Heat pump noise and the 37 decibel rule

An air source heat pump can usually be installed without a planning application, under permitted development rights. Those rights come with conditions, and one of them is that the noise reaching a neighbour must not exceed 37 dB(A) at any assessment position.

Establishing that is not a matter of standing in the garden with a meter. It is a calculation, set out in MCS 020, and MIS 3005-D requires it to be done before an installation proceeds on permitted development grounds.

What the calculation is

The whole thing is one line:

L_p = L_w + 10 log(Q ÷ 4πr²) − A_B

The sound pressure level at the neighbour equals the sound power level of the unit, plus a term for how the sound spreads out over the distance, minus whatever a barrier takes off.

The sound power level

A unit carries one sound power level wherever it is put, so this figure is read off the product fiche, the energy label or the MCS product database; it is not something worked out for the site. Domestic units are typically between 50 and 65 dB(A).

The common error in the whole exercise is using the sound pressure level instead. Datasheets often carry both, and the sound pressure figure is smaller, because it has already had a distance built into it. Substituting one for the other produces an answer that is roughly correct in appearance and wrong by twenty decibels. MCS 020 says that where a datasheet uses a vague term like "noise level", the manufacturer should be asked which it means before starting.

The figure for low noise mode must not be used, since nothing obliges the unit to stay in that mode when it is cold and working hard.

Distance and reflecting surfaces

Sound spreads over the surface of a sphere, so its intensity falls with the square of the distance. Every doubling of distance takes off about six decibels. This is the largest lever in the whole calculation and the cheapest to pull.

Q is the directivity factor, and it accounts for surfaces near the unit that stop the sound spreading in all directions and push it outwards instead. A reflecting surface is anything within a metre of the unit that extends at least a metre past it in two or more directions. The ground counts as one of these surfaces, and vegetation does not.

• One reflecting surface, typically just the ground: Q = 2
• Two, so ground mounted against a wall: Q = 4
• Three, so ground mounted in a corner: Q = 8

Each surface adds three decibels. A unit tucked into a corner is six decibels louder at the neighbour than the same unit standing in the open, which is the same as halving the distance twice over.

More than three reflecting surfaces, which is what a small lightwell amounts to, cannot meet MCS 020 at all.

Barriers

A barrier earns a deduction according to what it is made of and how much of the assessment position can still be seen over it.

Barrier	No view	Partial view	Full view
Type 1: masonry, or a solid fence 18 mm thick or more	10 dB	5 dB	0 dB
Type 2: a solid fence thinner than 18 mm	5 dB	2.5 dB	0 dB
No barrier	0 dB	0 dB	0 dB

The definitions are strict, and deliberately so. A barrier must have no cracks or gaps, and must extend at least a metre horizontally beyond each edge of the unit. Open fences, fences with gaps, fences in poor repair and vegetation of any kind count as no barrier whatever. A hedge is worth nothing in this calculation, however dense it looks.

Line of sight is judged from the top edge of the unit. No view means the assessment position is completely hidden. Partial view means it becomes visible if you move up to 0.25 m in any direction. Full view means you can see it from where you stand, and full view earns nothing however solid the wall is.

Assessment positions

An assessment position is a window or door of a habitable room in a neighbouring property, at ground floor and upper floors alike. Every one that could be affected has to be assessed, and each needs its own distance and its own barrier judgement.

The nearest position is not always the worst, and MCS 020 makes a point of saying so. A window eight metres away behind a solid wall can be quieter than one fifteen metres away with a clear view of the unit, because distance is worth six decibels a doubling and the wall is worth ten outright. An upstairs window frequently looks straight over a fence that shields the room below it.

More than one unit

Where two heat pumps serve the same assessment position, each is calculated separately and the results are combined logarithmically:

L_p,total = 10 log(10^L_p,1/10 + 10^L_p,2/10)

Decibels do not add like ordinary numbers. Two identical units are three decibels louder than one, not twice as loud, and ten identical units are ten decibels louder.

A worked example from the standard

MCS 020's own example: a unit with a sound power level of 60 dB(A), ground mounted against a wall so Q is 4, eight metres from a neighbour's window, with a brick wall between that leaves a partial view.

The spreading term is 10 log(4 ÷ (4π × 64)), which is −23.0 dB. The barrier takes off 5. So 60 − 23.0 − 5 = 32.0 dB(A), which is comfortably inside the limit.

Move the same unit to five metres, with the wall now hiding the window entirely: 60 − 19.0 − 10 = 31.0 dB(A). Closer, and quieter, because the barrier is doing more.

Put it fifteen metres away behind a slatted fence, which counts as no barrier: 60 − 28.5 − 0 = 31.5 dB(A). Nearly twice the distance of the first case, and no better, because the fence earns nothing.

When it fails

Failure at any assessment position means the installation is not permitted development on noise grounds, and would need a planning application. It usually does not mean the heat pump cannot be installed, only that the siting has to change.

In rough order of how much they are worth: move it further away, six decibels a doubling. Bring it out of a corner, three decibels a surface. Put a proper solid barrier in the line of sight, up to ten decibels. Specify a quieter unit, which comes straight off the top.

MCS publishes separate guidance on mitigating noise, which goes into acoustic enclosures and anti-vibration mountings. It is worth reading before accepting that a position is impossible.

Noise is one condition among several

Passing the calculation does not by itself make an installation permitted development. The other conditions cover siting relative to boundaries and roads, the size of the unit, and whether the building is listed or in a conservation area or a national park. Those vary between England, Wales, Scotland and Northern Ireland, and the local planning authority is the place to confirm them.

It is also worth remembering that permitted development is a planning concept, not a promise of quiet. A unit that passes at 36.9 dB(A) is legal and may still annoy the neighbour it passed against.