A mate of mine has a problem with his heating system. It is a new build house with a Glow worm Ultimate boiler and a Gledhill vented cylinder. The heating system is run in microbore. After having reached operating temperature, and run for a short while there is a loud rumbling noise ( sounds like the roof is going to collapse !). When this happens the boiler cuts out and needs resetting, I checked the f & e tank and the level had risen by about 4/5 inches and was warm, not boiling as I expected. The CH system uses a 28mm combined feed and vent pipe, could this be causing the problem ?