Noise is the objection neighbours raise and the internet inflates. So here are numbers. Two things to hold onto: decibels are logarithmic — every 10 dB sounds roughly twice as loud — and sound fades about 6 dB with each doubling of distance. Spec sheets quote sound power; what you hear at any real distance is sound pressure, and it is always lower.
What Do the Decibel Numbers Actually Mean?
A modern heat pump measures roughly 40–45 dB(A) heard from one metre and 35–40 from three — between a library and a quiet fridge. The planning limit is 42 dB(A) at the nearest neighbour's window. Normal conversation is 55–60. The full comparison:
| Sound | Level |
|---|---|
| Bedroom at night, rural Bucks | 25–30 dB(A) |
| Quiet library | 35 dB(A) |
| Modern fridge at 1 m | 38–42 dB(A) |
| Heat pump at 3 m (typical modern unit) | ~35–40 dB(A) |
| The 42 dB planning limit at a neighbour's window | 42 dB(A) |
| Normal conversation at 1 m | 55–60 dB(A) |
| Oil boiler burner, same room | 50–60 dB(A) |
Permitted development hangs on that 42 dB(A) line: the MCS 020 assessment must show the unit at or below it at the nearest neighbour's "assessment position" — effectively their closest habitable window. With a modern unit and a sensible position, most gardens pass with 5–10 dB to spare; where the geometry is genuinely tight, the fallback is a planning application, covered in our planning permission guide.
Why Are R290 Units Quieter?
R290 — propane — transfers heat more effectively than the refrigerants it replaced, so the unit hits the same output with a larger, slower fan and a compressor that loafs at part load. Slow fans are quiet fans: today's units run 6–10 dB quieter than 2018 models, which the ear hears as roughly half as loud.
The redesign brought quieter packaging with it — variable-speed compressors on rubber isolation, aerodynamic blades, insulated casings. Most 2026 units also ship a scheduled night mode that caps fan speed through bedtime hours, trading a sliver of output for near-silence exactly when it matters.
Where Should the Unit Go? Placement Tactics That Work
Face the discharge away from bedroom windows, use distance (every doubling cuts 6 dB), avoid enclosed corners, mount on a ground-level base rather than a wall, and screen with slats or planting without boxing the unit in. Siting decides more of the heard noise than the hardware does.
Siting decides more than hardware. The tactics we use, in rough order of value: face the unit away from bedroom windows — yours and the neighbours'; fan noise is directional and pointing the discharge at a lawn instead of a window is worth several dB. Use distance — every doubling from the listener drops 6 dB, so 3 m beats 1.5 m by more than any premium unit beats a budget one. Avoid acoustic corners — a unit tucked between two hard walls reflects sound back at the listener; one open side is enough to let it dissipate. Mount on mass, not walls — a ground-level base on anti-vibration feet transmits nothing into the building; wall brackets on bedroom walls are how the "humming house" stories start. Screen with care — a slatted fence or planting between unit and neighbour trims line-of-sight noise, but never box the unit in: it needs to breathe, and a starved unit runs harder and louder. Since the 1 m boundary rule was scrapped in May 2025, there's far more freedom to pick the acoustically right spot rather than the legally forced one.
Noise Questions
Will I hear it inside my own house?
From a ground-mounted unit, effectively no — brick attenuates 45 dB+, and a unit reading 40 dB(A) outside is below your home's night-time background before it reaches the sofa. Indoors, the cylinder and pump make about as much sound as a boiler's pump did: a faint circulation you'll hear only from the airing cupboard.
What's the defrost noise people mention?
On cold, damp mornings the unit briefly reverses to melt frost off its coil — a change in tone, a whoosh, sometimes a gurgle, lasting a few minutes. It's normal operation, not a fault, and no louder than the unit's ordinary maximum; it just sounds different, which is why we mention it at handover before you hear it in November.
My neighbour is worried before we've even installed — what do I show them?
The MCS 020 calculation, which we produce at the survey: it names their window, the predicted level at it, and the 42 dB line it sits under. Failing that, invite them to stand next to one of our installs — the demonstration usually ends the conversation, because the birds are louder.
Does noise get worse as the unit ages?
Not if it's serviced: the wearing parts that could rattle — fan bearings, mounts — are checked annually, and refrigeration circuits don't degrade audibly. A unit that grows a new noise mid-life has a specific fault (usually a mount or a fan obstruction), which is a repair call, not a characteristic.
Hear the Answer for Your Own Garden
Every survey includes the MCS 020 noise assessment and a siting plan, so you'll see the predicted level at your nearest neighbour's window before anything is ordered — and we'll happily arrange for you to stand next to a running unit first. The full installation picture is on the air source heat pump page; the grant that pays for most of it is on the Boiler Upgrade Scheme hub.