I have been called in to a job to look at a heating system that has a serious pumping over problem. It is a two story house with boiler on ground floor, pump and three way valve in airing cupboard on first floor. The cold feed and vent tee into the common flow in airing cupboard. (seperatly both 22mm) The system has been flushed and shows no signs of any blockages, but as soon as the pump goes on in either heating or hot water it shoots out the 22mm vent pipe even when cold. I reckon I have been called in to sort a problem that never has been right since installation as the water in expansion tank was very brown. Any suggestions, I no it's the weekend but I would be gratefull for any comments. Will making the expansion pipe higher make any difference?