The short answer
Position the indoor unit high on a wall with a clear path to throw air across the room, away from obstructions and the bed’s direct draught; position the outdoor unit on a solid, level mount with free airflow, away from boundaries and bedroom windows to manage noise. The pipe run between them should be as short and direct as practical, and the outdoor siting must respect planning and noise conditions. A good installer designs both positions during the survey.
Where the two boxes go decides how well your air conditioning cools, how quiet it is, and whether it stays within the rules. Poor positioning causes uneven cooling, draughts, noise complaints and condensate problems — all avoidable at the survey stage. This guide covers the practical and regulatory factors for both the indoor and outdoor units. It is general information; your installer should design the exact positions for your property.
Positioning at a glance
- Indoor unit High on wall, clear air path
- Outdoor unit Level mount, free airflow
- Pipe run As short and direct as practical
- Noise Keep condenser from boundaries/bedrooms
- Drainage Condensate must fall away cleanly
- Rules Respect planning & noise conditions
Indoor unit: aim the air, avoid the draught
A wall-mounted indoor unit works best high on the wall, where it can throw cooled air out and across the room and let it circulate naturally as it falls. Avoid positions where furniture, curtains or a partition wall block the air path, and avoid blowing a cold draught straight onto a bed or a desk — comfort drops sharply when air hits you directly, and people often turn the system off rather than feel the chill. In bedrooms, aim the unit along the room rather than at the pillow. The indoor unit also needs a route for its condensate drain to fall away cleanly, so it cannot sit anywhere that forces water to run uphill.
For room-specific advice on getting the throw and capacity right, read air con for the bedroom and air con for a conservatory, where glazing and solar gain change the picture.
Outdoor unit: airflow, access and noise
The outdoor condenser rejects the heat removed from the room, so it must have free airflow around it — cramming it into a tight enclosure or hard against a fence makes it recirculate its own hot air and lose efficiency. It needs a solid, level mounting (wall brackets, a ground stand or a flat roof), space for an engineer to reach it for service, and a position that keeps noise away from your own and your neighbours’ bedroom windows. Vibration mounts and a sensible location do more for perceived noise than anything else, and they protect your relationship with the people next door.
Keep the pipe run short
| Factor | Why it matters |
|---|---|
| Short pipe run | Less efficiency loss, less refrigerant, lower cost |
| Indoor height | Better air throw and circulation |
| Outdoor airflow | Stops hot-air recirculation, keeps efficiency |
| Service access | Cheaper, easier maintenance later |
| Condensate fall | Prevents leaks and water damage |
The indoor and outdoor units are linked by refrigerant pipework, and longer runs cost more, can need extra refrigerant and slightly reduce efficiency. Where practical, position the two units so the pipe route is short and direct, ideally back-to-back or close to it. That is one reason a good survey looks at both ends together rather than treating the indoor and outdoor positions as separate decisions.
The rules that constrain siting
Outdoor siting is not purely a comfort choice. Permitted-development rights depend partly on where the unit sits — for example, units facing a road above the eaves and positions near boundaries can take you outside permitted development — and a noisy unit near a neighbour can attract a statutory nuisance complaint. Conservation areas and listed buildings restrict visible siting further. Always reconcile the ideal technical position with the legal one before fixing anything. Read do you need planning permission before the brackets go up, and if you rent, see air con for a rented property.
Common positioning mistakes to avoid
- Blowing the indoor unit straight at the bed or sofa, causing a cold draught.
- Boxing the outdoor unit into an unventilated cupboard or tight corner so it recirculates hot air.
- Choosing a position that forces the condensate to run uphill, guaranteeing leaks.
- Placing the condenser against a neighbour’s bedroom wall.
- Ignoring service access, making future maintenance awkward and costly.
Most of these mistakes are made to save a little time or a slightly longer pipe run on the day, and every one of them costs more later in poor cooling, noise complaints, water damage or expensive call-outs. The fix is simple: insist that positioning is decided at the survey, on paper, with the air path, the drainage fall, the outdoor airflow, the noise lines and the planning conditions all considered together. A position that is right on every count is almost always available; it just has to be chosen deliberately rather than settled for once the van is parked and the clock is running.
Design both positions at survey
Let your installer plan the indoor and outdoor positions together — for airflow, noise, drainage and the rules — before any brackets are fixed.
Frequently asked questions
Where should the indoor air con unit go?
High on a wall with a clear path to throw air across the room, not blocked by furniture and not blowing directly onto a bed or desk. It also needs a clean condensate-drain route.
Where should the outdoor unit be placed?
On a solid, level mount with free airflow around it, accessible for servicing, and positioned to keep noise away from bedroom windows and boundaries while respecting planning conditions.
Does the distance between units matter?
Yes. Longer pipe runs cost more, may need extra refrigerant and slightly reduce efficiency, so the indoor and outdoor units should be positioned for a short, direct route where practical.
Can positioning affect planning permission?
Yes. The outdoor unit’s siting affects permitted-development rights and noise nuisance, with extra limits in conservation areas and on listed buildings, so check the rules before fixing it.
Sources & further reading
- Planning Portal — siting conditions for outdoor units
- gov.uk — statutory nuisance (noise) guidance
- Mitsubishi Electric — indoor/outdoor unit siting technical guidance
- Daikin — installation siting and airflow technical data
This guide is general information, not a site-specific survey or a substitute for a quote from an F-Gas-certified installer. Installation, servicing and refrigerant handling are legally restricted to F-Gas-certified engineers.