The short answer
Through-the-wall air conditioners keep all their parts in one indoor unit that draws and expels outside air through two holes in an outside wall. They need no outdoor unit, but are noisier indoors and usually less efficient than a split system.
On this page
These units are fixed to the inside of an outside wall, high or low, and look a little like the indoor unit of a split system. The difference is that the compressor and both coils are inside the same casing, so outside all you see is a pair of grilles.
How they work
One hole lets outside air in to carry heat away from the hot coil, and the other lets it out again, taking the heat from the room with it. Room air passes over the cold coil and back into the room, and the two airstreams never mix.
Many models can also heat by running in reverse, gathering heat from the outside air. Some use an electric heater instead or as well, and heating output in cold weather varies a lot between models.
The holes are usually round and often around 15–20 cm across, depending on the model. Power needs vary too, so check the manual; some plug into an ordinary socket. Water taken out of the air is often dealt with inside the unit, but many models also need a drain, particularly for heating.
What fitting one involves
The refrigerant circuit is sealed at the factory, so there are no refrigerant pipes to connect on site. The main work is:
- checking the wall is suitable, since each model has limits on wall thickness and construction
- cutting the two holes accurately, avoiding lintels, pipes and cables, and finishing them properly inside and out
- fixing the mounting bracket and the outside grilles
- arranging the electrical supply and, if the model needs one, a drain
Cutting large holes through an outside wall is skilled work, and the holes are hard to undo neatly if the unit is ever removed. Any repair to the refrigerant circuit needs a qualified engineer.
Permission before you drill
- In a flat or a rented home, you'll usually need permission from the freeholder or landlord before making holes in an outside wall. See flats and leasehold homes.
- A listed building needs listed building consent, as listed buildings and conservation areas explains.
- In a flat or a conservation area, or if your home's permitted development rights have been removed, check with your local council first.
- Scotland, Wales and Northern Ireland have their own rules.
Compared with a split and a portable
| Through-the-wall | Wall-mounted split | Portable | |
|---|---|---|---|
| Outdoor unit | None, just grilles | Yes | None |
| Compressor | In the room | Outside | In the room |
| Refrigerant work on site | None | Yes | None |
| Holes in the wall | Two large | One small | None |
| Moves between rooms | No | No | Yes |
What they do well
- No outdoor unit, so nothing on a wall bracket, balcony or ground stand. That helps where an outdoor unit isn't practical or allowed.
- Fixed and tidy, with no hose through a window, so the windows close normally.
- No refrigerant connections to make on site.
- Heating as well as cooling on many models.
Where they fall short
- Noise indoors. The compressor is in the room, so they're noticeably louder than the indoor unit of a split system, especially at night.
- Noise outdoors. Sound also comes out through the grilles, which matters if a neighbour's window is close.
- Efficiency. They usually use more electricity than a split system for the same cooling or heating.
- Output. Most suit a single room.
- Cold weather. Heating performance on cold days varies widely, and some models lean on an electric heater.
- Permanent holes. The grilles are visible outside, and the holes are hard to reverse neatly.
If you can fit an outdoor unit, a wall-mounted split is quieter indoors. If you can't make holes in the wall at all, a portable air conditioner needs only a window.
Related guides
- Portable air conditionersA portable air conditioner is a self-contained unit on wheels that cools a room and sends the heat outside through a hose, usually out of a window. It needs no professional installation, but it is noisier and less effective than a fixed split system.
- Wall-mounted split systemsA 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.
- Flats and leasehold homesIn a flat or leasehold home, fitting air conditioning usually needs written consent from the freeholder under the lease, and the planning rules for flats differ from those for houses. Freeholder consent and planning are separate questions.
- Listed buildings and conservation areasFitting air conditioning to a listed building needs listed building consent, and permitted development doesn't cover it. In England, conservation areas and other protected areas still have permitted development, but with stricter rules, including on walls and roofs facing a road.