Hi there, I've got a hundred year old 2 1/2 story house with a hot water radiator system, which has a circulation pump. I'm guessing that pump was added somewhere along the line. All of my radiators work fine except for the third floor one. It doesn't appear to heat at all, although the third floor is surprisingly not that chilly. A while ago to test it out I turned the thermostat all the way up, and when I did that, one of the pipes going into the floor got very hot, up until the L bracket. But under normal circumstances it seems to be the same temperature as the rest of the rad. Oh, and the one difference between this rad and the rest of the house is that this has both pipes going into the floor on the same side of the rad, in the rest of the house all the rads have a pipe on each end. The system pressure is set to 19psi. Anybody have a clue how to fix this?

Make sure there is no air in the rad. There should be a small valve for letting out the air. Air will prevent the water from flowing in that rad. My house is the same. The air which the system naturally produces seem to only go to the one rad. Because the lines are all in parallel there is no force to move the air along and besides it is downhill from there.