boiler mate ii & suprima 30

Hi I drained down a central heating system to install a new radiator , the problem I have is that initially the system would not fill up. I thought that i had solved the problem by filling up the system by using a hose filling up thru the filling and expansion tank this must have freed something up as after a while the system started filling thru this exp tank without the hose and by the ball valve . The rads bled as normal , when i fired up the boiler everything seemed to be in order until loud noises started to come from both the boiler and the Boilermate II ( like in the film Titanic). The flow in at the bottom of the Boilermate II is very hot and the flow from the Suprima 30 is very hot but the return back to the boiler only has a slight temp rise if any! Tried a few local plumbers but they didnt have a clue please please can anyone help? thanks Jimmy ps the hot tap runs cold