Radiator problem

Shep

I live in a ranch home with hot water baseboard heating. (Cast iron radiators) My problem is I have 2 radiators on 1 line of the loop that won't stay warm. The rest of the radiators on the system stay hot. I've bled this particular radiator for hours. It is also the farthest one and the longest one in the system. From the main suppy loop to these there is a 6" rise, then a 12' copper feed line, then a 8" rise thru the floor to the long radiator which then feeds the 2nd smaller one, then completes the loop back to the main trunk line. When I bleed it I get cold water for a while, then the smaller 2nd radiator gets hot first and will back feed from the return end of the loop. The larger one will then get hot after bleeding it,but will get cold soon after I stop. I replaced the air reducer valve on the boiler and tried bleeding it in the "quick fill" mode. I've heated the suppy line to this radiator with a torch (MAAP GAS) to warm the water, but it seems  it is like" vapor locked" or the system won't push the hot water up thru this line up thru the radiator. I know the line is filled because I get a constant flow of cold water, not air, when I bleed it. Have you encountered this before? Could you offer a solution before I go crazy!! Thanks

icesailor

Radiator problems:

The piping that comes from the boiler and ends up back there, is it one size that goes around the perimiter of the under house and has seperate smaller pipes rising off this main pipes? Do all radiators have this piping arrangement?

Was anything done in the recent past where someone moved a radiator and reconnected it?  If the main pipe (the "Main") is copper, where the two pipes going to a radiator, does one copper tee look different from the other? Does one have a groove around the main body often painted red? If the "main is steel, are both the tees on the main that have risers going to the radiators the same in appearance? Is one different? The two radiators that do not work, are they connected together with only one take off on the main to one radiator and the other radiator is connected to the main?

Check this and post pictures if you can. It's important to see the tees and the connections.

icesailor

Radiator problems:

The piping that comes from the boiler and ends up back there, is it one size that goes around the perimiter of the under house and has seperate smaller pipes rising off this main pipes? Do all radiators have this piping arrangement?

Was anything done in the recent past where someone moved a radiator and reconnected it?  If the main pipe (the "Main") is copper, where the two pipes going to a radiator, does one copper tee look different from the other? Does one have a groove around the main body often painted red? If the "main is steel, are both the tees on the main that have risers going to the radiators the same in appearance? Is one different? The two radiators that do not work, are they connected together with only one take off on the main to one radiator and the other radiator is connected to the main?

Check this and post pictures if you can. It's important to see the tees and the connections.

Shep

reply to Icesailor

Thanks for your reply. To answer your q's, the "main" trunk line is all 11/4" copper. All the feeds are 3/4" copper. None have the grooved red stripe.(Please explain this type of fitting) 

The line was opened in the summer to install a "Super Vent" in the return at the boiler. I have tried running the system with the Super Vent both closed and opened, thinking that I was loosing pressure with this opened and there wasn't enough pressure to push the water thru the feed. These 2 radiators are fed from the larger to the smaller then back to the main. They are in the middle of the series of radiators. The room next to and before this room in line is set up the same way with about the same total pipe length. It is working fine although I do notice a "moving water" noise when the system starts up. I've bled this line and am getting straight hot water from it. Could the system be starving for water or is this a natural noise? The basement is finished so it would be hard to get pictures of the whole system but I was able to determine that all the system is the same.