Air con noise: what to expect indoors and outside

Every quote lists a decibel number. Most buyers do not know how to read them. Here are the numbers in plain English, plus what you can do at the install stage to make either quieter.

Indoor unit noise

Modern domestic split units run between 18 and 45 dB(A) depending on brand and fan speed. Rough interpretation:

• 18-22 dB on low fan: quieter than a whisper across a room. Sleep-friendly.
• 25-30 dB on medium fan: about the level of a quiet library. Barely noticeable.
• 35-45 dB on high fan: about a normal refrigerator. Audible but not disruptive.

Most units spend 90% of runtime on low or medium fan. High fan only kicks in for the first 5 to 10 minutes when the room is being brought to setpoint.

The number that matters for bedrooms

The low-fan decibel figure is what counts for a bedroom install. Marketing quotes always mention it because it is the best number the unit will ever produce.

• Under 22 dB on low fan: sleep-optimised. Mitsubishi Electric MSZ-LN, Daikin Perfera.
• 22-25 dB on low fan: fine for most bedrooms. Panasonic Etherea, Fujitsu LU.
• 25-30 dB on low fan: fine for living rooms, borderline for bedrooms.
• Over 30 dB on low fan: never buy this for a bedroom.

Outdoor unit noise

Outdoor condenser units run between 46 and 55 dB(A) at one metre. About the level of a normal conversation. What that means at the boundary depends on distance:

• At 1 metre: 46-55 dB
• At 3 metres: 40-49 dB
• At 5 metres: 36-45 dB
• At 10 metres: 30-39 dB

Doubling the distance drops noise by about 6 dB. That is why installer position of the outdoor unit matters more than the badge on the front.

The neighbour complaint problem

The biggest single cause of air con neighbour disputes is the outdoor unit sitting under a neighbour’s bedroom window. The unit does not sound loud during the day. At 2am when a compressor cycles on, it is very audible from 5 metres away.

Two prevention steps:

1. Position the outdoor unit as far from any bedroom window as the pipe run allows. Yours and the neighbour’s.
2. Ask the installer about “night mode” or low-noise outdoor units. Daikin and Mitsubishi Electric both make quieter outdoor options that add 5-10% to install cost but drop noise by 3-6 dB.

Compressor cycling patterns

Understanding when the unit gets loud:

• First 5-10 minutes after switch-on: full-load ramp-up. Loudest phase.
• Next 20-30 minutes: gradually easing back as the room reaches setpoint.
• Once at setpoint: compressor cycles on and off every 10-20 minutes for short bursts.

The cycling is what most people notice at night. Set to 22 in summer rather than 18 and the compressor cycles less because it hits setpoint faster and stays there longer.

Install decisions that reduce noise

Three specific choices at the install stage:

1. Rubber-isolated wall brackets on the outdoor unit. Not rubber pads under it - proper isolated brackets. Adds £30. Cuts wall-transmitted vibration to near zero.
2. Clean pipe runs with no kinks. Kinked copper pipes cause refrigerant gurgling that you hear inside. A tidy install is a quiet install.
3. Anti-vibration mount on the indoor unit wall plate. Adds £15. Stops the indoor head buzzing against the wall when the fan runs high.

Total spend for a much quieter install: about £50. Any installer who does not offer these is not thinking about noise.

What no installer can fix

Three noise sources built into the technology:

• Compressor start-up thump when the outdoor unit powers up. Lasts 1-2 seconds.
• Defrost cycle rumble in heat mode when outside is below 5 degrees. About 30 seconds every 45-60 minutes.
• Fan pulse from the outdoor unit while running.

None of these can be eliminated. All of them can be mitigated by position and by choosing a premium unit.

Buying advice for noise-sensitive rooms

• Bedroom: Mitsubishi Electric MSZ-LN or Daikin Perfera. Under 20 dB on low fan.
• Home office: any mid-range brand. Under 25 dB acceptable.
• Living room: standard brand fine. Under 30 dB comfortable.
• Kitchen or utility: any brand. Noise is not the priority.

Get three quotes with noise in mind

Fill in the quote form and mention “quiet” or “bedroom” in the comments if noise is the priority. Installers will note it. Three fixed quotes back within 24 hours.