@jamespa 

Thanks again for your valuable insights (and to everyone else).

1:
I'll ask octopus why they didn't use a 2-port volumiser instead of a 4-port buffer. 

Just read this:
https://renewableheatinghub.co.uk/volumisers-in-heat-pump-systems-does-placement-matter/

Maybe they've placed the volumiser on the return side, and that's why had to use a buffer? The installer said something about the placement of the buffer being why they went with that instead of a volumiser.

2.
I don't know if they've replaced a low pressure loft fed system with a mains fed system, I don't remember them mentioning anything like that. I just know the knocking/thumping/pulsing noise is very annoying.

3.
The outside unit is apparently operating as expected, based on their telemetry data. WRT the noise, they said that turning the secondary pump down would reduce demand on the outside unit and it would be less noisy. It's still a nuisance. 
The unit is not on any anti-vibration pads as far as I can see. It just sits on a concrete slab, which sits on a cement/agregate (?) base.

Octopus gave me no indications about noise being an issue with heatpumps, especially given I have a small courtyard garden at the back of a terraced house. Wish I'd been on this forum and done more research, I'm very pro-green tech and believed a lot of people who say their heat pump is quiet enough to sit next to in the garden.

Posted by: @razz

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Maybe they've placed the volumiser on the return side, and that's why had to use a buffer? The installer said something about the placement of the buffer being why they went with that instead of a volumiser.

sorry but thats just nonsense.  A volumiser can go in flow or return.  Flow is arguably better but return is nearly as good and in either case it functions just as well as a volumiser, the difference is a relatively minor comfort issue which you may well not notice unless you have fancoils. 

More BS from the installer I am sorry to say.

Posted by: @razz

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I don't know if they've replaced a low pressure loft fed system with a mains fed system, I don't remember them mentioning anything like that. I just know the knocking/thumping/pulsing noise is very annoying.

From the picture you showed they definitely fitted a new mains fed system.  Obviously I dont know if the old system was loft fed or mains fed (you may know).  The key point though is that the banging is nothing to do with the heat pump as such, the water system is separate (other than for the coil in the cylinder which will not be the problem)

Posted by: @razz

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WRT the noise, they said that turning the secondary pump down would reduce demand on the outside unit and it would be less noisy.

Very likely more Bow Locks Im sorry to say.  The demand on the outdoor unit is dependent on the outside temperature, ie how much your house loses.  Changing the buffer to a volumiser (and making the necessary adjustments to flow temperature) will however reduce the demand on the ODU because it will be able to operate at a lower temperature which is less hard work for the ODU.  Whether this will be enough to solve the problem it is impossible to say.

There is a circumstance in which reducing the secondary pump speed might reduce the demand which is if it has been set up badly, ie too fast, in the first place, however like changing a buffer to a volumiser this will work only if the necessary adjustments are made to the weather compensation curve.

I detect BS.

Posted by: @razz

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The unit is not on any anti-vibration pads as far as I can see. It just sits on a concrete slab, which sits on a cement/agregate (?) base.

Thats very unusual for heat pumps, notmally they are put on antivibration pads.  However the picturs I have seen of cosy heat pumps dont have pads, maybe they are internal or maybe they have decided its not necessary.

Posted by: @razz

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Octopus gave me no indications about noise being an issue with heatpumps, especially given I have a small courtyard garden at the back of a terraced house. Wish I'd been on this forum and done more research, I'm very pro-green tech and believed a lot of people who say their heat pump is quiet enough to sit next to in the garden.

Indeed so to the second sentence, although they can get a bit noisy if its very cold.  If yours hasnt quietened down now that its warmed up then thats definitely disappointing, in current weather I can barely hear mine from 2m away (I could hear it when it was -5 outside).

@razz  You should perhaps take a look at and interrogate this  A Cosy owner who has solved excess noise problems with some very simple DiY case mods.

We are having very similar issues with our Cosy12 installation.  Our main concern is the constant humming inside the house that permiates to every room: far from the actual system. It sounds like the red pump shown in this post is running constantly and sending the vibration noise through the existing pipework.

Sorry if this has been answered already in this forum but is there a solution to this or something specific we should be getting Octopus to fix/change?

Thanks 

@dedwards, welcome to the forums. Can you please post a photo showing the positioning of your red Grundfos circulation pump?

@dedwardsards I've had the Octpus people come in and have a look, and they turned the pump down, but no luck.From what I can tell, sometimes the noise happens even when the pump is off. 

How's the noise from the external unit itself? As you saw in my post, it can get pretty horrendous especially if it's running hard. It just seem unsuitable for a small garden like mine. Did Octopus say anything about the noise level before installation?

@editor pictures below. We tried installing sound proofing before realising that the sound was propagating through the pipework.

@razz 

The heatpump itself often operates well above the 40-60 decibels quoted on the Octopus website: often hitting 73+dB but this isn't our primary concern as it is away from our living space. 

The Grundfos circulation pump is running constantly even when the house is well above the target temperature.

Octopus are yet to send out anyone to have a look at the system and communcations shine putting the order in has been less than ideal. 

@hook Apparently REDS were known for unique sound profiles!

@sukh-sukh-sukh-swwils  I have a Cosy 6 and it’s quieter than my old gas boiler, but it does have a particular frequency when it ramps up to its max when it heats water. 

Normally it’s silent. 

For anyone not familiar with this issue: there are two completely different types of noise with heat pumps, and they need to be understood separately.

Airborne noise is the normal fan/compressor sound you hear through the air. Modern units like the Cosy are genuinely very quiet in this respect - most people have no issues with airborne noise.

Structure-borne noise is a different beast entirely. This is when vibrations transfer from the heat pump into the building structure (through mounting brackets, pipe connections, wall penetrations, etc.). These vibrations then travel through walls, floors and ceilings, and certain surfaces resonate at specific frequencies - essentially turning parts of your house into a speaker.

The telltale signs you're dealing with structure-borne noise:

• Low-frequency hum or buzz that you almost feel rather than hear
• Sound that "pops out" at different spots around the room or house
• Worse when touching walls or lying in bed
• Standard dB(A) meters show acceptable levels but the noise is still maddening

Here's the critical bit: most sound meters and microphones have low-frequency filters that completely remove the exact frequencies where structure-borne noise occurs. So you can have "compliant" measurements while someone is genuinely being driven mad by the vibration.

So please, when someone reports this kind of issue, don't dismiss it just because your airborne measurements look fine. Structure-borne noise is real, it's measurable (with the right equipment), and it absolutely needs addressing differently than airborne noise. I have 10,000% sympathy, it is absolutely maddening. 

Another loud Cosy 12 install here. We're confident the spec is wrong and not what is being seen in reality. We have a complaint in but so far being told it's fine and a 'you' problem. Neighbours are livid. Honestly the more complaints that go in the less they can claim it's one person being sensitive. There is no way it should be anywhere near as loud as it is, regardless of AV mounts, Flexi pipes etc. There is a fundamental noise issue with these units.

Posted by: @livinggreen

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being told it's fine and a 'you' problem.

This is what I've experienced too. The latest email I received claimed "the sound levels are higher for you as it was measured so close to the unit". Their documentation states that the pump should be "around 40 - 65 decibels within one metre": I took the measurements from 1.5m and it was well above 70dB.