How loud is air conditioning? Indoor units, outdoor units and the neighbours

How loud is the indoor unit?

Around 19 to 20 dB(A) on its quietest setting and 40 to 42 dB(A) at full fan speed, on the makers' sound pressure figures for three current 2.5 kW models:

Which end of that range you hear depends mostly on sizing. A unit big enough for the room can hold it on a low fan speed; one that is too small spends hot evenings near the top. When comparing models, ask for both figures, the lowest setting and full speed, rather than a single headline number. The bedroom guide covers sizing for a quiet night. Sources: Mitsubishi Electric; Daikin; Fujitsu.

How loud is the outdoor unit?

About 46 dB(A) at 1 metre in cooling for a 2.5 kW outdoor unit. That is the figure on both Mitsubishi's MUZ-LN25VG2 and Daikin's RXM25R9, and Daikin's rises to 47 dB(A) in heating. Those are sound pressure figures: what you would hear a metre away. The planning test starts from a different number, sound power, which describes the total sound the unit gives off wherever you stand; Daikin gives 58 dB(A) for that outdoor unit. Sound pressure falls as you move away, while sound power stays the same. Sources: Mitsubishi Electric; Daikin; IOA and CIEH.

How does the 37 dB(A) planning test work?

It is a sum the installer does before fitting, under MCS 020a, and permitted development depends on the answer (the planning guide lists the other conditions). The answer has to come out at 37 dB(A) or below at the assessment point, which is 1 metre in front of the centre of the nearest window or door to a habitable room in the neighbouring property, leaving out reflections from that wall. The steps:

1. Take the outdoor unit's sound power from its energy label or data sheet. A figure for any low-noise mode may not be used.
2. Count the hard surfaces within a metre of the unit, the ground included. Each extra surface, up to three, pushes the result up; more than three, as in a narrow lightwell, fails outright.
3. Measure the distance to the assessment point and take off the reduction for that distance.
4. Allow for a barrier: 10 dB off if a solid barrier completely hides the assessment point from the top edge of the unit, 5 dB if it is hidden but moving 25 cm would reveal it, nothing if any part of it is in view.

A council form written for the original MCS 020 sets these steps out; MCS 020a removes that form's later steps and clarifies the rules on barriers, hard surfaces and the assessment point. Sources: Grant UK; IOA and CIEH; Copeland Borough Council; MCS.

What changed when MCS 020a replaced MCS 020?

The allowance for background noise went. Under the original method the installer combined the unit's level with an assumed background of 40 dB(A) and compared the total with a 42 dB(A) limit. MCS's consultation response removed those steps, numbers 7, 8 and 9 of the old standard, and set a single overall limit of 37 dB(A) for the unit alone. It also said it would add the maker's sound power for MCS-certified units to its product directory, so the starting figure can be checked. Older articles still quote 42 dB(A); for a unit fitted now, the number is 37. Sources: MCS; Copeland Borough Council.

What makes an outdoor unit quieter?

• Distance. The Institute of Acoustics and the CIEH put siting first: as far as practicable from the windows that matter. In open air, each doubling of distance takes roughly 6 dB off.
• Fewer hard surfaces. A unit tucked into a corner bounces more sound towards a window than one against a single wall.
• A solid barrier. Imperforate, at least 10 kg per square metre and breaking the line of sight to the window, while keeping the maker's clearances.
• Anti-vibration mounts and flexible pipe connections. They stop vibration travelling into the wall and coming out as noise indoors, in your home or next door.
• A quieter model. Where the margin is tight, choose a lower sound power.
• Quiet settings, used honestly. Daikin's Silent button lowers its outdoor unit by 3 dB(A), which helps at night, but the planning sum must use the normal figure.

Enclosures are a last resort, because they can make the unit draw in its own exhaust air. A rear wall or side return, as in the drawing, is often the place to start.

Sources: IOA and CIEH; Copeland Borough Council; Daikin.

What if a neighbour complains about the noise?

Passing the planning sum does not settle everything. Noise from premises that is prejudicial to health or a nuisance is a statutory nuisance under section 79 of the Environmental Protection Act 1990, and a council must take reasonably practicable steps to investigate a complaint. GOV.UK's test is whether the noise unreasonably and substantially interferes with the use or enjoyment of a home, or injures health or is likely to. Where a council finds a statutory nuisance it must serve an abatement notice; ignoring one can lead to prosecution and daily fines, and an appeal goes to the magistrates' court within 21 days. Sources: Environmental Protection Act 1990, section 79; GOV.UK.

How does Redbridge handle a noise report?

In stages. Redbridge Council asks residents to report noise through its online form and to keep a diary of each incident. It sends an advisory letter first, and an officer attends once there have been three or more confirmed reports about the same address within seven days. Formal action can follow as an abatement notice or a fixed penalty. A resident can also go to the magistrates' court directly under section 82, after giving at least three days' written notice for noise. The council's examples are music, licensed premises, building work, alarms and animals; air conditioning is not named, but the law covers noise from any premises. Neighbours over the borough line deal with their own council. Source: Redbridge Council.

Is there a standard for noise at night?

The World Health Organization's night noise guidelines of 2009 set a target of 40 dB for night-time noise outside homes, with 55 dB as an interim target where that cannot yet be met. That is a yearly average aimed mainly at traffic and community noise, not a limit for one appliance, and it is measured differently from the planning sum. It does give the 37 dB(A) figure some context. How we choose outdoor positions on Ilford streets is covered on our homepage for air conditioning in Ilford. Sources: WHO Europe; Grant UK.