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Two Pipe Steam System with Air Vents

MPL15654 Member Posts: 45
I have a customer who's been having issues with her heating system since she moved in. She wants to replace her boiler because her existing one is 30 years old. She has a lot of issues with some radiators getting hot, some won't get hot at all and it seems like it's also a balancing issue. Upon looking at the system i realized she had a combination of 1 pipe and 2 pipe steam. The 2 pipe steam has radiator valves at both ends, air vents, and no traps. The really strange thing is the radiators that she has the problems with are the "2 pipe system" and it's piped very strange. The returns come from the radiator, drop down to the floor of the basement and then loop up and tie back in the steam main. That doesn't make any sense to me, i thought for sure that the condensate return would need to tie back into the return on the boiler, and they don't. I really don't want to change her boiler unless i get this resolved and i feel like these pipes need to be disconnected and re-piped into the return of the boiler for it to work properly. Any thoughts on this? Thanks


  • Jamie Hall
    Jamie Hall Member Posts: 23,276
    That one's been around

    for awhile.  But it's described in The Lost Art, of which I hope you have a copy.  What makes it a bit strange is the water seal in the "outlet" side of the "2 pipe" radiators (I like to think of them as 1 1/2 pipe...).

    You have a really critical dimension here: the height from the steam main, where those returns tie in, in relation to the radiator.  On first thoughts (it's late at night...) you may have difficulties if that distance is less than about 28 inches for each pound of steam pressure.

    I'll have to think about this one some more.  Meantime, go look it up in Lost Art...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    STEAM DOCTOR Member Posts: 1,966

    Cant say that I have ever seen something like what you are describing. Maybe the piping between the return side of the radiator and the main is supposed to be some sort of steam trap. Any piping below the main will be filled with water. This will in effect, create a steam trap. The water above the main will return through the main IF(as Jamie said) there is enough height in the piping to create enough pressure to overcome the steam pressure in the main.

    Try removing the air vents and see what happens.

    This system might be a good candidate for a vaporstat(to ensure low steam pressure in the main).

    You could also try to put an air vent in the return piping above the height of the steam main(you just need to drill and tap a 1/8" hole). This will give the benefit of steam pressure in the return to overcome the pressure in the steam main.
  • JStar
    JStar Member Posts: 2,752

    The return piping definitely forms a water-seal trap. The air vents on the radiators will now act as the only means of air release from the radiators, so it's critical that they are sized and working properly. This piping arrangement should not be a problem as long as the water seal is low enough to hold bback the pressure from the steam main. Definitely install a Vaporstat on this job to make sure that this will be guaranteed.
  • MPL15654
    MPL15654 Member Posts: 45

    It's really strange like i said, the returns drop down to the floor and than loop up and tie back into the steam main. The basement isn't finished but the only thing i was thinking was maybe originally these lines were running along the floor (or underground) and then were tied back into the return of the boiler. I don't see any unions or newer looking fittings, but the house is over a 100 years old so this could have been done 30 years ago. And what i'm trying to figure out is if i should tell these homeowners that i need to cut out where it ties back into the steam main and plug that tee and than run those returns back to the boiler with new piping.
    STEAM DOCTOR Member Posts: 1,966
    Step by step

    The return configuration wont affect the heating unless the condensate backed up enough to block the inlet piping of the radiator. It is possible but not likely(famous last words).

    You have to attack this step by step. First step is to remove the air vents and fire the boiler.If you don't get heat, then try to crack a union by the radiator inlet. Be careful. Water or steam may come out.

    You could replace the returns. However, it might not be necessary any may not cure any problems.

    If the returns are backing up and blocking the radiators then a vaporstat and hefty main venting might be a better idea.
  • Charlie from wmass
    Charlie from wmass Member Posts: 4,322
    keep the pressure

    low with a vapor stat. check the returns and make sure they warm up as if they do not it means they are clogged. If they are clogged clean them if they used tees on the bottom or install tees to clean them out. There are ways to do things right you may have not seen but they work, the water leg works and I have used it several times even on other styles of systems. It is helpful when stopping baseboard from banging when it is piped into a pipe system.
    Cost is what you spend , value is what you get.

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