Combi boiler - bang when hot water opened

[woytaz]

Hi All! I hope you can give me some advise.
I had combi boiler (Viessman vitodens 100-w 30kW) installed about half year ago. Recently I start hearing one single bang when hot water is opened at any of taps around house. It sound like someone in other room will knock a radiator (not relly loud but you can hear it). After investigating a bit I found it's probably coming from heating circut flow as also when that happens I can see brief jump of pressure on a pressure gauge.
So it only happens when any tap is opened regardless heating is on or off. After closing hot water tap and opening again straight after it's not doing it - only after leave it for some time.
Or maybe it is totally normal with those boilers (as I had previously backboiler with open vented system)?
When I called my plumber (which was installing this boiler) he said (without seeing it) that he never had this kind of problem and start coming with all different possible ways to solve it, but I'm not really keen on spending my money to trial each option.

 

[ShaunCorbs]

Is there a shock arrestor fitted looks like a 3” silver ball on the cold under the boiler on the pipework

 

[king of pipes]

As Shaun has said a shock arrestor should be installed I put a double check valve before it seems to sort out the issue. Kop

 

[woytaz]

No, there is no arrestor. Seems like you sure about this so I will give it a try. Thanks for help. Will update after fitting it.

 

[woytaz]

King of pipes, what is this white device in upper right on your pic?

 

[ShaunCorbs]

King of pipes, what is this white device in upper right on your pic?

Room stat receiver

 

[king of pipes]

Eph combi pack 4 wireless receiver bud this one was wired for open therm , as I said fit a double check valve inline and then the shock arrestor that should cure it . 😉👍

 

[woytaz]

Unfortunately shock arrestor didn't resolved it. Double check valve and then shock arrestor close to boiler. Have checked charge on shock arrestor and it's around 3.5 bar.
Nothing changed. Any other ideas?
I'm not sure if that is relevant, but this boiler is in the loft. Also I'm leaving next to water tower and guess I have pretty high mains pressure (but never measured it). Maybe should put some pressure reducing valve?

 

[king of pipes]

Yes definitely a pressure reducing valve on the incoming supply I'd recommend set to 3 bar Kop.

 

[woytaz]

So can I fit that PR valve just before double check valve or maybe past it?

 

[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.

 

[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.