Combi boiler - bang when hot water opened

[woytaz]

No, I have not contacted with Viessmann yet, as I wanted to cover everything regerding installation first (arrestor and PRV). Now I can see the problem still there I will contact them. We will see how it go. I will surely mention to them that there is more of those cases with those boilers so they cannot say it's all fine with it. I will surely update here if I get anywhere with it.

 

[John Coen]

Have you spoken to Viessmann about your problem? When I had first visit from Viessmann engineer as a warranty call he did some checks on the boiler etc. He basically said that everything is fine but that he would recommend change of the diverter valve motor if I remember it correctly. At least this is what he said he would put in his report. However, the second visit from Viessmann by another engineer was that he only changed the gauge. So know I am wondering whether there is something to do with that diverter motor or valve itself but Viessmann is kind of ignoring it.

I am going to go ahead with installing the PRV valve anyway as I think it would still benefit my whole installation anyway as the mains pressure in my house is absolutely ridiculously strong. I am planning on doing it after the King's coronation bank holiday. I will certainly post an update of how it went.

Diverted valve is on sealed heating system and shouldn't be affected by mains pressure

 

[andrew77]

Diverted valve is on sealed heating system and shouldn't be affected by mains pressure

Thanks for explaining that. That gives me some piece of mind that at least this is not what might be causing it. I am having GAS engineer coming tomorrow to investigate. I measured mains water pressure in my home and at midday it was around 4.5 bar so that it's definitely something to address.

 

[John.g]

Its almost mandatory now to install a expansion vessel on the mains at the combi boiler as some PHE were failing, especially on combis with preheat due to the expansion having no where to go with a closed HW tap.

Not sure, but KOP will know if some/all shock arrestors just have a spring instead of air to take the shock?.

 

[andrew77]

So I had the Pressure Reducing Valve installed (i have it set at 2.5 bar max). I also have the mini expansion vessel on the cold feed. Unfortunately the problem is still there.

All the people I spoke to they seem to be lost at trying to identify what the problem might be. I am planning to contact Viessmann again. They need to figure it out especially that I am not the only having that problem.

I started wondering if this is could be caused by hard water (I live in London in an area with hard water), but it's guessing again.

 

[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.