Loud clonking and pressure dial bouncing on baxi combi.

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Very similar symptoms to this other thread on our Baxi duo tec Combi 40 e: https://www.diynot.com/diy/threads/boiler-pressure-going-mental.304352/

I've heard it take a few times but never got to it in time, it only lasts for a minute or so. Today I heard it - sounded like someone running along a scaffold plank from downstairs.
Up close, it was more like there was something inside the the boiler. Knocking away very loudly but you could kind of hear the sound move around and it was more from the bottom than the top.
At the same time the pressure dial was bouncing +/- .5 bar every time there was a knock like something in a bad sci-fi film.

However it didn't sound like kettling to me and the temperature was steady around 70... We've had kettling before with a blocked heat exchange and there was more of a roaring/bubbling sound as the temp reached 90, this just sounded more like something banging it with a spanner.

We've had all sorts of recent issues but this has never happened when the plumber was here. I rushed to get a video but it stopped just as I started recording... I'm hoping it's an obvious clue what exactly is wrong?

The boiler continued working as normal after this 'episode' which as I say was only 1-2 minutes.
 
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Filling loop being the two black 'taps' on the right hand side underneath? If so they are definitely turned off (I checked) - are they a part that could fail and be stuck open though? The pressure dial is (apart from when it went crazy) holding steady at just over 1 bar.

Prv presumably is the pressure vessel so I can picture the noise could be it literally physically shifting about in response to sudden pressure changes?

Sounds like good info for the plumber, I just wish I'd got it all on video. Thanks.
 
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what is the ;pressure gauge at when its doing this and do you have to top it up afterwards at all
 
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what is the ;pressure gauge at when its doing this and do you have to top it up afterwards at all
It was bouncing around it's 1.5ish bar so probably between 1 and 2. It was just acting like you were shaking it making the needle wobble, in time with each bang from inside.

Afterwards the pressure was still good. I don't know exactly what it was at before but no obvious drop remembering where the dial is. I didn't have to top it up. It's been working fine all day today.
 

CBW

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Prv = pressure relief valve, not pressure vessel. Do you have any dripping hot water taps?
 

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