high-end refurbishments | Ceiling void and access for filters | Only grilles visible |
If a wall unit isn't the right fit, the ceiling air conditioning unit article covers cassette, ducted and suspended options.
High on the wall, usually with at least around 150mm clearance below the ceiling so the unit can draw air in through its top grille. The manufacturer sets the exact minimum clearances for each model, and they should always be followed.
Yes. The pipework then runs along walls in trunking, or through a loft, floor void or cupboard, to reach the outside. If the condensate cannot drain by gravity, a small condensate pump is fitted. An external wall gives the neatest result.
A single wall-mounted split system is usually installed and commissioned in one day. Longer pipe runs, difficult access or multi-split systems can take longer.
Yes. Modern wall-mounted splits are reversible air-to-air heat pumps, so they provide efficient heating as well as cooling. Their heating efficiency is shown as SCOP on the energy label.
We recommend an annual service for domestic systems, with more frequent visits for heavy commercial use. Clean the filters regularly, every few weeks when the system is working hard.
A wall-mounted split is only as good as its placement and installation. UK Aircon's REFCOM-registered engineers survey the room, the pipe route and the outdoor position before recommending a size and model. Every installation carries our 5-year warranty on parts and labour. Find out more about our domestic air conditioning installation service, with installs from £1,800, or ask for a free quote.
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