Combi boiler - bang when hot water opened

[John.g]

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

 

[andrew77]

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

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?

 

[John.g]

Yes, on the expansion vessel.

 

[andrew77]

Yes, on the expansion vessel.

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.

And to set the pressurise of the expansion vessel to 2 bar, the procedure I am guessing would be as follow:

1. Close the stopcock valve.
2. Open the cold water tap to empty the water so that there is also no water in the pipe that the expansion vessel is installed on.
3. Use a tool (I don't know the proper name but I am guessing it's some kind of pump with a manometer) that can measure the pressure and effectively release some air from that expansion vessel until the gauge on the tool will show 2 bars.

 

[John.g]

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.

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.

 

[andrew77]

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.

Yes, it is. The other member has the same problem.

Thanks for explaining it. So, it all looks like I have everything installed what it's needed. It's just some balancing that needs to be done now on that mini expansion vessel. I can easily do this myself. I will post a reply with update.

 

[John.g]

No guarantees that it will cure the problem but worth a try anyway.

 

[ShaunCorbs]

4 bar seems very high how did you get this info ?

 

[andrew77]

4 bar seems very high how did you get this info ?

It says Precharge: 4.0bar hence my thinking.

 

[andrew77]

Ok, so I adjusted the pressure in that mini expansion vessel (2.0bar at the Pressure Reducing Valve and 1.8bar at the mini expansion vessel). It did help a bit as the bang it's slightly less louder, however it did not fully cure it. I think at this point I will probably get Viessmann to investigate or I might wait for the annual service and then have them investigate it.

 

[woytaz]

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.

 

[andrew77]

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.

@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.

I will be contacting Viessmann and asking them to do exactly the same to my boiler. I will 100% mention that this is not an isolated case.