I've been having this same issue with my Vaillant Mag 325. The problem turned out to be a bad Diaphragm. It had a hole in it so the water pressure was not pushing up on it as it should so it didn't sense that there was enough water flowing through it. I repaired it for now with a rubber glove and ordered a new diaphragm from www.heating-parts.co.uk. Hope this helps.
J
July 2008
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