Heat pump noise: the 37 dB(A) rule and how to site a unit

Are heat pumps noisy?

Not usually enough to matter, and in England the rules make sure of it. To be permitted development, the calculated sound level from the outdoor unit at the nearest neighbour’s habitable room must not exceed 37.0 dB(A) under the MCS planning standard. That is a calculated level at the window, not a reading at the unit: the unit itself can be louder close up. Position, distance and any barrier decide the answer, and the calculator below shows how. In terraces and flats, where neighbours are close, the calculation is the first thing to check; the guide to heat pumps for flats covers those cases.

The limit and what it measures

The noise rule that matters for most homeowners in England sits in MCS 020 a), the Microgeneration Certification Scheme’s standard for air source heat pump sound calculation for permitted development installations. Issue 1.1 is dated 14 April 2025. It sets out a calculation that confirms whether the permitted development noise limit of 37 dB LAeq,5mins, shown in the standard’s checklist as 37.0 dB(A), would be met. The Order itself is covered in heat pump planning permission; this page is about the noise condition.

Three details of the definition are worth knowing:

• Where it is measured. At an “assessment position”: 1 metre outside the centre point of any door or window to a habitable room of a neighbouring property, measured at right angles to the plane of the door or window. The effect of the facade is ignored.
• Which rooms count. Rooms used or intended for sleeping or living, but not rooms used solely for cooking, bathrooms, corridors, laundry rooms, hallways or utility rooms.
• Which neighbours count. Any building used as a dwelling, hotel or residential institution. Where the unit goes on a block of flats, other flats in the same block count, but not the owner’s own flat.

The installer must do one calculation for each assessment position that could be affected, keep one copy and give you another. That is a document you are entitled to ask for before you pay a deposit, and it is a good test of how seriously an installer takes siting. The standard adds that installers are under a duty to ensure compliance for any installation done as permitted development, MCS may impose penalties if they do not, and an installation that fails the standard may be subject to enforcement action by the local planning authority.

Where the unit goes is a planning question as much as a noise one: heat pump planning permission sets out the permitted development limits and when the noise calculation decides the answer. Siting it further from the house adds pipework, which is one of the line items on the heat pump installation cost page.

The calculation, step by step

MCS 020 a) is a short desktop calculation. The eight steps in the standard are, in plain terms:

1. Date. The date of the assessment is recorded, and re-checked if time passes before installation, in case a neighbouring building or barrier has changed.
2. The assessment positions. Every potentially affected window or door, on all sides and at each floor. The nearest is not always the loudest: a further window with a clearer line of sight can come out worse.
3. Sound power level. Taken from the manufacturer’s data. The low-noise mode figure is not permitted.
4. Directivity (Q). A count of reflecting surfaces within 1 metre of the unit that extend at least that far beyond it in two or more directions: Q2 for one (including the ground), Q4 for two, Q8 for three. A unit with more than 3 reflecting surfaces, such as one in a small lightwell, will not meet the standard.
5. Distance. A straight line from the centre of the unit to the assessment position, to the nearest 0.1 metre.
6. Barrier correction. Subtracted if a solid barrier sits between the unit and the window (see the matrix below).
7. The sound pressure level. Calculated with the formula below.
8. The check. The result, rounded to 0.1 dB, must be equal to or lower than 37.0 dB(A) at every assessment position.

The step 7 formula is: L_p = L_w + 10 · log₁₀(Q ÷ (4π r²)) − A_B, where L_w is the sound power level in dB(A), Q the directivity factor, r the distance in metres and A_B the barrier attenuation.

How far is far enough?

The table turns the formula round: the straight-line distance at which a unit reaches 37.0 dB(A) with no barrier, for three sound power levels and the three reflecting-surface cases. The sound power of 60 dB(A) is the value MCS uses in its own worked example; check a real unit’s figure on its energy label or the MCS product database. These are arithmetic on the published formula, not measurements.

Look along the rows: each extra reflecting surface adds roughly 3 dB, which is why a unit tucked into a corner needs about 1.4 times the distance of one on an open wall. Down the columns, each 5 dB of sound power adds a factor of about 1.8 to the distance needed. That is the case for choosing a lower sound power unit when the neighbour is close. A quiet unit on an open wall can pass at about three metres, as the first cell shows, while a louder unit in a corner needs several times that distance, or a barrier.

Barriers, reflecting surfaces and two units

A solid barrier between the unit and the window reduces the calculated level, but only if it meets the standard’s definition. A Type 1 barrier is a solid brick or masonry wall, or a solid fence, at least 18 mm thick with no cracks or gaps, extending at least 1 metre horizontally beyond either edge of the unit. A Type 2 barrier is a solid fence thinner than that, again with no gaps. Open fences, fences with gaps, vegetation and walls in poor repair count as no barrier. The reduction depends on the view from the top edge of the unit to the window:

Two units on a detached house are summed logarithmically at any assessment position they share: two identical units at the same level give a result 3 dB higher than one, which is why the second unit can turn a pass into a fail. The standard’s own guidance for a failed calculation is that it may be possible to make adjustments, such as moving the unit, and MCS has published separate guidance for installers on mitigating noise (18 November 2025).

Beyond the rule: vibration, nuisance and flats

Passing a 37 dB(A) calculation is not the whole story, and the best-informed source on what else matters is a joint Professional Advice Note from the Institute of Acoustics and the Chartered Institute of Environmental Health (2026). Its design advice is practical:

• Location. Put units as far as practicable from sensitive neighbours, while keeping to the manufacturer’s clearances so airflow is not restricted.
• Barriers. A barrier works if it breaks the line of sight and is imperforate with a mass of at least 10 kg per square metre.
• Vibration. A unit rigidly fixed to a building can pass vibration into the structure, which re-radiates as noise indoors and can be annoying even at low levels. Use vibration isolation mounts and pipe connections.
• Enclosures. Manufacturer-approved enclosures can reduce noise but risk air recirculation and poorer performance, so they are a last resort.
• Your own household. Bedrooms and garden space of the house the unit serves count too, and the occupier may be a tenant rather than the owner.

The same note says cumulative impact from many units is unlikely to be a problem for detached, semi-detached and terraced houses on recent research, and is only necessary to consider for courtyards or flats; see heat pumps for flats and terraces. It also sets out alternative assessment routes for installations where permitted development does not apply for non-acoustic reasons (for example, a listed building): an installer-led MCS 020 a) calculation, then a desktop assessment by a suitably qualified acoustician against the same 37 dB(A) criterion, and a full assessment in exceptional cases such as tranquil areas.

If a complaint comes after installation, the route is different. GOV.UK says councils investigate noise that amounts to a statutory nuisance under the Environmental Protection Act 1990, and must serve an abatement notice if one exists or will occur; the notice can be delayed for up to seven days while informal resolution is attempted. Complaints about a defective installation are covered in complaints and consumer codes.

What to ask the installer

• A copy of the completed MCS 020 a) table for each assessment position, with the sound power level and its source.
• Where the unit will stand, how many reflecting surfaces it has and the distance to the nearest neighbouring windows, including upstairs windows.
• What mounting is proposed: ground-mounted on a base, or wall-mounted, and what vibration isolation is used.
• What happens if the calculation fails: a different position, a quieter model or a barrier.
• How the noise from the unit during defrost and at maximum output has been considered, since the standard’s sound power figure is measured under standard test conditions and may not be the noisiest operating condition.

The order of events from survey to commissioning is in the installation process, and the checklist for comparing quotes is here. Source note: figures on this page come from the documents listed at the end, read on 8 October 2026.