Just moved house and the heating system doen't work properly.When heating has been on for about 10 mins, the boiler clunks and bangs and hot water is discharged into header tank, it then sucks air into the system. It's an open system, fully pumped with 3-way valve. I have checked the pump and flushed the pipework through, all radiators get hot initially but 3 upstairs collect air very quickly. A friend who 'knows a bit' says that the cold filler pipe and the vent are in the wrong place. Can somebody tell me where the filler and vent should be, in relation to the pump, and flow and return pipes. At present the filler is on the return and the vent is just below the pump on the boiler flow. I have now way of finding out if the sysyem has ever worked properly. Any help would be appreciated.