hi dave thanks for the info . i,m a chippy and not to clever with plumbing speak, is the cold pipe you mention the one that goes from exp tank to the boiler
we have a potterton suprima boiler which is located in the airing cuboard,when the full system is on we can hear the water flowing round,when the rads are off we can hear the water running in the airing cuboard only louder.is it air? should there be some sort of automatic vent in the airing...