How noisy is air conditioning, and where should the outdoor unit go?

Modern indoor units are quiet. On their lowest fan setting many are around the level of a whisper, which is why they’re often fitted in bedrooms. They get louder on higher fan speeds, when cooling a hot room quickly.

Outdoor units make more noise, mainly from the fan and compressor. Inverter models slow down once the room reaches temperature, so they’re loudest when they first start and quieter for most of the time they run.

Manufacturers publish two kinds of figure, and they aren’t directly comparable:

• Sound pressure, in dB(A), usually measured at 1 metre. This is closest to what you’d hear standing near the unit.
• Sound power, in dB(A), which describes the total noise the unit gives off. It’s always a higher number than sound pressure.

When comparing quotes, compare the same kind of figure. Every 10 dB increase sounds roughly twice as loud, so a few decibels makes a real difference.

• Away from bedroom windows, both yours and your neighbours’. Noise at the nearest neighbour’s window often decides whether planning permission is needed.
• Not tucked into a tight corner or narrow passage, where walls reflect and amplify sound and restrict airflow.
• With clear space around it for air to flow in and out, as set out in the manufacturer’s instructions.
• Where condensate water can drain safely, and not onto a path where it could freeze in winter.
• Where an engineer can reach it for servicing.
• Within the maximum pipe length for the system, so it isn’t too far from the indoor units.

• Anti-vibration mounts on wall brackets or ground stands stop vibration travelling into the building.
• Ground-mounted units on a solid base usually transmit less vibration indoors than wall-mounted ones.
• Many systems have a quiet or night mode that limits the outdoor unit’s speed.
• Acoustic screens can help in some positions, but they must not block airflow.

Siting also affects planning. See our planning permission guide for more.