When I was a kid, we were always having to bleed the upstairs convectors, which were tapped off the loop that ran through the large-tube Tuttle & Bailey baseboards downstairs. Before I moved out, I replaced the bleeders with float valves and all was well. It was only then that someone told me that the old ones were hygroscopic, and it occurred to me that we were probably over-tightening them. I'll bet a lot of people do the same thing and wonder why they have no heat.