Regarding our combi boiler?

I had a similar problem with my Worcester Bosch Heatslave boiler. Check your overflow pipe. It is likely that pressure is building owing to a blocked pipe to the expansion tank. When this occurs the sealed central heating system jettisons water when the pressure builds during heating. If it cannot expand into the expansion tank it will reduce the pressure by venting it via the vent pipe. Put something under this pipe to collect any water to prove that this is happening. Simple to cure. Remove flexible pipe to expansion tank and remove blockage. It worked for me.

ChrisD
January 2011