I have a vaillant arotherm plus too 10kw. Mine gets more audible as well when it’s colder outside. Can’t be heard from inside the house, only when I’m outdoors near the pump.
It’s my understanding that most if not all heat pumps are the same.

Appreciate this is an old post but I’m experiencing the same thing in the current cold snap. At freezing temperatures, my Arotherm Plus 7KW unit makes a sustained loud buzzing noise. I measured this at about 65db and it’s audible inside the house. The buzzing is not audible during regular operation and is definitely not the unit entering defrost. Annoyingly however it doesn’t do this all the time. It is well above the stated noise levels for the unit so I’m assuming it is a fault…? Anyone else experiencing this? Thank you. 

@tallmarc stated noise levels are made at a defined outside temperature not the worst case which is what we currently have in the depths of winter, mine is equally noisy at sub zero temps as its working flat out 24/7, I would suggest its probably performing normally they are just noisy in winter, they are especially noisy if just turned on and are trying to get the water up to set temperature same after a defrost for the same reasons.

Our 10kW Grant Aerona also gets noticeably louder when cold. There's a few reasons why:

1. The ASHP is working very hard, the compressor frequency is very high nearly all of the time. That creates more noise and vibration. It also tends to cause other things such as panels, grilles and covers to rattle in sympathy. ASHPs are built to a price, there's acoustic lagging around the compressor, but not on the cover panels or other sheet steel that can start to vibrate.

2. The evaporator coil is icing up. That increases the velocity of the air through it, which makes more noise. The fan is trying to move a large volume of air through very narrow gaps that are increasingly restricted by ice.

3. Sound travels further in colder weather than warmer weather. It's just one of the quirks of the physics of sound.

Last night was the first time my wife complained to me she could hear our ASHP from the bedroom, keeping her awake. The unit is one floor below and the other side of a cavity wall stone & block wall. I could hear it too. However, now the cold spell has ended we'll revert to switching the unit off 9pm - 4am and hopefully that'll be the end of that conversation 😉 It was running 24/7 last week while temperatures never rose above freezing point.

Posted by: @gemiar

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I have a vaillant arotherm plus too 10kw. Mine gets more audible as well when it’s colder outside. Can’t be heard from inside the house, only when I’m outdoors near the pump.
It’s my understanding that most if not all heat pumps are the same.

My 7kW Arotherm does the same.  Its especially noticeable during recovery from defrost, when the compressor is going at 100%, and at this point its has a tonal component and is just slightly annoying.  Fortunately defrost recovery is 5 mins once per hour or thereabouts.  A near neighbour of mine, with the same model, reports similarly.

Turning on NR mode kills the noise, but also compromises recovery performance too much.  Obviously limiting compressor modulation to only 60% (which is what NR does) is going too far.  I have tried reducing 'max compressor current' from 16A (the default) to 13A (the minimum available), which appears to reduce the max compressor modulation to 90%.  This makes quite a difference and almost fixes the problem without materially compromising defrost performance. 

Unfortunately there doesn't seem to be a way to adjust the max compressor modulation to somewhere between 90% and 60%.  I have a suspicion that ~85% would do the job, but that option isn't available (unless I have missed something). 

I have actually just asked Vaillant about this and will post any response.

My heat pump is surrounded on 3 sides by hard surfaces and I think the tonality might actually be a resonance effect.  The distance between two of the surfaces (which are parallel to each other) is exactly twice the wavelength of the principal tone - a bit suspicious to say the least.  I'm trying to find a reasonably priced sound absorber (it needs to be an absorber not just a barrier) with a tolerable appearance suitable for outdoors.  Solflex (solflex.eu) do something which appears absolutely perfect and will ship to anywhere in the EU, which is not helpful!  I have yet to find an equivalent product available in the UK.

@jamespa nice analysis 

have you considered something like https://rubbermatting-direct.co.uk/products/sound-proofing-and-deadening-rubber-sheet-a?variant=44408298537208&country=GB&currency=GBP&utm_medium=product_sync&utm_source=google&utm_content=sag_organic&utm_campaign=sag_organic&cq_src=google_ads&cq_cmp=21831003036&cq_term=&cq_plac=&cq_net=g&cq_plt=gp&gad_source=1&gbraid=0AAAAADyPPoi_IjUULmYLiHLjdTemaRja1&gclid=EAIaIQobChMIr8PakObyigMVEJZQBh0ZZCzKEAQYAiABEgJhh_D_BwE  

or neoprene?

@gary This was at -1.1 so not a particularly taxing operating temperature. The pump was also heating the house at the time, not defrosting or heating the hot water. It made enough noise to be heard in two rooms away through the double glazing.

I am a bit shocked as one of the myths that I have tried to counter when showing people the pump - I support the Nesta Visit-a-heat-pump scheme - is that heat pumps are noisy. It usually isn't but it now appears that, unless faulty, the Arotherm Plus sometimes it operates at a much louder level than the stated specifications suggest.  

Posted by: @judith

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@jamespa nice analysis 

have you considered something like https://rubbermatting-direct.co.uk/products/sound-proofing-and-deadening-rubber-sheet-a?variant=44408298537208&country=GB&currency=GBP&utm_medium=product_sync&utm_source=google&utm_content=sag_organic&utm_campaign=sag_organic&cq_src=google_ads&cq_cmp=21831003036&cq_term=&cq_plac=&cq_net=g&cq_plt=gp&gad_source=1&gbraid=0AAAAADyPPoi_IjUULmYLiHLjdTemaRja1&gclid=EAIaIQobChMIr8PakObyigMVEJZQBh0ZZCzKEAQYAiABEgJhh_D_BwE  

or neoprene?

Thanks for the suggestion.

This appears to have a solid shiny surface and be designed to reduce transmission.  I think what I need to do in my situation is reduce reflection, which needs a broken-up surface.  I dont think either this or neoprene this it the right product in the particular use case I have (and am loathe to buy and install until I have reasonable confidence it is).

I suspect it’s not reflective to audio waves and rough surface matting is available too. Patterned door mat perhaps ☺️

Posted by: @judith

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Patterned door mat perhaps ☺️

Quite possibly.  As I mentioned above the 'ideal' product seems only to be available in the EU.  There is a UK product which looks like grass, (and therefore also like a door mat) which I could believe might work.  Much more expensive than the EU product though.  Still looking.

Googling noise with ashps and found this technical document https://www.apexacoustics.co.uk/wp-content/uploads/2023/10/NOISE-FROM-ASHPS-WHAT-WE-KNOW.pdf#page5

it identifies several noise related issues