Planning and grants
Heat pump sound level at the assessment position (MCS 020 method)
- where
- heat pump sound pressure level at the assessment position (dB(A))
- sound power level of the heat pump (dB(A))
- directivity factor
- distance to the assessment position (m)
- barrier reduction (dB)
- level combined with background (dB(A))
- assumed background level (dB(A))
- margin below the limit (dB)
- permitted development limit (dB(A))
Explain this equation
Checks a heat pump against the MCS 020 noise test for permitted development, at the nearest neighbour's window or door.
L_w is how much sound the unit puts out. That sound spreads over the surface of a sphere of radius r, 4πr², so the level falls with distance. Walls and the ground reflect sound back, which the directivity Q allows for: 2 for one surface, 4 for two, 8 for three. A solid barrier knocks off 0, 5 or 10 dB. Decibels cannot be added directly, so the heat pump and background are converted back to energy, added, and converted again.
This is the free-field spreading law for a point source, with the standard logarithmic addition of two sound levels.
Sense check: a 55 dB(A) unit with Q = 4, 4 m away and partly screened, gives about 33 dB(A); with a 40 dB(A) background the total is about 40.8, just under 42. Doubling the distance takes off 6 dB.
Watch out: use the highest sound power figure, not a quiet mode, and measure to the nearest habitable room window or door, not the boundary.
Works out the noise from an air source heat pump at the neighbour’s nearest habitable-room window, the check needed for permitted development under MCS 020 (now MCS 020(a)).
Enter your values to see the result.
Good to know
Q counts reflecting surfaces: Q2 for a unit on the ground in the open, Q4 against one wall, Q8 in a corner. The assessment position is 1 m from the centre of the nearest habitable-room window or door of a neighbouring dwelling. A solid barrier that hides the position completely allows −10 dB; one where you would need to move up to 25 cm to see it allows −5 dB; otherwise 0. MCS combines the result with an assumed 40 dB(A) background: a heat pump level up to about 37 dB(A) keeps the total within 42 dB(A).
Common mistake
Using the low-noise or night-mode sound power. MCS 020 requires the highest sound power level the unit can produce.
Limits
This card follows the original MCS 020 Issue 1.3 steps. MCS 020 reads the distance term from a table rounded to whole dB and combines levels with a rounded table, so its answer can differ from this exact calculation by a few tenths of a dB near the limit. Since 20 September 2025 permitted development needs the revised MCS 020(a) calculation, which tightens the definitions of solid barriers, reflective surfaces and the assessment position and covers up to two units: treat this card as a quick check and use the current MCS 020(a) tool for the formal record.
Sources
- MCS 020 Issue 1.3 MCS planning standards for permitted development installations (heat pump sound calculation: sound power, directivity Q, distance, barrier, 40 dB(A) background, 42 dB(A) limit)
- MCS 020(a) (2025) heat pump sound calculation for permitted development, replacing the heat pump part of MCS 020; mandatory for permitted development heat pumps from 20 September 2025; revised and more detailed method (solid barrier, reflective surface and assessment position definitions; up to two units), with the same 42 dB(A) limit
- Copeland Borough Council: Heat pump noise level calculation form (MCS 020 Notes 3–7: Q2/Q4/Q8 reflecting surfaces, −5/−10 dB barriers, 40 dB(A) background, 42 dB(A) limit)
Results are calculations from the published methods and figures shown, and depend on the data you enter. Follow MCS standards, manufacturer data, BS 7671, the ENA connection rules and Building Regulations. System design and DNO applications need a competent, certified installer.