Combi boiler - bang when hot water opened

[woytaz]

I just had a thought. To be fair, that bang is not straight away after opening a hot water tap. You open the tap, water starts flowing and after maybe 3-4 seconds you can hear that bang. So it doesn't look like it is typical water hammer. My feeling is that this bang happens when boiler shuts central heating flow to prioritise hot water. Also that pressure jump on central heating when that happening... You don't think it might be something to do with boilers internal expansion vessel?

 

[king of pipes]

I'd still protect the Pipework and appliances from incoming high pressure I have experience with a similar estate I work on at night when no water is being used pressures can reach 8 bar and drop to 3-4 bar in the day , the heating switching shouldn't cause banging unless the system is not connected correctly is there a auto bypass valve fitted ?? Maybe a few pictures would help us here . Kop

 

[woytaz]

Hi. So there is a photo of instalation and a video of how it looks when it's happening. I have checked mains pressure today and it's only 3 bar during a day.

 

[king of pipes]

Does this always happen every time the hot water is turned on ? or occasionally?

 

[woytaz]

Pretty much every time. Just when you open hot water it's doing it first time. Then when you close it and open within short time then it's not happening again, untill you close it long enough for boiler to rest completly (something around 5-10 minutes) then it happens again after opening hot water. I hope you can understand what I mean Regardless if central heating running or not.

 

[king of pipes]

I'd say the pressure builds to a greater pressure than what's behind it a inline prv set to 3 bar should help

 

[king of pipes]

This explains it better than I can hope it helps

 

[woytaz]

Should I install that PRV just after water meter to cover whole house installation or should I put it close to the boiler?

 

[king of pipes]

One to protect the whole house from high water pressure would be better but if you want to just protect the boiler then in the cold supply to it.

 

[andrew77]

Hello, I am newbie in this forum. I had exactly the same boiler installed around November 2022. It was all fine for some time but then exactly the same issue started happening. I had the little shock arrestor added but that did not helped. I had Viessman checked the boiler and they said that basically everything is fine. They also replaced the little gauge.

I am really curious whether you managed to solve the problem? It clearly something is to do with the pressure and this only happens when I open any hot water tap. Similarly to yours I had open vented system converted to sealed one. I have also installed Aladdin auto vent valves to all radiators.

I also noticed that the pressure reading on the gauge is always higher by about 0.3 bar than on the electronic display but that might be just red herring due to where the electronic gauge actually measures the pressure.