Our header tank split and emptied overnight which I didn't discover until the following morning when the heating failed,the boiler lights showed that it had overheated and switched off.I installed a new header tank but the boiler now operates for 50 seconds and turns off with a knocking sound that suggests to me air in the system.Bled all radiators to no avail.( Christmas ruined)

Terry, December 2009

depending on which version you have there is either an auto air vent or a manual one which is on the return pipe right hand side. but make sure your pumps working

darren, December 2009