Combi boiler - bang when hot water opened

[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.

I did some testing in my house to see whether the problem is caused by a high mains pressure which it's a case at my property.

I closed quite a lot the main stopcock valve to reduce the flow of water (I had it fully opened). I made sure that the boiler is on for the central heating. I then flushed the water in the toilet so that I would have a running water for some time. While the cistern was filling up I would then quickly open the hot water tap in the kitchen. Surprisingly no bang when the boiler would switch from central heating to hot water. So as @king of pipes suggested I will also be installing PRV valve. Any suggestions which one would be a good one? I especially want to make sure that once the flow of water is stopped there is no pressure creeping up little by little. How would you also determine if you perhaps need 2 in series (cavitation)?

 

[woytaz]

I have installed PRV just after water meter to cover whole house installation and nothing changed. Still the same problem. I even reduced pressure to 2 bar. I think it must be something with boiler itself, maybe diverter valve because it's only when it redirect heating from centrall heating to hot water. Also I have noticed that it's not happening when centrall heating is off for long time.
@andrew77 - from video I guess you don't have your boiler in the loft? As I was also considering if that is creating a problem.
I think I will be contacting Viessmann aswell.

 

[andrew77]

I have installed PRV just after water meter to cover whole house installation and nothing changed. Still the same problem. I even reduced pressure to 2 bar. I think it must be something with boiler itself, maybe diverter valve because it's only when it redirect heating from centrall heating to hot water. Also I have noticed that it's not happening when centrall heating is off for long time.
@andrew77 - from video I guess you don't have your boiler in the loft? As I was also considering if that is creating a problem.
I think I will be contacting Viessmann aswell.

That's interesting that it did not fix the problem. My boiler is installed in the kitchen on the ground floor. I live in a 2 storey house (ground floor + first floor) so the pump would be pushing centra heating water up. I also noticed that this is not happening after central heating is off for some time.

I have been also wondering whether it's the diverter valve and I wonder whether because of the high pressure it would effectively get damaged. I am not sure what it's the design of it but there might be a spring inside it that no longer has enough tension to keep it in place.

What I also noticed is that when a central heating goes on there is a kind of hammering noise at the very beginning. I don't know how to describe it so I attached it. The same noise happens when the centra heating goes off. You can hear the noise at about 5 seconds into the recording.

 

[woytaz]

I have the same sound comming from it, but so far I have only noticed it when I run hot water then close it. Boiler then keeps running itself for a minute or so and just before it completly rest - then that sound appear.

 

[andrew77]

I have the same sound comming from it, but so far I have only noticed it when I run hot water then close it. Boiler then keeps running itself for a minute or so and just before it completly rest - then that sound appear.

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.

 

[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.