Where the outdoor unit goes: planning, noise and condensate

What the unit actually needs

• Clear airflow in and out. It draws air through the coil and discharges it, and both paths must be unobstructed. Manufacturers specify minimum clearances at the front, sides, rear and above.
• Not recirculating its own discharge. A unit in a tight corner or an enclosed space breathes its own hot exhaust, which raises operating pressures, cuts efficiency and eventually trips it out.
• Structural support. Wall brackets fixed into sound masonry, or a ground stand on a firm level base. A unit on a rendered lightweight wall or a fence panel is a problem.
• Anti-vibration mounts, which matter more for noise transmission into the building than most people expect.
• Maintenance access, all round. A unit tucked behind a shed that has to be dismantled to service it will be serviced badly.
• Condensate consideration. In heating mode the outdoor unit produces condensate and defrost water, which drains out of the base. Directly above a path, that freezes.
• Protection from the worst weather, without enclosing it. Under an overhang is fine if clearances are met; inside a box is not.

Noise, which is the real issue

A modern inverter condenser is not loud in absolute terms. What makes it a problem is that it is a continuous mechanical sound, outdoors, running for long periods, frequently in the evening when everything else is quiet.

The positions that cause disputes:

1. Facing a neighbour’s bedroom window across a narrow gap. The single most common cause of complaint.
2. In a narrow side passage between two houses, where sound reflects off both walls and is amplified rather than dispersed.
3. In an enclosed courtyard or lightwell, for the same reason.
4. On a party wall, where vibration transmits structurally into the neighbouring property – which is a different and worse problem than airborne noise.
5. Close to your own bedroom window, which people only discover in the first warm night.
6. Above a neighbour’s garden seating area.

Sound behaves predictably. It reduces with distance, it reflects off hard surfaces, and it is worse at night when background noise falls. A unit that is inaudible in the daytime can be clearly audible at two in the morning when everything else has stopped.

The mitigations, in order of effectiveness: site it somewhere else (by far the best), increase the distance, use anti-vibration mounts, fit an acoustic barrier that does not obstruct airflow, enable the unit’s night or quiet mode, and as a last resort an acoustic enclosure designed to maintain clearances.

Planning permission

Domestic air conditioning units are generally permitted development in England within limits, and there are meaningful exceptions. The permitted development rules for air source heat pumps are the relevant framework and the conditions typically concern:

• Size of the unit.
• Position – distance from boundaries, and restrictions on positions on or near a principal elevation fronting a highway.
• Height, particularly on a roof or a flat roof.
• Noise, with a limit at a defined assessment position.
• Number of units on the property.
• Listed buildings, where consent is required regardless.
• Conservation areas, article 4 directions and similar designations, where permitted development rights may be restricted or removed.

The honest advice: if your property is listed, in a conservation area, or the only sensible position is on a front elevation or close to a boundary, check with your local planning authority before committing. The conditions change and the local authority is the definitive answer for your property.

For commercial premises, planning is required more often, and local authorities commonly attach noise conditions requiring an acoustic assessment – see commercial air conditioning.

Ground or wall mounted

Ground mounting is generally preferable where there is room – easier to service, less vibration into the building, and easier to screen sensibly at a distance. Wall mounting wins where ground space is not available or where there is a security concern.

Practical checks before committing

1. Stand in the position at nightImagine a continuous low mechanical sound there. Then stand in your neighbour’s likely bedroom position and imagine the same.
2. Check the clearances physicallyWith a tape, against the manufacturer’s figures – front, sides, rear and above.
3. Check what is aboveA unit under a low overhang recirculates its discharge.
4. Check where the condensate and defrost water will landNot on a path that will ice up.
5. Check the wall or the baseStructurally adequate, and not a lightweight or rendered panel.
6. Check maintenance accessCan somebody get to all four sides?
7. Check the pipe route from thereTo the indoor unit, within the system’s length limits.
8. Talk to the neighbour if it is close to a boundaryBefore rather than after. It costs nothing and it prevents almost every dispute we hear about.