Well, it depends on anyone's viewpoint really. There are already two cut off devices in the system and the Boiler Safety Valve IS the secondary cut off: a) pressurestat b) Pressure Relief / Boiler Safety / Over Pressure - Valve. Failures in the former are more commonly manifested as failure to pressurise, rather than to allow an over pressure situation, but they do occur one way or the other from time to time, which is why we have a Safety Valve. Failures in the latter are rare OR, the pressurestat is usually good enough that a situation of over pressure seldom occiurs, so the safety valve is rarely "tested". Either way, rare enough. I could be wrong but think you may find the failures were caused by one or other faulty parts, rather than through a "common service malfunction" in use if you get the distinction? Happy to be corrected if not. Incidentally, one does not (atleast in my neck of the woods), "check and service" a Boiler Safety Valve. If for some reason it's suspect (perhaps if its really old, or leaking, OR like when someone who is not in the know has tried to remove or service it and we cant be sure the pre set manufacturer setting hasnt been tampered with), we change it over for a new one. That is the extent of the check and service. Hope that helps.
