As far as I know this occurs when a valve/ tap closes very quickly. Can you narrow it down to a particular valve or tap? I assume it occurs on the Hot water piping as the ASHP is a closed system. Since you mention the supply perhaps I am wrong and it occurs on the cold supply side?

I agree with@jancood and incidentally it is unlikely to be anything to do with the ashp so unlikely an ashp specialist can help any more than any other plumber.  It could however be a result of upgrading from a low pressure water system to a high pressure one, which may well have been done when the ashp was installed, which is normal practice.

If you can provide more details when it occurs/what triggers it and anything else at all that you observe it may be possible to tie down the likely cause or suggest some tests which allow it to be tied down.  Also did you previously have a low pressure hot and or cold water system (with a tank eg in the loft).

I have a LG heat pump. Installed in 2020.

After installation, it was very noisy at times. After call back with installers they installed a bypass on the hot water heating:

• Controlled by a manual valve which by trial & error is probably closed approx 20%
• that pushes part of the flow directly back to return (rather than via the hot water tank). The heat is not wasted therefore (maybe a slight efficiency loss only)

Possibly the pump on the heat pump is too powerful vs the pipe runs & tank? Other heat pumps may have better controllable pump speeds.

In our controls we can reduce pump speed but afaik that would reduce it for the heating as well as hot water.

So the bypass seems a better solution in our case. Might it help you?

Posted by: @tim441

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I have a LG heat pump. Installed in 2020.

After installation, it was very noisy at times. After call back with installers they installed a bypass on the hot water heating:

 

From OPs description I had the impression (quite possibly wrongly) that the water hammer is associated with the domestic hot/cold water not the heating water.  

OP needs to clarify this by telling us more about his/her observations!

Thanks everyone for your replies.

Initially the hammer occurred only when the farm was drawing water for their barns - 8am ish and 4pm ish. It would last for approx 3-10 mins. Whilst it was happening the water pressure would drop right down. However, if we turned our cold taps on, the noise would stop, so it was bearable.

For the past few months we’ve had an issue with much louder hammer that occurs when our next door neighbour (not the farmer) uses their shower, which we understand to be a power shower. This can happen at any time they wish to shower (which is perfectly reasonable)  and can go on for as long as 25 minutes. The hammer is much louder than when water is being used by the farm. And the noise isn’t reduced by us turning on the cold taps.

Before the new system was installed we had a cold water tank in the loft. 

@jaynieq ahhhh... so sounds like its not really a heat pump issue ..  more associated with the pressurised system which is "live" connected. Whereas the old system was effectively disconnected via the tank in roof.

HHopefully a plumber on this forum can advise e.g. if it needs an extra valve... or something 

Posted by: @jaynieq

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Thanks everyone for your replies.

Initially the hammer occurred only when the farm was drawing water for their barns - 8am ish and 4pm ish. It would last for approx 3-10 mins. Whilst it was happening the water pressure would drop right down. However, if we turned our cold taps on, the noise would stop, so it was bearable.

For the past few months we’ve had an issue with much louder hammer that occurs when our next door neighbour (not the farmer) uses their shower, which we understand to be a power shower. This can happen at any time they wish to shower (which is perfectly reasonable)  and can go on for as long as 25 minutes. The hammer is much louder than when water is being used by the farm. And the noise isn’t reduced by us turning on the cold taps.

Before the new system was installed we had a cold water tank in the loft. 

That's really helpful.

Yor last sentence confirms it's not to do with the ashp as such.  When your ashp was installed you were also upgraded from an unpressurised (indirect) system to pressurised (direct) dhw and pressurised cold water throughout.  This is a normal thing to do and brings your water system up to date with modern practice.  It also means you get higher pressure water at taps and it's all potable.  Generally this is regarded as positive.

Unfortunately, whilst previously most of your water system was isolated from your neighbours by the tank, now it isn't and it's causing you a problem.

It's absolutely nothing to do with the ashp so you don't need an ashp specialist, what you need is a plumber who is good at tracking down and solving water hammer problems in older properties.

Maybe it's worth having a chat with the neighbours and seeing if there are any obvious bits of pipework between the properties that could be the cause, or whether replacing some with mdpe (plastic) might help.

Sorry but I don't think I'm going to be more helpful than that, but hopefully it enables you to ask the right questions to find the right type of plumber to fix it.

@tim441 thank you 🙏

@jamespa thank you - again! 😊

@jaynieq I wish you luck with this one.  I think the main thing I am telling you is to focus (when discussing with any plumber) on the water hammer and the fact that you went from an indirect system to a direct system.  That's the trigger, not the ASHP.  The latter is irrelevant so probably best not even to bring it up.  If your neighbour also experiences the noises they may have an incentive to help.

Probably a good idea to nail down the triggers if you havent already and be sure you are able to cause the hammer if anyone visits.  Also consider making a recording for them.  Its nigh on impossible to fix a problem if you cant experience/reproduce it!