It seems that, unbeknown to me, there was amains pressure problem in our street. Many had similar boiler issues and some houses even lost water altogether. Anyway the water board had rectified the problem and things have returned to normal for me. It seems there was nothing wrong with the boiler at all! Thanks for all your help anyway....at least I'll be even better informed the next time I have problems...
Sorry to bring this thread back from the dead but I have exactly the same problem with exactly the same boiler.
I was wondering Fraz if your problem has stayed away since the mains water pressure returned to normal?? I was banging my head against a wall until I read this post! I have noticed though my electric shower pressure seemed lower and also the washing machine was making unusual noises, these things happened a couple of weeks back.
I think I overfilled the other day because the prv released water but since then the system has stayed at 1.5bar when operating and 0.5bar when not??
Thanks for adding this topic. I found it really useful to diagnose my boiler problem.
Not that I'm able or I even try to fix it myself. However it helped me understand what the engineer upstairs, dealing with my boiler for the third day in a raw, is doing or not doing.
Other than the suggestion about the expansion vessel, which is probably the most common cause, another common possibility is that the pressure relief valve is broken. This can be double checked by looking at whether the draining pipe is dripping (in my case the boiler draining pipe was just above the porch roof).
I probably hit both problems, the first caused the second: the expansion vessel, being too empty, didn't control the pressure which jumped to over 3 bars when the heating was on and opened the valve which kept leaking even after the pressure was restored to a low value.