Acoustic assessment for ASHP

“Acoustic assessment” can describe three different pieces of work for an air source heat pump:

1. the MCS 020 a) desktop calculation used for permitted development in England
2. a site-specific noise assessment submitted with a planning application
3. investigation of sound or vibration after the heat pump is running

They answer different questions. A passed MCS calculation is not a substitute for a planning report where an application is required, and neither automatically diagnoses an installed rattle, hum or vibration problem.

Permitted development: the MCS calculation

As at 22 July 2026, a new air source heat pump relying on England’s Class G permitted development right must comply with MCS 020 a) Issue 1.1 as well as all the other planning limits.

This is a prediction, not a sound survey. It uses the proposed unit’s sound power level, distance, nearby reflecting surfaces and a prescribed barrier allowance to predict the level one metre outside neighbouring habitable-room doors and windows.

Issue 1.1 requires every assessment position to be at or below 37 dB LAeq,5mins, ignoring the neighbouring facade effect. The detailed method, formula and barrier table are covered in the MCS 020 entry.

The installer should complete the calculation for the actual model and final location before installation, keep one copy and give one to the customer. If the model, position, barrier or surroundings change, the evidence has to be checked again.

MCS 020 a) deliberately simplifies the acoustic problem. It does not measure local background sound and does not apply corrections for tonality, impulsivity or intermittency. In particular, the standard does not add the 5 dB tonal correction claimed in some guidance.

Planning application: a site-specific report

If the proposal cannot use permitted development, the local planning authority decides what evidence the application needs. Contact it before commissioning measurements or modelling because local validation requirements and planning policies vary.

An authority may request an assessment using BS 4142:2014+A1:2019, which BSI currently lists as the current edition. BS 4142 rates and assesses industrial and commercial sound using outdoor levels and considers the context in which it is heard. Unlike the MCS calculation, it can account for background sound and audible characteristics such as tone or impulsivity.

A planning report may therefore need:

• a site plan, proposed unit location and all relevant noise-sensitive receptors
• the unit’s operating modes and sound power data at suitable design conditions
• representative background sound measurements, with dates, times, weather and equipment details
• predicted levels at each receptor and the propagation assumptions used
• corrections for audible acoustic character where applicable
• uncertainty, context and the assessor’s reasoned conclusion
• any enclosure, barrier, mounting or control mitigation included in the design
• an installation or commissioning check if the authority requires one

BS 4142 is a method, not a universal planning limit. A local policy might seek a rating level at or below background, below background by a stated margin or another outcome justified for the site. The authority’s written requirement controls; copying a threshold from another council is unreliable.

Because this work involves measurement selection, operating scenarios, acoustic character and professional judgement, a competent acoustic consultant is normally appropriate. Ask the planning authority to agree the scope before paying for the survey.

Designing out noise before installation

MCS’s noise-mitigation guidance starts with product and location choices. Useful measures include:

• selecting a unit with a lower applicable sound power level
• increasing its distance from neighbouring habitable rooms
• avoiding corners, small enclosed spaces and other reflective arrangements
• facing the unit away from sensitive rooms where practicable
• adding a qualifying barrier or purpose-designed acoustic treatment without obstructing airflow
• isolating vibration from walls, floors, pipework and brackets
• retaining manufacturer clearances and service access

Quiet modes and time controls can sometimes reduce impact, but they can also reduce heat output, hot-water recovery or efficiency. MCS 020 a) does not allow the low-noise-mode sound value as the calculation input, so a quiet schedule cannot rescue a failing permitted development calculation by itself.

Any enclosure, compressor treatment or retrospective modification should be agreed with the manufacturer. A measure that causes recirculation, restricts defrost drainage, compromises safety or invalidates the warranty is not a sound design solution.

After installation: identify the mechanism first

If an installed heat pump is noisier than expected, diagnosis starts by comparing it with the approved design and commissioning evidence:

• assessed model and position
• continued presence and condition of any fence or wall used in the calculation
• loose or obstructed fan guards, panels and internal components
• vibration transmitted through the base, brackets, pipes or flexible connections
• the operating mode in which the issue occurs: space heating, hot-water production, defrost or a fault condition
• changes to controls or operating temperatures
• maintenance status and the condition of the outdoor coil and fan

Record the dates, duration, weather, operating mode and where the sound or vibration is perceived. A phone recording can help describe timing, but it is not a substitute for calibrated evidence where planning or enforcement decisions depend on sound levels.

The remedy depends on the mechanism. Airborne fan sound, compressor tone, rattling panels and structure-borne pipe vibration are not solved in the same way. The installer and manufacturer need to diagnose the equipment before an enclosure or other mitigation is selected.

Complaints and statutory nuisance

Permitted development and MCS compliance do not prevent a local authority from investigating a possible statutory nuisance under Part III of the Environmental Protection Act 1990.

Where possible, give the installer or owner a clear record of the problem and an opportunity to inspect it. If that does not resolve a persistent issue, the relevant council explains how to report a noise nuisance and what evidence it needs. The council’s nuisance investigation is separate from checking whether the original planning route was valid.

BS 4142 can inform an acoustic investigation, but BSI does not present it as a test that by itself decides whether a legal statutory nuisance exists. That conclusion belongs to the statutory process and the circumstances of the case.

Pre-installation acoustic evidence

For a permitted development proposal, retain:

• the completed MCS 020 a) table for every assessment position
• drawings and photographs locating the unit, receptors, reflectors and barriers
• the source of the sound power figure
• the calculation result for the final model and location
• confirmation that the remaining Class G conditions have been checked

For a planning application, also retain the authority’s agreed scope, the complete acoustic report, the approved drawings and every noise-related condition. Make sure those documents become part of the installer’s specification rather than sitting separately in the planning file.

Related entries

• MCS 020 heat pump noise calculation
• Planning constraints for ASHP
• Outdoor unit siting
• Sound power level
• Noise assessment
• MIS 3005 heat pump standard