rick wrote:



Anybody experienced such a fault €“ thought I would ask as this is
dragging on for so long, about the only items not changed are gas valve
and fan.


Many moons ago we had and Ideal condensing boiler that started lighting
explosively. Turned out to be a faulty gas valve that was letting gas into
the combustion chamber before the "pilot igniter" had lit. This boiler had
a two stage gas valve, the first stage admitting gas to an electronically
lit pilot and then the second stage is supposed to only open once a flame
has been detected in the pilot light.

Does your boiler have a similar system? I suspect that they've all moved
over to "single stage" ignition of the main burner but you never know...

Tim

--
Please don't feed the trolls