What is the quietest air conditioning unit for a London home?

Two kinds of quiet

When people ask for the quietest air conditioning, they usually mean one of two things. Either they want to sleep next to the indoor unit, or they are worried about the outdoor unit annoying a neighbour, the managing agent or the council. These are measured differently, and the best unit for one is not always the best for the other.

In a London flat or terrace you need both to work. The bedroom unit has to whisper at 3am, and the outdoor unit has to sit a few metres from someone else’s window without causing a complaint.

Reading the indoor figure

Manufacturers publish a sound pressure level for each indoor unit at each fan speed, in dB(A). For a bedroom, the useful number is the lowest speed, often labelled quiet, silent or night mode. The full power figure is what you hear on a hot afternoon with the room heating up, which is less important.

A few points help when comparing:

• compare units of the same cooling size, because a bigger unit run slowly can be quieter than a small unit at full power
• check the figure is for the indoor unit you are buying, not a smaller model in the same range
• remember that decibels are logarithmic, so a few dB makes a real difference to how it sounds

Placement inside the room changes what you hear too. A unit above the bed head blows air past your ears. A unit on the wall facing the bed, high up, sends air across the ceiling and is easier to sleep with.

System type	Where the compressor sits	Indoor noise
Ducted split	outside, fan hidden in a ceiling void	usually the quietest in the room
Wall mounted split	outside	very quiet on low fan
Floor console split	outside	quiet, fan nearer head height
All in one (monobloc)	inside the room	noticeably louder
Portable	inside the room	loudest

Reading the outdoor figure

For the outdoor unit, look for the sound power level, not the sound pressure level. Sound power describes how much noise the unit gives off at source. It is the figure used in the MCS 020 planning standard, which permitted development relies on. That calculation works out the sound at 1 metre outside the nearest neighbour’s habitable room window, and the result must be 37 dB(A) or lower. Planning Geek’s guide to Class G sets out how it works: distance, the number of reflecting walls around the unit and any solid barrier in the line of sight all go into the result.

That is why the quietest outdoor unit on paper can still fail if it is boxed into a corner of a small lightwell, and why a slightly louder unit can pass in an open spot behind a solid wall.

Why Westminster sets a higher bar

The City of Westminster is dense, with mansion blocks, mews and offices close together, and much of it is in a conservation area. The council’s noise technical guidance asks that, in places where background noise already exceeds World Health Organization levels, new plant should be 10 dB below the minimum external background noise level, measured 1 metre from the most affected façade of the nearest noise sensitive property. If the noise has tones or is intermittent enough to attract attention, the target becomes 15 dB below.

In plain terms, the unit should be well under the quietest sound already in that street at the quietest time it runs, which is often the middle of the night. In a quiet Marylebone courtyard, that is a demanding target. It is common for an application to include an acoustic report, an enclosure or screen, and a night time setting that limits the outdoor fan speed.

Other central boroughs take a similar line. The Kensington and Chelsea Local Plan says plant such as air conditioning units with an unacceptable noise and vibration impact will be resisted. You can read more about the local position on our Westminster area page.

Ways to make any system quieter

The model is only part of the answer. On London jobs, these make the biggest difference:

• Distance: every extra metre between the outdoor unit and the nearest window helps
• Barriers: a solid wall or close boarded fence that hides the unit from the window cuts the sound
• Avoid tight corners: a unit surrounded by walls bounces sound back out
• Anti vibration mounts: rubber feet or brackets stop hum travelling through brickwork into the building
• Night settings: many outdoor units have a low noise mode that caps fan and compressor speed at night
• Right size: an oversized unit switching on and off is more noticeable than a correctly sized one running steadily

Inside, a unit sized properly for the room can run at low fan speed most of the time, which is quieter than a small unit working flat out.

What to ask before you buy

Ask the installer for the indoor unit’s lowest fan speed figure and the outdoor unit’s sound power figure, in writing. Ask where they propose to put the outdoor unit and what the calculated level is at your neighbour’s nearest window. If you are in a flat, your managing agent will probably want the same numbers. If the answer is “it’s very quiet, don’t worry”, ask again.

Our split system air conditioning page explains how these systems work. If noise from the outdoor unit is your main worry, read our guide to outdoor unit noise and neighbours.

Quiet in a heatwave

One last point. On the hottest London days, every system runs harder, and both indoor and outdoor units get louder. A system sized with some headroom for a 2022 style heatwave, when the Met Office recorded 40.2°C at St James’s Park, will stay quieter on those days than one that only just copes with a normal summer. That is worth discussing at the survey if you are in a top floor flat or a south facing room.