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.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 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)?