Vaillants Combicompact VCWGB242EH?

I cannot comment on the flue fan running because my system does not have one. But my boiler also fires for a few seconds when a tap is opened and closed on the cold supply on the same mains pipe as the boiler. It appears that the pressure fluctuation on the cold main triggers the ignition system for a few seconds and then the boiler shuts down when it finds it is not needed. You could investigate from this angle. If the circulation pump keeps running for any great length of time this is probably not the answer. In my case it is just for 10 seconds or so after each firing and seems to cause no real problem. Hope this helps.

Robert
December 2008