can anyone help me ive got a vented old system and suffer from venting (heating the refill tank) the pump looks ok and is working, the hot water-central heating and hot water valve works ok. ive bled everything but when you put heating on even on hot water only it just bangs and vents to the refill tank and you can hear water falling back from the pump causing airation. is this likely to be a blocked, scaled up heat exchanger? any help would be gratefully appreciated. thanks. its a thorn apollo 15/30b boiler