Discuss Combi boiler - bang when hot water opened in the UK Plumbers Forums area at PlumbersForums.net
By DCV you mean directional control valve (double check valve)? If yes, then I don't have it installed at all. I think it's the other person on this thread who has it installed. Also, by setting the air end pressure to 2.0 bar you mean setting it on that little expansion vessel?Can't say I agree with installing a DCV as a E.vessel/shock arrestor should allow water to move both ways. I would remove the DCV and also set the air end pressure to 2.0 bar
Thanks. I just had a look. That expansion vessel is precharged to 4 bars so if I am guessing correctly if the water needs to expand it needs to do it against the membrane that is pressurised to 4bars and this is effectively hard to do. When pressurised to 2bar it will be basically easier for the water to push that inside membrane. I hope my understanding is correct.Yes, on the expansion vessel.
Just noticed that its your boiler water pressure gauge thats jumping? re post 34 above, you need a tyre pressure gauge to stick on the air end schrader valve on the expansion vessel, you can then reduce the pressure to 2 bar by pressing the pin on the schrader valve to release some air until pressure gauge reads 2/2.2 bar.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.
Yes, it is. The other member has the same problem.Just noticed that its your boiler water pressure gauge thats jumping? re post 34 above, you need a tyre pressure gauge to stick on the air end schrader valve on the expansion vessel, you can then reduce the pressure to 2 bar by pressing the pin on the schrader valve to release some air until pressure gauge reads 2/2.2 bar.
It says Precharge: 4.0bar hence my thinking.4 bar seems very high how did you get this info ?
@woytaz thanks for the update. It's funny because the very first Viessmann engineer (I had 2 different engineers from Viessmann checking this) that came actually mentioned that in his report he would mention that the diverter valve motor should be replaced. On the second visit another engineer came and he only replaced the gauge and did nothing to the diverter valve or the diverter valve motor itself. I am guessing that you probably had the whole set replaced.Ok, sorry, but I was bit busy in my life recently and I have forgotten a bit about this thread. So, I finally contacted Viessman, engineer just came and replaced diverter valve. It's just still fresh but so far so good. No banging noise after opening hot water when CH running. We will see if it's gonna stay like this, but surely it was a diverter valve problem. The engineer said that for some reason this valve was closing/openning too quickly where it should do it slowly. So @andrew77 , you need to push them to replace diverter valve, tell them about my case which is similar.
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