The short answer
Quiet indoor units are often around 19–26 dB(A) on their lowest fan speed and louder at higher speeds, while outdoor units are typically 45–50 dB(A) at 1 metre. How loud it seems also depends on where the units are and how hard they're working.
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Noise is one of the most common worries about air conditioning, and one of the easier ones to check before you buy, because manufacturers publish figures for both units. The numbers need a little care: they come in two kinds, they change with fan speed, and a unit on a specification sheet isn't quite the same as a unit on your wall.
Indoor units
A wall-mounted indoor unit is quietest on its lowest fan speed, where quiet models are often around 19–26 dB(A). At higher fan speeds it's noticeably louder, often 40 dB(A) or more at the top speed. The compressor sits in the outdoor unit, so most of what you hear indoors is air moving through the unit.
How loud it is from day to day depends on how hard it's working. On auto, the fan speeds up when the room is far from the temperature you've set and slows as it gets close. A unit that's too small for the room spends more time on high speed. Most units also have a quiet or night setting that keeps the fan low at the cost of some output, covered in sleep and quiet modes.
You may also hear quieter sounds that are normal: a soft trickle or hiss as refrigerant flows, clicks as plastic parts expand and contract with temperature, and the louvre moving. Noises: what's normal and what isn't goes through the ones that need attention.
Outdoor units
The outdoor unit holds the compressor and a larger fan, and it's the louder of the two: typically 45–50 dB(A) sound pressure at 1 metre. It also gets louder when it's working hard, such as on a very hot afternoon or a cold winter morning. Many have a quiet setting that limits the fan and compressor, again at the cost of some output.
In heating mode, the outdoor unit defrosts from time to time in cold, damp weather. You may hear a whoosh as the system switches over and see steam rising from the unit. Both are normal.
Where the unit sits makes a real difference to how loud it is at a window:
- Hard surfaces reflect sound. A unit on the ground against a wall is louder than one standing in the open, and one tucked into a corner is louder still.
- Distance helps. Sound drops away as it spreads, so a few extra metres from a window matters.
- Solid barriers help. A solid wall, fence or purpose-made screen that blocks the line of sight between the unit and a window reduces the noise that reaches it.
- Vibration travels. Anti-vibration mounts help stop a hum being carried into walls and floors.
Reading noise figures
Noise figures come in two kinds, and it's easy to compare the wrong ones.
| Figure | What it describes | Where you'll see it |
|---|---|---|
| Sound pressure, dB(A) | The level at a set distance | Brochures, specification sheets |
| Sound power, dB(A) | The total noise the unit gives off | Energy labels, noise calculations |
Sound power figures are higher than sound pressure figures for the same unit, so check which kind you're looking at before comparing two units. Check too that the figures are for the same fan speed and the same mode, since cooling and heating figures can differ.
The scale isn't linear. A difference of 2 or 3 dB is barely noticeable, while 10 dB sounds roughly twice as loud. Decibels explained shows how the scale works. Published figures are measured in controlled test conditions, and a room with hard floors and bare walls can sound a little louder than a furnished one.
Noise and your neighbours
In England, fitting an outdoor unit under permitted development means meeting the MCS 020(a) noise standard: no more than 37 dB(A) at 1 metre from the nearest window or door of a habitable room in a neighbouring home. The calculation uses the unit's sound power level, the hard surfaces close to it, the distance to the window and any barrier. Scotland, Wales and Northern Ireland have their own rules. Noise rules and your neighbours explains how the calculation works.
Keeping noise down
- Choose a unit that's the right size for the room, so it isn't running flat out.
- For a bedroom, compare the figures for the lowest fan speed.
- Place the indoor unit so the airflow isn't aimed at your bed or sofa. If you're a light sleeper, a side wall keeps the fan further from your head.
- Keep the outdoor unit away from bedroom windows, yours and your neighbours', and out of corners where possible.
- Use a quiet or night setting when you don't need full output.
- Keep the filters clean and the outdoor unit clear of leaves and clutter, so the system runs as designed.
Related guides
- Decibels explainedA decibel (dB) is a unit on a logarithmic scale, so adding 10 dB means ten times the sound energy and sounds roughly twice as loud. Air conditioner figures are usually in dB(A), which reflects how sensitive hearing is to different pitches.
- Noise rules and your neighboursIn England, a heat pump fitted without planning permission must be no louder than 37 dB(A) at 1 metre from a neighbour's nearest habitable-room window or door, worked out with the MCS 020(a) calculation. Councils can also investigate noise complaints separately.
- Sleep and quiet modesSleep mode usually shifts your setting by a degree or two over the first hours of the night, warmer when cooling and cooler when heating, and turns the fan down. Quiet mode slows the fan to cut noise, at the cost of some output.
- Where the outdoor unit can goAn outdoor unit needs a firm fixing, free airflow, service access and somewhere for defrost water to drain. On a house in England it can usually go on a wall, the ground or a flat roof without planning permission if set conditions are met.