Does this mean jezzy that you changed the pump only to find that the boiler is still behaving the same. YES.
So,it could have been or still is? a defective Gas valve. Why did you drain down if as you say the pump valves are fine to isolate the pump? I,m at a loss here[/quote]
I didn't drain down, i just shut off the pump valves and changed the pump as the old one had seized, then i opened the pump valves back up, but there didn't seem to be water circulating, and the boiler was still cutting out after about 20 secs, which led me to ring a mate who actually told me the problem before i told him (as he just changed a gas valve on one last week). This led me to think it could be the valve, but before i get one and find it isn't the gas valve, i just wanted to pick your brains to see what you all thought first. Yes pump valves could be stuck, but i don't think so as they weren't tight, unless the heat from the old pump has fudgered them also. Pump runs continuously so if he's bled a few rads and pipes were iced up last week, this could have taken the old pump out?