You say that you have refitted a new rad removed old for new(yes) how did you bleed your system? you should all ways start on your top floor with a sealed system,if not sealed start on the bottom floor this will stop air locks? also you should check your pressure gauge it should be between 1 bar 1.5 bar after bleeding there could be reset on your boiler? look at your instructions some times this is the case. good look
steve
October 2007