i currently have a vailant combi boiler which has run 10 radiators perfectly for tha last 9 years, i have recently built an extension which has 2 extra radiators. the plumber i used disconnected all the central heating when i first started and a couple of days ago reconnected the central heating i am now getting 10 radiators red hot and 2 cold and the plumber is telling me its the boiler, i have tried knocking all the hot radiators off to try and force the pump to pump to the cold radiators but to no avail.the radiators in question one is the nearest to the boiler the other is one off the new radiators upstairs, i think i am being ripped off by the plumber to the price of a new boiler.