Heat Pump Noise: How Loud Are They and What Does 42 dB Actually Mean?

Heat pump noise is one of the most-Googled objections to installing an air source heat pump — and one of the most misunderstood. The short answer is that modern units are unobtrusive: a typical air source heat pump (ASHP) produces around 40–50 dB at one metre, comparable to a domestic fridge or soft rainfall. The longer answer involves understanding why the planning system focuses on 42 dB specifically, and what affects how loud a heat pump actually sounds in your garden. This guide covers both.

What 40–50 dB actually sounds like

Decibels are logarithmic, so each 10 dB step represents roughly a doubling in perceived loudness. To put heat pump noise in context:

30 dB — a quiet whisper, rustling leaves
40 dB — a library, a domestic fridge humming, soft rainfall
42 dB — a quiet room with an appliance on — the MCS planning threshold
50 dB — a quiet conversation, a moderately busy office
60 dB — normal speech at close range, a dishwasher on its quiet cycle
70 dB — a vacuum cleaner, a busy road 10 m away

A modern ASHP at 40–45 dB is genuinely quiet — quieter than many people expect from a machine heating a whole house. At typical garden distances of 3–5 metres, the perceived level drops further still, because sound pressure halves (falls 6 dB) each time distance doubles in open air.

The 42 dB rule: what it means under permitted development

Since 28 May 2026, every air source heat pump installed under permitted development in England must comply with MCS 020 — the Microgeneration Certification Scheme’s planning standard for heat pump noise. The key condition: the calculated sound pressure level at the centre of the nearest neighbour’s habitable room window must not exceed 42 dB(A).

This matters because it determines whether you need full planning permission. If your MCS-certified installer’s noise calculation comes in at or below 42 dB(A), the installation proceeds as permitted development (PD) — no planning application, no waiting, no fee. If it exceeds 42 dB(A), full planning permission is required before work starts.

The 42 dB limit is assessed at the neighbour’s window, not your own. Your installer takes the manufacturer’s quoted sound power figure, applies corrections for distance and any acoustic barriers (walls, fences), and produces a predicted sound pressure level at that reference point. MCS 020 also assumes a background noise level of around 40 dB(A), which explains why the threshold is set close to typical suburban ambient levels.

Before May 2026, installers could use MCS 020 or an “equivalent” acoustic method. From 28 May 2026, MCS 020 is the only accepted route for permitted development in England. If you are in Wales, Scotland, or Northern Ireland, check with your local planning authority, as rules differ slightly.

Sound power vs sound pressure: what manufacturers quote

Manufacturers publish two different noise figures, and confusing them leads to false expectations. Sound power level (L_WA) describes the total acoustic energy the unit emits — it is a fixed property of the machine, independent of where you stand. Sound pressure level (L_pA) is what you actually hear at a given distance — always lower than the sound power figure. A unit with a sound power level of 58 dB(A) might register roughly 45–48 dB(A) sound pressure at 3 metres in open air.

MCS 020 uses sound pressure at the neighbour’s window for compliance. When comparing models on spec sheets, use the sound power figure as a consistent baseline; your installer converts it to a predicted sound pressure level using the MCS 020 method.

Factors that affect how loud it feels in practice

The measured dB rating is only part of the story. Several site-specific factors can make a heat pump sound louder or quieter than the spec sheet suggests:

Reflective surfaces. Hard walls, fences, or paving directly behind or beside the unit reflect sound back, raising the effective level. Positioning the unit away from corner “acoustic traps” reduces this.
Distance from bedrooms. Even a well-spec’d unit will feel intrusive if the outdoor unit is directly below a bedroom window at 1 metre. Good positioning — ideally with 3–5 metres between unit and habitable windows — makes a large difference.
Night-time perception. Background noise falls sharply after 11pm, so a unit that is inaudible at noon can become noticeable at midnight. Heat pumps on smart schedules often reduce output at night, which helps.
Operating load. At full heating output in very cold weather, the fan and compressor work harder and the unit is louder. Low-load operation — mild days, part-load — is quieter. Inverter-driven compressors (now standard on most UK models) modulate continuously and avoid the noisy start/stop behaviour of on-off units.
Vibration transmission. Poor bracket installation can transmit vibration through the building fabric. Anti-vibration mounts are cheap and should be standard on any quality installation.

Quietest heat pump brands in the UK

Most modern inverter-driven ASHPs from major manufacturers comfortably meet the 42 dB(A) MCS threshold in typical UK installation scenarios. Among models commonly installed in 2026, a few stand out for low noise:

Mitsubishi Ecodan — consistently among the quietest units available; the standard range operates from around 45–49 dB(A) sound pressure at 1 m. If noise is a primary concern, the Ecodan is often the first recommendation from acoustic specialists.
Daikin Altherma 3 — rated at approximately 48 dB(A) at 1 m in most configurations; widely installed across the UK and MCS 020 compliant in the majority of domestic settings.
Vaillant aroTHERM Plus — carries Quiet Mark certification and posts a sound power figure in the 54–58 dB(A) range; louder than the Ecodan or Altherma at close range, but its high SCOP efficiency offsets the premium for many buyers.

For an in-depth ranking of efficiency and value across models, see our best air source heat pump guide. If you are still weighing whether a heat pump makes financial sense for your home, our honest heat pump worth-it assessment covers the full cost picture — including noise as one of the practical objections to address.

Planning permission: when noise triggers a full application

If the MCS 020 noise calculation exceeds 42 dB(A) at the neighbour’s window, you cannot rely on permitted development and must apply for planning permission. This is more likely in these situations:

Semi-detached or terraced properties where the only viable position for the outdoor unit faces the shared boundary, leaving little distance between unit and neighbour’s windows.
Conservation areas — additional restrictions apply. In England, an ASHP in a conservation area cannot be installed on a wall or roof that faces a highway. The garden-facing wall is typically the only permitted option, which may bring the unit closer to a neighbour.
Very small gardens where no position achieves adequate separation distance from the nearest habitable window on any boundary.
High-output units in large homes — a 16 kW unit needed for a large detached house has a higher sound power rating than a 5 kW unit fitted to a flat; the MCS 020 calc may not pass for the bigger unit in tight spots.

A full planning application for a domestic ASHP is not unusual — local planning authorities assess them routinely. A specialist acoustic report from an MCS-accredited consultant can support the application and sometimes propose mitigation measures (acoustic barriers, revised positioning) that bring the installation into compliance without refusing permission.

Ground source heat pumps: effectively silent

If noise is a genuine deal-breaker for your site — perhaps a small urban garden right beside a neighbour — a ground source heat pump sidesteps the issue entirely. The working machinery (compressor, pump) is indoors, typically in a utility room or plant room, and the ground loop circulates silently underground. There is no outdoor fan unit and no external noise to measure. The trade-off is higher installation cost (£15,000–£30,000) and the need for sufficient land for the ground loop.