Monday 19 February 2024

A whole street of heat pumps - noise levels

Heat pumps make noises, of course they do. The question is, will their noise be a nuisance if every house on the street has one? The short answer is - not usually. Here I explain why we are not disturbed by ours, and why a whole street of heat pumps does not sound much louder than a couple. In urban areas, other noises are much more noticeable. In rural areas, you might be disturbed by yours but not so much your neighbour's.

We don’t find our heat pump a problem.
Our heat pump is just by the kitchen window but we don’t hear it at all, except when it is going full pelt and then it is quieter than the refrigerator. This is with the window closed but we do not open it when the weather is cold enough that the heat pump is working. Heating the house with the window open would be silly. We do sometimes go outside but if it is warm enough for me to do some gardening the heat pump is not making a noticeable racket. We don’t use ours to heat hot water but even if we did this would only be for a short period in the day.
Heat pumps are less of a nuisance than dogs barking, traffic, music …
A recent survey [1] of neighbours of homes with heat pumps found that very few were considered a nuisance. The survey had a low response rate: out of 3050 letters sent out, only 139 responded. This suggests the rest were not concerned. Invitations were sent to households living close to a heat pump in an urban area and the survey asked what sort of sounds were noticeable during the day and during the night. During the day, dogs came top with 44% of respondents noticing them. Traffic was next, also shouting, alarms, radio, TV and music. The neighbour’s heat pump got only 5 mentions (4%) and only in one case was the noise frequent.
That was for day-time noise – what about night time? Perhaps you will be surprised but the results for the heat pump were unchanged – the same 5 mentions and only in one case frequent. 

Heat pump noise normally dissipates quickly
Noise does vary depending on the surroundings. In most cases, a heat pump is situated on or near the ground with a wall close behind. Under these circumstances, noise dissipates in all directions except behind and below the heat pump – which means it recedes quite quickly.
For a heat pump mounted close to a wall, the sound dissipates in all directions except behind and below.

Mechanical problems can lead to annoyance. 
If your heat pump is noisy, especially if seems to 'whine' there is probably something wrong with it that needs fixing. I heard of one case where it turned out the heat pump was assembled incorrectly and once the problem was diagnosed it was easily fixed. Having said that, neither the installer nor the manufacturer wanted to know about the problem so getting a diagnosis was difficult.
Even if the whole street has one, the noise is hardly worse than two or three.
Noise is measured in decibels (dB) which is a log scale because that is largely how our ears perceive it. We notice quite small increases in volume in quiet noises – but not so much for louder noises [2]. Under the decibels measure, doubling the sound pressure increases the decibels by 3, no matter if it is from 20 to 23 or 60 to 63. So if you and your neighbour both have a heat pump making the same level of noise, you could get up to an extra 3 dB which would be noticeable but hardly ear splitting. 
In fact the increase in sound is considerably less than that because your neighbour’s heat pump would be further away from you. Suppose you were standing 2m in front of your heat pump. (You would not want to be closer than that if it was working hard because of the arctic breeze.) Your heat pump has a maximum sound power level of 58 dB (Mitsubishi Ecodan [4]). Combined with the background level of 40 dB, the total sound pressure would be 47.8 dB. Now add in a neighbour’s heat pump 6m away: the result is 48.2 dB. Add in another neighbour the other side: 48.5 dB. Add another two, both 12m away: 48.7 dB. The sound level is creeping up, but more and more slowly as they are further away. Adding 4 neighbouring heat pumps has added less than 1 dB to the sound level – and this simple calculation ignores solid barriers between like fences and walls which would also reduce the noise.
Increasing sound levels as you add more heat pumps, 6m apart, in the order indicated. From 1 (47.8 ) to 5 (48.7) adds 0.9 dB.

On the other hand, if heat pumps are placed in an urban canyon with reflective barriers on both sides – then you would notice the noise far more because it would have less volume to dissipate into.
If your house is smaller and your neighbours are closer, then the increase in sound is greater. If the neighbours are 4m apart (a tiny terrace) then going from 1 to 5 heat pumps the increase in dB is 1.8: twice the increase with the neighbours at 6m. However, a smaller house will need a smaller amount of heat so the heat pumps will not be so noisy to start with.
Permitted development, allows only 2 dB increase at the nearest neighbour’s window.
The permitted development rules are based on planning guidelines that the heat pump should not increase the noise level for your nearest neighbour by more than 2 dB. So allowing for ‘quiet urban’ background noise of 40 dB, that means the combined total must be no more than 42 dB. With the Ecodan I mentioned before, assuming your neighbour’s window is 6m away, the total would be 41.9 dB so within the limit.
In a rural area, the background level is lower but houses are further apart.
In a rural area the background noise may be much less than 40 dB. Suppose it was 30 dB [3]. Then the combined sound, 6m from the heat pump, is 38.2 dB – a lot more than the permitted level of 32 dB. Of course, in a very quiet area your neighbour is likely to be much further away than 6m. If they were 20m away then you would still be within the permitted limit.
Size matters.
Generally speaking, producing more heat makes more noise and our heat pump is not noticeable unless it is working hard. However, for a given amount of heating power, making the heat pump physically larger can reduce the noise. This is because a larger heat pump can have a larger evaporator and fan and a bigger fan can rotate more slowly for the same air flow. The permitted development rules restrict the physical size of the heat pump (to 0.6m3). This is a problem for manufacturers of high power heat pumps (> 10kW) trying to comply with the noise regulations as well [1].
In summary
Heat pumps do make a noise but it is probably not nearly as bad as you think, at least as long as they are sensibly installed and working normally.
  • Heat pumps can be noisy when they are working hard but then you are not likely to have the windows open.
  • Annoying noises due to mechanical faults should be fixed.
  • A whole street of heat pumps will seem barely louder than two or three, barring urban canyon effects
    • Urban canyons are usually caused by high rise buildings and heat pumps are unlikely to be suitable for other reasons.
  • In rural areas, heat pump noise will be more noticeable but this is normally offset by greater distances between neighbours.
  • Bigger heat pumps with larger, slower fans are generally quieter. Manufacturers would like less restriction on physical size for high power heat pumps.
[1] Review of Air Source Heat Pump Noise Emissions, Permitted Development Guidance and Regulations
[2] The non-linearities of the Human Ear
[3] Comparative Examples of Noise Levels
[4] Ecodan R32 Ultra Quiet PUZ Monobloc Air Source Heat Pump

1 comment:

  1. Arithmetic with noise levels is a bit weird. I should have explained that the heat pump sound power level is the amount of power that goes to make sound (effectively all at distance 0m) while the sound pressure level is what our ears detect, at an arbitrary distance from the HP. Sound pressure levels are lower than the sound power level and decrease with more distance.


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