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One pipe radiator on a 2 pipe vapor system

I'm working on a vapor steam system in a 3 story house which hasn't been maintained in 15 yrs. The homeowner called us and said the 3rd floor radiators weren't heating and the other half of radiators weren't heating. The 3rd floor radiators were added over 20 yrs ago and they are 1 pipe radiators with vents at the radiators. The other radiators in the house are 2 pipe with traps.
I broke open the union on the main return after the thermostatic trap and before the air eliminator to drain the return to see the return was clogged. Rusty water came out and ALL the radiators started heating. We dialed the PRESSURETROL down as low is it would go, cleaned the Return Trap and Air Eliminator. We checked the traps at the radiators and found they were good, no steam passing through.

We put the union back together and cycled the system. The 3rd floor radiators never heated back up and the other half of the radiators were cold again. I took the air vent off the 3rd floor radiator and blew in the radiator and could hear condensate gurgle down and steam started to flow to that radiator.

I suspect the PRESSURETROL is the problem since I've backed the pressure out till the screw fell out. What else should I check, water level of loop to boiler level-what dimension should that be? (Pitch of lines is good, check valves are working). I can get the radiators to work when the end of return main is open to atmosphere. But as soon as the system is closed it stops working. The owners said the system worked when they bought the house 15 yrs ago but didn't do any maintenance.

Second, what are the options for the 3rd floor can 1 pipe radiator be put on a 2 pipe system and work?



  • JStar
    JStar Member Posts: 2,752
    Sounds like water is being trapped in the supply piping to that radiator. The higher pressure would eventually move the trapped condensate. You did the same thing by blowing into the radiator.

    It should be able to work, as long as the pipe sizes are correct.
  • Jamie Hall
    Jamie Hall Member Posts: 22,915
    If it works when the end of return main is open to the atmosphere, but not otherwise, two things come to mind. First, is there any venting on that end of return main? Having it open to the atmosphere is the same as having a nice big vent on there. Second, is there any way that boiler pressure can get into that line? Are there loop seals, perhaps, which aren't sealed any more due to a change in water level? A wet return somewhere which isn't wet any more for the same reason?

    There is no reason why a one pipe with air vent radiator shouldn't work on that system -- provided that the condensate can drain back and the steam can get to it.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England