Hello to everyone! I have a two-pipe oil-fired hot-water boiler system in a two story house that uses radiators. The radiators on the first and second floor heat just fine, however, there are two radiators in the basement that will not heat. Each radiator comes off of the supply using a monoflow T, and returns back to the supply (not the return) using a monoflow T. Both radiators are one of the first to come off of the supply. The monoflow Ts are installed correctly. The radiators are always ice cold. The pipes from the main running horizontal over to the radiators are warm, but nothing goes down the verticle. There are bleeders right at the elbows going down to radiators. I have bled the radiators and have no air. Now once in a while when the boiler is under a heavy load, one of the radiators will get hot, the other one remains cold. However, after trying to bleed them on last time (I have a manual fill for the boiler) I must have put too much water in the system because the next morning, the pressure was up around 30+ psi, but guess what...both radiators were hot! Why will they work at the extreme pressure but not at 12-18psi? One thing is that the monoflow Ts are only separated by about 6 inches or pipe. What can I do to get flow through these radiators?
A. Remove the pipe between the mono tee's and replace it with the next size smaller.
B. Replace your current circ, with one that is capable of over coming the resistance of pushing hot water down.
C. Pipe them up as their own zone, with their own circulator.
Increasing the pressure keeps the air in solution and flowing with the water. when you lower the pressure air comes out of solution it sounds like you are getting airbound. you need to look closer at the piping. Are both the bleeders at the highest point of the piping on the radiator supply and return?