MCS 020(a) heat pump sound check — permitted development
MCS 020(a) sound calculation
Method from MCS 020(a) Issue 1.0: Lp = Lw + 10·log(Q / 4πr²) − barrier. Each assessment position must come out at 37.0 dB(A) or less, rounded to 0.1 dB. Check every neighbouring habitable-room window or door, on every side and floor.
Job
Heat pump
Use the A-weighted sound power level (Lw) from the MCS Product Directory, ErP fiche or energy label. Not the low noise mode figure, and not a sound pressure figure.
Assessment positions
1 m out from the centre of a neighbour's habitable-room window or door, square to it. Measure from the centre of the heat pump in a straight line, to 0.1 m.
Result
How to pick the inputs
Directivity (Q)
Count reflecting surfaces within 1 m of the heat pump. The ground counts; plants and hedges do not. A surface only counts if it reaches at least 1 m past the edge of the unit in two or more directions.
Q2 = one surface (just the ground, or one wall if wall-hung) · Q4 = two (ground + wall) · Q8 = three (ground + two walls, a corner). More than three does not comply.
Barrier
Type 1 is brick or masonry, or a solid fence 18 mm or thicker. Type 2 is a solid fence thinner than 18 mm. Both need no gaps and must reach 1 m or more past the unit to each side. Fences with gaps, vegetation or anything in poor repair count as no barrier.
Line of sight: stand at the assessment position. No view of the unit = full reduction. If you can see the assessment position by moving within 0.25 m of the top edge of the unit, it is a partial view. Type 1: −10 / −5 / 0 dB. Type 2: −5 / −2.5 / 0 dB.
This checks the sound condition only. Permitted development has other conditions, and the rules in Scotland, Wales and Northern Ireland differ — check before relying on it outside England.
