Resources / Heat pumps
Noisy Heat Pump: What’s Normal and What’s Not
Outdoor units are quieter than older kit, but they are not silent. Here is how to tell ordinary operating sound from a real problem — and how siting decisions on South West plots make or break neighbour peace.
Quick Answer
A modern air source heat pump will make a soft fan whoosh that rises with load, plus occasional changes of tone during defrost. That is normal. What is not normal is structure-borne vibration through walls, a pure tonal whine that dominates a quiet night, or a unit that got clearly louder after install.
On Devon and Cornwall plots — terraces, courtyard cottages, tight village streets, exposed coastal walls — siting and mounting matter as much as the datasheet dB(A) figure. A quiet unit in a hard reflective courtyard can still annoy; a slightly louder unit on a well-chosen wall with proper anti-vibration mounts often does not.
View printable checklist Planning aid — not a quote
What you actually hear
The outdoor unit has a compressor and a fan. Both get busier when the heating load rises — which is why the same installation can sound almost forgettable in mild weather and more present on a cold evening.
| Sound | Usually means |
|---|---|
| Steady soft whoosh | Fan moving air — normal under load |
| Tone change for a few minutes | Often defrost in damp cold — normal if brief |
| Slightly louder on cold nights | Higher output demand — expected, within reason |
| Rattle, buzz through walls, pure whistle | Mounting, vibration, debris or a fault — investigate |
Manufacturer noise figures are measured under controlled conditions at a stated distance. Real gardens have walls, fences and corners that reflect sound. The number on the sheet is a starting point, not a guarantee of how it will feel outside your bedroom window.
Normal vs problem — a practical split
- Broadband fan noise that fades into the background
- Load-related increase on cold days
- Short defrost tone changes in damp weather
- Quiet overnight modes reducing fan speed when demand allows
- Vibration you can feel in floors or joinery
- Tonal whine or drone that cuts through conversation
- Rattling panels, loose brackets, debris in the fan
- Sudden step-up in noise after works or a cold snap with no recovery
Vibration is often worse than “loudness”
Airborne whoosh is one thing. Structure-borne noise — energy travelling through the wall, floor or pipework — is what turns a tolerable unit into a neighbour complaint or a 2 a.m. problem in your own house.
Common causes:
- •Unit hard-mounted without proper anti-vibration feet or rails
- •Uneven base or rocking slab
- •Pipework rigidly clipped so vibration transmits indoors
- •Wall brackets on a lightweight or poorly detailed structure
Fixing mounts and flexible connections is often more effective than arguing about a 2 dB difference on a datasheet.
Siting on Devon & Cornwall plots
South West housing throws up the same constraints again and again: narrow terraces, granite courtyards, listed frontages, side passages shared with neighbours, and coastal winds that both create noise and limit where you can put the box.
| Situation | Noise risk | Design response |
|---|---|---|
| Hard courtyard / granite walls | Reflections amplify sound | Avoid corners that focus noise; consider orientation and distance |
| Under bedroom / neighbour window | Night-time annoyance | Prefer a different elevation if possible; night modes help but do not invent silence |
| Tight side passage | Tunnel effect + poor airflow | Clearances matter for noise and efficiency |
| Exposed coastal wall | Wind noise + weather on the unit | Robust mounting; do not starve airflow with decorative screening |
| Village terrace | Close receptors either side | Early neighbour-aware siting; check planning / PD rules |
Decorative enclosures that look tidy on a planning drawing can make noise and defrost behaviour worse if they block intake or discharge. Airflow is part of the noise story, not only the acoustic story.
Acoustic modelling — when a “quiet enough” guess is not enough
Datasheet sound power or sound pressure at one metre is only a starting point. What neighbours hear depends on distance, height, reflections, barriers and background noise. Acoustic modelling (or a structured noise assessment) estimates level at the nearest sensitive windows instead of hoping the brochure number is enough.
What a simple assessment is trying to answer
- •How loud is the unit at the nearest habitable room window (yours or next door)?
- •Does that level sit within the limits used for permitted development noise checks (where those rules apply)?
- •Would moving the unit, changing height, or adding a compliant barrier change the result enough to matter?
Inputs that change the answer
| Input | Why it matters |
|---|---|
| Sound power of the unit | Manufacturer data at stated conditions — not a site measurement until someone measures |
| Distance to receptor window | Level falls with distance; a few metres can decide pass/fail |
| Height and line of sight | Direct path vs screened path |
| Hard courtyard reflections | Granite, render and glass can add level versus open garden |
| Barriers / fences | Only count if they truly break line of sight and are dense enough — lattice rarely helps much |
| Operating mode | Max heating vs night mode — assessments should use a realistic or worst credible mode |
MCS 020 and permitted development context
For many domestic air source heat pumps, permitted development routes use a published noise assessment method (commonly referred to in the industry via MCS 020-style guidance) to show calculated levels at the nearest neighbour assessment position are within set limits. Installers who work under PD should be able to explain whether your proposed position was assessed and what receptor was used.
That process is not the same as a full BS 4142 environmental noise survey for a contentious commercial site — but it is more than “the unit is 42 dB so it will be fine.” If the calculator only works with an optimistic position, the real wall you planned to use may fail.
Conservation areas, listed buildings, flats and non-standard plots may sit outside simple PD assumptions. In those cases planning officers can still care about noise even when a spreadsheet is not the formal route.
The noise calculation for permitted development
For many domestic air source heat pumps in England, permitted development needs a signed MCS 020 a) sound calculation on file before work starts. It is a fixed method — not a full acoustic survey and not “the brochure says 42 dB so we are fine.”
- •Start with the manufacturer’s sound power (highest figure — not quiet/night mode).
- •Adjust for distance to assessment points, nearby reflecting walls, and any real solid barrier.
- •Assessment points are typically 1 m outside neighbouring habitable room windows and doors — not the middle of your garden.
- •The calculated level must meet the PD limit of 37 dB LAeq,5min at those positions (as set out in the current standard).
Pass — that position can satisfy the PD noise condition (other PD rules still apply).
Fail — move the unit, change the setup, or apply for full planning instead of relying on PD.
Ask your installer: “Can I see the MCS 020 assessment for this exact position?” On tight terraces and stone courtyards in Devon and Cornwall, the spreadsheet often decides the wall. Always use the current MCS 020 a) document — the method was updated (Issue 1.0, 2025).
When to push for a proper look
- •Terrace or semi with windows close to the only practical wall
- •Hard reflective courtyard (common in stone / granite South West stock)
- •Neighbour already sensitive or previous complaints on the street
- •Installer cannot show how PD noise limits were checked for the chosen spot
Acoustic modelling will not fix a unit hard-bolted to a bedroom wall. Use it to choose and defend a position — then still install mounts, clearances and pipe isolation properly.
Planning, permitted development and neighbour reality
Many domestic air source heat pumps can fall under permitted development rules when limits on size, position and other conditions are met — including a noise assessment against published methods (see acoustic modelling above). Conservation areas, listed buildings, flats and non-standard sites often need a closer look. Building regulations also cover installation standards more broadly; noise between properties is usually handled through planning/PD and good practice rather than a single “Part X” homeowner checklist.
Noise and siting are part of how those rules are applied in practice; they are not only an after-install courtesy.
Even when planning is straightforward, neighbour relationships are not a PDF. A unit that is legal but sits under someone else’s bedroom window is still a poor design choice. Good installers treat receptor distance and reflection as design inputs, not complaints to manage later.
Always confirm current rules for your address with the installer and, where needed, the local planning authority — especially in historic cores across Devon and Cornwall.
Why it seems louder in cold weather
Load goes up, fan and compressor work harder, and defrost cycles appear in damp cold. That is the same physics as our cold weather performance guide — more output demand, more outdoor unit activity.
A system that is undersized or forced to high flow temperatures may run nearer maximum more of the time, which keeps noise elevated. Fixing the thermal design can calm the soundscape as well as the bill.
If noise is bothering you — check in this order
- Is anything loose — panels, debris, vegetation in the fan?
- Are anti-vibration mounts present and actually isolating the unit?
- Is pipework transmitting buzz into the structure?
- Has the unit iced up or is it in constant defrost?
- Is night mode / quiet mode enabled where available?
- Is the system short-cycling or stuck at high output because of design limits?
- Would a different position have been realistic — and is retrofit relocation proportionate?
Start with mounting and airflow before assuming the compressor is failing. Many “noisy heat pump” call-outs are installation details.
Honest bottom line
Expect some sound. Do not expect silence. Judge the install by vibration, tone, siting and how it behaves at night in cold weather — not only by a single dB number on a brochure.
On tight South West plots, where you put the unit and how you mount it will do more for neighbour peace than chasing the absolute quietest model and then boxing it into a reflective corner.
Frequently asked questions
Are newer heat pumps much quieter?
Generally yes compared with older generations, but load, siting and mounting still dominate real-world experience.
Can I put a fence or box around it to hide the noise?
Solid boxing often makes airflow and noise worse. Any screen must respect manufacturer clearances; acoustic design is not the same as hiding the unit.
Is night mode enough for a terrace?
It helps when demand is low. On a hard cold night the unit may still need to work. Siting remains the bigger lever.
When should I call the installer back?
Vibration through the structure, sudden new noises, constant defrost-related noise with poor heating, or clear breach of what was promised at survey.
Next step
Get a realistic planning band for your home, then use the checklist when you speak to installers.