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Hello again, SJB060685.
Photocell !!! That was the problem. Once you'd suggested the Control Box and Photocell as being common problems on the Riello units, I sent a text to my engineer.
He was doing his own research too, and enthusiastically returned two days ago, quickly changing the photocell. That did the trick! However, not before he'd tried a couple
of photocells . . . one being analogue and the other being digital. I'm not sure as to which one he ended up fitting, but I'll find out hopefully.
Thanks again, SJB060685. Hopefully this thread will help others too.
Best wishes.
Alan.

Glad it's sorted. The Riello burners use analogue and digital control boxes, the correct is required for each type.
 

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