Heat Pump Noise Calculator

Under MCS 020 a) (Issue 1.1, 14 April 2025) the predicted sound pressure level at every assessment position must not exceed a fixed 37.0 dB(A) LAeq,5min (ignoring the façade effect). This is an absolute limit — it is no longer "background noise + 5 dB". The 37 dB(A) figure replaced the previous 42 dB(A) limit and is mandatory for permitted-development ASHP installations in England from 20 September 2025.

MCS 020 a) Step 7 predicts the sound pressure level at the assessment position as LP = LW + 10·log10(Q ÷ (4·π·r²)) − AB. LW is the A-weighted sound power level from the datasheet, Q is the directivity (number of reflecting surfaces), r is the distance in metres to the assessment position, and AB is the barrier attenuation. The assessment position is 1 m external to the neighbour’s nearest habitable-room window or door.

Sound power. MCS 020 a) Step 3 is explicit: use the A-weighted sound power level (LWA) of the heat pump, not the sound pressure level — these are two different quantities. Datasheets and EU product fiches quote sound power (typically 55–65 dB(A) for a domestic ASHP). Do not enter a "sound pressure at 1 m" figure or a low-noise-mode value, or the result will be wrong.

Q accounts for sound reflecting off nearby hard surfaces (MCS 020 a) Step 4). One surface (the ground only, e.g. on a stand) is Q2 (+3 dB); two surfaces (ground plus one wall — the most common case) is Q4 (+6 dB); three surfaces (a corner) is Q8 (+9 dB). More than three reflecting surfaces, such as in a lightwell, fails MCS and is not permitted development.

MCS 020 a) Step 6 sets the barrier attenuation AB by barrier type and line of sight from the top of the unit to the assessment position. A Type 1 barrier (solid masonry/concrete or a fence ≥ 18 mm thick) gives 10 dB with no view, 5 dB with partial view and 0 dB with full view. A Type 2 barrier (solid but < 18 mm thick) gives 5 / 2.5 / 0 dB. If you can see the assessment position over the barrier you get no credit. The maximum barrier allowance in the standard is 10 dB.

Most effective is increasing the distance to the assessment position (every doubling of distance removes about 6 dB). Then: fit a Type 1 solid barrier that fully obscures line of sight (up to −10 dB), select a unit with a lower sound power rating, reduce the number of reflecting surfaces around the unit, and use anti-vibration mounts. If you still exceed 37 dB(A) you must apply for full planning permission rather than rely on permitted development.