Are Air to Air Heat Pumps Noisy? Decibels Indoors and Out

Are air to air heat pumps noisy?

Not usually. Indoor wall units on their quietest setting are typically rated in the low 20s dB(A), about as loud as a whisper or rustling leaves, rising on higher fan speeds. The outdoor unit is louder, but in England it must meet a 42 dB limit at a neighbour's nearest window or door to count as permitted development.

Typical levels

Source	Typical level	Comparable to
Indoor unit, quietest setting	Around 20 to 25 dB(A)	Whisper, rustling leaves
Indoor unit, high fan	Around 35 to 45 dB(A)	Quiet office, fridge hum
Outdoor unit at 1m	Often mid-40s to mid-50s dB(A)	Quiet conversation
Limit at neighbour's window (England, permitted development)	42 dB	Quiet library

Check the manufacturer's sound pressure and sound power figures for the exact model. Sound drops with distance and behind walls or fences.

The 42 dB rule

In England, for an air source heat pump to be permitted development, the installer must follow MCS 020 and the noise at 1m from a neighbour's nearest door or window must not exceed 42 dB. Since 2025 there's no longer a fixed 1m distance from the boundary, but the noise test still applies.

How to keep it quiet

Put the outdoor unit away from bedrooms, yours and the neighbours'.
Use anti-vibration feet or mounts, and avoid fixing to thin walls that resonate.
Avoid corners and enclosed spaces where sound reflects.
Pick models with a night or quiet mode.
Size it properly. An undersized unit runs flat out, which is louder.

Questions

How loud is an air to air heat pump indoors?

On the quietest fan setting, typically low 20s dB(A), about a whisper. On high fan it's more like a quiet office.

Will my neighbours hear the outdoor unit?

They might, a little. In England it must stay within 42 dB at 1m from their nearest window or door to be permitted development, which is quieter than a normal conversation.

Sources

Checked 8 November 2026.