TL;DR:

  • Air source heat pumps typically run at 40–55 dB at one metre — comparable to a quiet conversation or a refrigerator
  • Where you position the unit matters more than which unit you buy: keep it away from bedroom windows and neighbouring properties where possible
  • Under Permitted Development rules, the noise level measured at your neighbour’s nearest window or door must not exceed 42 dB — most modern units meet this, but placement is critical

One of the most common reasons people hesitate over an air source heat pump is noise. And it’s a fair concern — unlike a boiler hidden in a cupboard, a heat pump sits outside your house, running for extended periods. But the reality of heat pump noise is often misunderstood, both by people who dismiss it as irrelevant and by those who assume it’ll be constant and disruptive.

Here’s what the noise actually sounds like, what the rules are, and what you can do if noise is a genuine concern for your situation.

What Do Heat Pumps Actually Sound Like?

The main noise from an air source heat pump comes from its fan — a large, slow-turning fan that draws air across the refrigerant coils. This produces a steady, low-frequency hum. The compressor adds a mechanical sound underneath it. Neither is particularly sharp or intermittent — it’s more of a constant background presence than something that starts and stops abruptly.

At one metre distance, most modern units run at around 40–55 dB. For context:

  • 40 dB is roughly a quiet library or a refrigerator humming in the kitchen
  • 50 dB is a normal conversation or light rain
  • 55 dB is the level many manufacturers list as maximum output for their quieter models

Sound levels drop with distance. At 5 metres, a 50 dB unit is closer to 34 dB — you’d struggle to hear it over any background noise. At 10 metres, it’s barely perceptible even in a quiet garden.

The operational profile also matters. Heat pumps run longer cycles at lower output rather than short bursts at high power (which is the opposite of how a boiler works). On a mild day, the fan may run almost continuously but quietly. On a cold day, it runs harder. Most units cut off completely during defrost cycles.

The Permitted Development Rules

In England and Wales, installing an air source heat pump under Permitted Development rights (meaning no planning permission required) comes with conditions, including a noise limit: the sound level must not exceed 42 dB(A) measured at one metre from the nearest window or door of the neighbouring property.

This is the standard that installers and the MCS (Microgeneration Certification Scheme) use. Before installation, an MCS-certified installer should carry out a noise assessment that includes the proposed position and the nearest relevant neighbours. If the assessment indicates the limit would be exceeded, you either need to choose a quieter unit, change the position, or apply for planning permission rather than using Permitted Development.

Scotland has its own planning rules — check with your local council if you’re in Scotland, as the specifics differ slightly from England and Wales.

How Installation Position Affects Noise

This is where most of the real-world variation comes from. The same unit can be almost unnoticeable or a constant irritation depending on where it’s placed and what it’s near.

Avoid these positions if possible:

  • Directly beneath or adjacent to bedroom windows — either yours or a neighbour’s
  • In a corner or alcove where sound reflects off surrounding walls (this can raise the effective level noticeably)
  • Pointing directly at a neighbouring property’s outdoor sitting area

Better positions:

  • Along a side return or side fence where the unit has open space ahead of it
  • On the side of the house furthest from any neighbouring bedrooms
  • Raised on a wall bracket if ground-level placement would direct airflow toward a fence or wall at close range

Some installers will also use acoustic enclosures or barriers — specially designed screens that deflect sound upward rather than toward neighbouring properties. These can reduce the perceived noise level by several decibels when placement options are limited.

Anti-Vibration Mounting

Separate from airborne sound is the structural vibration that a heat pump can transmit through walls and floors. If the unit is mounted directly on an external wall of the house without vibration isolation, the compressor vibration can travel through the structure and be heard inside — sometimes more distinctly than the airborne noise outside.

An MCS installer should fit anti-vibration mounts as standard. If you’re retrofitting or finding vibration is a problem post-installation, aftermarket anti-vibration pads and mounts are available. This is worth raising explicitly with your installer before work begins.

Noise From Neighbours: What You Can Do

If you’re on the other side of this — a neighbour has installed a heat pump and the noise is bothering you — the options depend on whether the installation was done under Permitted Development and whether the 42 dB limit was met.

Your first step is to contact the homeowner directly and calmly. Most noise issues have practical solutions: repositioning the unit slightly, fitting an acoustic screen, or adjusting the operational schedule so it runs less during late evening hours. Installers can usually make adjustments.

If that doesn’t resolve it, contact the local council’s environmental health team. They can measure the noise and, if the installation doesn’t meet the Permitted Development conditions, require the homeowner to address it.

Heat Pump Sound in Practice

The honest answer is that most people barely notice their heat pump after a few weeks. It becomes background noise in the same way a boiler firing or a fridge humming does. The installations that cause ongoing problems are almost always ones where the unit is badly positioned — too close to a window, bouncing sound off a wall, or vibrating against a poorly mounted bracket.

Get the position right during the planning phase, use an MCS-certified installer who carries out a proper noise assessment, and make sure anti-vibration mounts are included. Those three things handle the vast majority of noise complaints before they happen.