I live in France but have not had any success finding an answer here.
Irregularly the boiler fails to restart when the water temperature falls. This happens more and more often. The hot water from taps works ok. Extraordinary but if one taps the middle of the lower panel the boiler starts up with a jerk. Sometimes also the start button when moved slightly has an effect. I have noticed a small spark at the bottom near the panel end??
What I have done so far: professional have been called in and have changed, the thermistors, the fan, the thermocouple solenoid -successively in different visits. I have cleaned - not very professionally - the printed circuit board. Maybe its that or the start button, but I am reluctant to go out and spend more money if that is not the reason.
If any one can help I would be most grateful

David Steele, February 2007

It's a dry joint on the PCB. I had a similar fault with the boiler stop-starting, so I dropped down the control panel and removed the back of it to reveal the PCB (it's held on by around 6 screws if I remember correctly). I was lucky that the fault occurred straight away and I could see where the sparking was happening, and was able to easily resolder the joint. This completely cured the fault.

Be careful when removing the back of the panel where the controls are, since it will be live with 240v. I would remove the power to the boiler while you are taking it apart, and only power it back up when you've finished getting the cover to the pcb off. Then restart it & fire it up and watch what happens on the PCB.

Gary, May 2007

Check the PCB transformer soldered connections, looking at the rear of the PCB the bottom left connection could be dry and giving erratic switching

alan, March 2007

The problem you have David, is none of the previous "repairs" where needed. you have a "Dry joint" on the pcb. usually the relay switch. replace board and no more problem..............

clfgas, February 2007