First floor radiator keeps spitting. It isn’t leaking – water is spewing through air valve. Just replaced it, which seems to have improved it, but not solved it. An cellar examination reveals that it is at the end of a straight shot from the boiler, with 2″ pipes reduced to 1″ about five feet from valve. Other radiators on second a third floor don’t have smae problem, but they are higher up and there is a 90 degree branch where their pipe branches off (and even so they are markedly hotter than front risers). Thoughts? Wisdom? Sarcastic coments?