I have just replaced hot water cylinder in the attic, connected all pipework including flow and return to baxi barmuda GF back boiler, filled cylinder up header tank full hot water taps are running fine, however the back boiler has an air block as I changed all valves on the radiators and before I fit radiators back I was going to flush out the pipework so water is nice and clean, however there is no flow now to raidiator pipes and I am sure the back boiler has an air blockage, how can I get this blockage free so water will run through pipework to rads, is there a valve to let air out on the baxi barmuda GF back boiler, HELP.