The water in header tank nearly explodes hot after you can hear strange noise probably from the pump. I changed the pump, drained water from the system cleaned it, tried to put some cleaning chemical stuff, not solved. 30 min after turning the boiler on water gets to the header tank very hot (steamy) and condenaste on the ceiling. I checked all rads bleeded them, they full of water, the pump (you can hear) is working nice till 30 min, after starts making noise, it is very hot.

wvolvo, October 2009

Pumps PULL water not push -are you sure the pump is fitted ( see arrow on body ) to pull water from the return side of the loop ( going back to the boiler
I know you said the radiators appear free of air but I woul try draining water out of the stopcock - usually beside boiler
As this is a so very dangerious fault and recently a child was scalded by a tank bursting in a loft above the bedroom - call in a plumber

pc6493, October 2009