We have just spent a small fortune having two replacement pumps fitted to the central heating system (one above the boiler and one next to the hot water cylinder) and having the system pressure cleaned. Now we are back in the heating season we find that every time the heating fires up (not the hot water on its own) then the overflow from the feed & expansion tank gushes until a radiator is bled (which can take up to 10 minutes). Our latest plumber says this is an inherent fault in the system, drawing in air due to the negative presuure at the tank, and the only real solution is to reconfigure the pipework with a single pump. True or false? Is there any other way of improving the situation, by installing self bleeding valves say?