I know, i know...this problem has been posted time and time again but i just can't find my specific problem. i'm using a gas-fired back boiler and durning heating phase pipes start banging; a lot. i live in a convereted bungaloo and when i turn on the attic radiator the downstairs rads don't heat, if i close attic radiator, they do. no airlock in rads or pump. and pump is working fine. it sounds like there is a lot of air in the system but the expansion arrangement is working. Know issue: the pump is not connect to an overrun circuit and i'm sure this caused the problem and when the overrun circuit was removed the expansion pipe would expel hot water and steam once the boiler stopped but that doesn't happen any more. how can i rectify this? why is the expansion tank not topping up the system?