“It’s just kicked in now,” said Paul Eastwood, as a gentle sound rose like gathering breeze, “so if you want to come closer, you can hear it and feel it. It’s kicked in. I’ve forced it on. I said I want 30C inside in order to do this. This is me ramping it hard.”

One of the most frequently heard concerns about air source heat pumps (ASHPs) is that they are noisy. A school in Norfolk was, earlier this year, ordered to turn off its heat pumps because of complaints from neighbours about the noise. Famously,  Lord Willie Haughey, founder of City Facilities Management Holdings,  called them, in 2021, “window rattlers”. But the noise emitted from Mr Eastwood’s is not close to rattling.

A sound meter on my mobile phone, held half a metre from it, registers just less than 50 decibels, though we are outside and there is other ambient noise.

It’s worth comparing with other noise levels that impact on our domestic acoustic environments. A tram passing my open window reads more like 70 decibels. My fridge, when it cranks on is a sub-heat pump hum at under 50 decibels. My vacuum cranks the noise levels up to almost 80. But the real horror in my home is my washing machine, which I am about to replace for multiple reasons, whose spin cycle hits over 90 decibels.

Outside a Victorian tenement flat in Glasgow, another heat pump whirred into life with a noise so low that I found myself checking that it was the pump. “That’s it on,” said owner Dr Jane May Morrison. “That’s as loud as it gets.”

Has she had any complaints about the sound?

“I haven’t," she replied. "The neighbour’s window is right next to us and he hasn’t said anything to us at all. We’re on really good terms with the neighbours and they support what we’re doing here.“

At the edge of Musselburgh, Pat Hackett’s Vaillant heat pump registered around 60 decibels with the sound meter held up close (this is no controlled scientific experiment, just an impressionistic tour).  “People,” he said, “will say that heat pumps are noisy. I’ll say, ‘No, they’re not. I’ve got one - and it’s not noisy. But they’ll still insist, just because they’ve read or heard something, that they are noisy, as if, though I have one,  I mustn’t know.”

In Dunblane, Neil Kitching's garden-based Vaillant gave sound readings that were less than his kettle.

But, the noise levels created by many air source heat pumps could be enough, if close to a neighbouring property, or where there are multiple installations in a high density neighbourhood, for instance tenements, to exceed sound constraints. Current regulations restrict heat pump noise to a maximum of 42dB within one-metre of a neighbour’s windows..

A survey conducted as part of a Department for Energy Security and Net Zero (DESNZ) review of air source heat pump noise emissions found that most neighbours were not concerned by the noise.

“Sound emissions,” the review said, “from heat pumps were not reported as noticeable by most participants in the areas of focus for the research. A small number of survey respondents said they could hear their neighbour’s ASHP from inside their property. Of these, some reported they could hear the ASHP regularly, with others hearing the ASHP either sometimes or rarely.”

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It also noted: “Interview participants described noise from a neighbour’s ASHPs as a low hum and ‘rumbling noise’. The two interviewees expressed concern due to the potential for cumulative noise impacts arising from multiple installations, citing noise as one of the considerations for not installing ASHPs in their own property.”

A literature review conducted as part of this research also found that, though there was a low incidence of noise complaints, some did exist. “These arose,” it noted, “due to poor quality installations, including location and proximity factors.”

"The available evidence suggests that ASHP noise complaints appear to be relatively infrequent in relation to the total number of ASHP installations in the UK," the  DESNZ review noted. 

“Location and proximity of ASHP units to neighbouring properties emerged as a key cause of noise complaints. According to installers and LPAs, complaints usually centred around disturbed sleep and installers reported they were typically resolved through moving or replacing the ASHP.”

Poor installation quality was often a factor underlying noise complaints. “Simple modifications like rubber matting or acoustic enclosures were found to reduce noise impacts, highlighting the importance of proper installation.”

The story that heat pumps are noisy is not something that has come from nowhere.  There have clearly been problems with sounds from some heat pumps, as evidenced by the Lord Haughey's 2021 recording  made of a London unit which prompted his window rattling comment.

“I couldn’t believe it,” he said, at the time. “That was me standing eight feet away from the condensing unit. I’m building 356 apartments around the corner here. Can you imagine if I had to put in 356 of these?”

Meanwhile, noise does matter, and sensitivities to it vary. A 2018 survey by double glazing firm Everest found that one in six Britons has moved house because of noise pollution.

Increasingly, there has been a move by the industry toward tackling the noise pollution question, and developing new models with lower sound emissions. At the Mitsubishi heat pump factory in Livingston, is an anechoic chamber where unit noise levels are tested.

Their newest model, the Ultra Quiet Ecodan, advertises itself as “three times quieter than previous models, virtually eliminating planning restrictions”. The model, which has sound power levels of 58 dB, registers just 45 dB at one metre from the pump, dropping to less than 30dB at ten metres. This means, the model’s product information says, that “the pump can be located much closer to the assessment position and pass planning.”

The best way to get a sense of real noise levels from heat pumps is to listen to one, and some of the hosts I met through Nesta’s Visit A Heat Pump scheme said that it was a frequent request that they turn on the pump so it was possible to hear the noise level. But, in the absence of a visit,  it’s also possible through a tool on the Loco Home Retrofit website to get an impression of the noise levels and assess how they may impact you. 

Homeowners who want to be sure of getting a quieter heat pump, can also search for a low noise model at Quiet Mark, an offshoot of the Noise Abatement Society.

The Quiet Mark certification, said CEO and founder Poppy Skziler, means it is “one of the quietest models on the market for it's kW capacity”. These include models by Daikin, Vaillant, Samsung, and Worcester Bosch.

“Quiet Mark assess heat pumps," she added.  "On a kW like-for-like basis so we do not unfairly penalise larger, more powerful models designed for bigger heating systems. As the main part of an air source heat pump is sited outside of the house, a Quiet Mark certified model should have little to no impact, in terms of sound level to the homeowner, or neighbours, as long as the external unit is positioned correctly and well maintained.”

A recent survey by Quiet Mark found that 82% of people in the UK said it matters to them how loud an appliance is when making a purchasing decision. “We know,” said Ms Szkiler, “people are proactively seeking out quieter products and Quiet Mark helps people find third party expert assessed and verified products in an easy free product sourcing platform to support public need.