Hot water rad is warm, not hot

RadNewbie

Every few years, one hot water rad on second floor stops working well. I suspect it is because sagging floors have supply line running downhill...but not sure.



Bleeding the rad for half an hour succeeds in heating up supply pipe at problem rad and warms the upper end of the fins pretty well, but return pipe still seems room temp at best.



What I can't figure out is, I had a contractor address the problem heater on the second floor by adding a shut-off and spigot in the basement to the supply line leading to the problem radiator. At the time, I thought, "wouldn't you want to install the shut-off and spigot on the RETURN line so that any possible bubble could be forced out by draining at the return spigot  for the 2nd floor problem radiator down in the basement?" He replied that it didn't make any difference, because it was a loop: supply ==> rad ==> return. So even if you install the shut-off and spigot on the supply line, the water would run thru the entire system and force out any bubble. It seemed to work. He asked me to shut off the supply line going to the upstairs rad and we ran a hose from the spigot to a floor drain. He did something at the boiler and told me when to shut the spigot (to close the system once again) and turn on the supply flow again. Now I'm trying to figure out what he could have been doing at the boiler so that I know when to turn off the spigot for the supply line feeding the disabled rad -- and turn back on the supply shut-off to the disabled rad.

Would he have been opening or closing the reducer-valve that feeds the boiler? (Is that what you call the valve that ensures you have the correct psi going to the boiler?)



Another thing I can't figure out tho, is that when I ran the boiler and bled the problem rad for at least a half hour, both the supply and return lines in the basement were hot! I'm 99% sure that those two lines are the ones that lead to the problem rad -- but how could they both be hot if the rad is only warm in the upper half and the return pipe at the rad is only room temp after bleeding it for so long?



Sorry for being so long-winded.