I have a very badly installed (apparently by a HETAS qualified engineer!) oil fired central heating system. In order to resolve the problem with the vent for the system coming off the return, and travelling most of the way around the house, I would like to convert the system into a sealed non-vented system. Is there any reason why I cannot do this with the current boiler? Is it the case that I simple replace the expansion tank with an expansion vessel, and provide a means of pressurising the system from the mains?