Loud banging noise Vaillant

[dusan8]

I have same problem that has been posted on this youtube video:

If the boiler has the heating running and you turn on hot water anywhere in the house you can hear this sound.
If the heating is off and you turn on hot water, no sound.

I also had regular service in November, so 3, 3.5 months ago. They cleaned heat exchanger and everything that goes with it. Everything was fine for 2-3 days and then it started banging again.

The boiler is the same as the one from the video -> Vaillant VUI 240-7 r3 aquaBlock

If anyone knows what might be the problem or how to diagnose it, please help. Thank you.

 

[Johntheo5]

There may be a meter of non return valve on the cold feed to the combi which is causing pressure to build up in the plate Hx which may still heat up even with no HW demand.
Suggest (while CH on) just (crack) open a hot tap to give a flowrate of less than 1LPM (a trickle) which shouldnt cause a c/o to HW, feel this and see if cold/lukewarm or whatever, then say 30 secs later open the hot tap to its normal setting to cause c/o to HW and see if banging persists.

 

[dusan8]

Suggest (while CH on) just (crack) open a hot tap to give a flowrate of less than 1LPM (a trickle) which shouldnt cause a c/o to HW, feel this and see if cold/lukewarm or whatever, then say 30 secs later open the hot tap to its normal setting to cause c/o to HW and see if banging persists.

Yeah, tried this even after a minute or so, still you can hear loud bang when I nomally open the tap

 

[Johntheo5]

Can't find your exact model but the one below might be similar, it has a separate expansion vessel for the DHW and also a DHW pump and a PRV.
Could possibly be diverter fault or DHW PRV if you have very high mains pressure especially if boosted by another pump which seems unusual except that this model (Aquaplus) is fed from a CWST.? No, its only a circ pump tpo heat the store I think.

 

[dusan8]

UPDATE: Diverter Valve was the issue.
In Croatia, I paid 192€ for this, the valve itself was 142€.