I have recently installed a new potteton supreme 50. a boiler engineer checked it and bled all system. i've got three rads downstairs and two upstairs and 1 in loft room. Every week loft rad gets cold i switch off and open bleed valve on loft rad "nothing happens" if I turn system on and open bleed valve after about 30mins of gargle it finaly gets hot."great" a week later bioler starts to make sum hissing sounds and loft rad is only hot at bottom again. all other rads are fine,"well dinning room rad is little cold at bottom, which is the largest rad, Bathroom rad too hot" all rads have temp control except bathroom. Ive checked expansion tank,Its fine. gas man says bioler is fine. All rads are new except loft rad. WHERE IS THE AIR COMING FROM!. Once the system started to bang and water hist out of pipe above boiler. "has not done that for some time now. seem to spend every week bleeding loft rad. I have can anyone help me!