In message , Christian
McArdle writes
British Gas have changed the expansion chamber, the sensor, the
room thermostat and various electronics boards. Each engineer
thinks he's solved it but they haven't.

But at no point have they replaced the diverter valve?

Seems odd, as this behaviour screams diverter at me. I don't know the
boiler, though. Perhaps it doesn't have a conventional diverter.


If the boards hadn't been changed, I would have suspected the relay on
the Zagas 126 (sequence) board which drives the fan - a common failure

Yes, it could also be e.g. a knacked microswitch in the diverter valve.
As to whether it's a conventional 3 port valve or not, I can't remember
offhand

--
geoff