Viessmann boiler failure

[Yorkie]

Don't know if yours is a sealed system combi boiler (as opposed to an open vented) condensing boiler. If so, is it the metallic flexible filler loop hose, which is under mains pressure up to the re-pressurising filler tap, which you need to periodically turn on to re-pressurise the system then turn off when the dial indicates the correct pressure has been attained? It's unusual for then to fracture, but they're commonplace and the likes of Plumb Centre will have them and a simple task to replace. I'm puzzled when you say 'its coupled to the main burner' - the only think that goes to the burner is gas - not water. If the re-pressurising  pipe bursts, it doesn't stop the boiler from operating normally provided the re-pressurising valve isn't opened.  

Mains water pressure varies, but generally is about 75 PSI or in 'new money' 5 Bar. I would imagine that the test pressure of such hoses is at least double that. Most combi boilers operate on 1 - 1.5Bar and most pressurised systems leak slightly over time due to minute leaks on radiator valve,  and if the system pressure drops much below  1 Bar, the boiler will shut down.

There's a pic of a filler loop hose at this link:

https://www.youtube.com/watch?v=rr6k8eBqoOg


This video is a bit shouty, but useful:

https://www.youtube.com/watch?v=4meZgMAUecw&ebc=ANyPxKr_IPP-L3z2abA-I-PFYrYiTDNBYA80I7ogX0ewyPGqCqk5dl5dxHLxmVKQol1fLn64817L

Good luck with sorting it out!