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Water in radiator----puzzling!

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I live in a high-rise apartment building with steam heat, two-pipe system. When I turn "off" a radiator, I can hear water flowing inside the radiator for several minutes. Plus the return pipe gets super hot. When the radiator is the "on" position, I don't hear water inside. And the return pipe does not get hot, which is as it should be. I have apartments above me and I have a warehouse below me.

What's going on? Is there a way that steam is somehow getting into the return pipe (besides condensed water at the end of the cycle)? How can that only happen when that radiator is turned off? Could a bad trap upstairs trigger this? Or back pressure from down below?

Very puzzled. Thank you for your response.

Comments

  • ethicalpaul
    ethicalpaul Member Posts: 5,729
    edited February 2023
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    Don't turn off the radiator with the valve. Leave the valve fully open all the time. If you want to turn off a radiator, rotate the vent upside down or cap it.

    Thanks Jughne and sorry Saldanah, I missed it was a two-pipe

    And please don't post the same message in multiple forums, it makes @Erin Holohan Haskell 's job harder
    NJ Steam Homeowner. See my sight glass boiler videos: https://bit.ly/3sZW1el
    mattmia2Saldanah
  • Saldanah
    Saldanah Member Posts: 8
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    Yes, sorry ... I had meant to post here and couldn't figure out how to delete the other post. Lesson learnt.

    Isn't it customary to use valves to turn one or more radiators off when it gets too hot inside? I was just curious about how turning off the valve causes water to flow inside the radiator and the return pipe to heat up. Both of those outcomes seemed at odds with my understanding of how the system is meant to work.
  • JUGHNE
    JUGHNE Member Posts: 11,075
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    Paul, two pipe system.

    Sounds like steam in the common returns.

    Poster; does the return end of the rad get hot when valve is off or just the trap?
    ethicalpaulmattmia2Saldanah
  • Saldanah
    Saldanah Member Posts: 8
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    Thanks. The return end does not get hot when the radiator valve is shut. So the steam is getting into the return pipe from elsewhere ... can't figure out how steam is moving from the supply to the return. Plus how there is water coming into the radiator when it is off.
  • mattmia2
    mattmia2 Member Posts: 9,735
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    Do the radiators have steam traps? If the system has traps then one or more of the traps on the radiators or the mains is failed open. If it is a vapor or some other non-conventional system then we will need more information about the system. A picture of a radiator and the near boiler piping would be a start.
  • Erin Holohan Haskell
    Erin Holohan Haskell Member, Moderator, Administrator Posts: 2,325
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    Don't turn off the radiator with the valve. Leave the valve fully open all the time. If you want to turn off a radiator, rotate the vent upside down or cap it.

    Thanks Jughne and sorry Saldanah, I missed it was a two-pipe

    And please don't post the same message in multiple forums, it makes @Erin Holohan Haskell 's job harder

    I've merged both posts into one here. Thanks!

    President
    HeatingHelp.com

  • Saldanah
    Saldanah Member Posts: 8
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  • Saldanah
    Saldanah Member Posts: 8
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    I do not have a picture of the boiler piping. The return pipe is just a riser that runs from the floor to almost the ceiling of the apartment.
  • bburd
    bburd Member Posts: 923
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    I hope that radiator is supported from the wall structure, and not just hanging on the pipes!

    Bburd
  • Saldanah
    Saldanah Member Posts: 8
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    There are plates that transfer the load to the walls.

    My original question still is a puzzle. How is steam getting into the return pipe when--and only when--this radiator is turned off?
  • mattmia2
    mattmia2 Member Posts: 9,735
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    The steam is always in the return and it is coming from a failed steam trap elsewhere in the system.
  • Saldanah
    Saldanah Member Posts: 8
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    Thanks, but that is the puzzling thing. When this radiator is turned on, the return pipes do not get hot so evidently there is no steam passing through the pipes. When the radiator if off, the pipes get hot.
  • mattmia2
    mattmia2 Member Posts: 9,735
    edited March 2023
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    When the radiator is on the steam from the supply closes the steam trap on the radiator and traps air in the return so the steam that shouldn't be in the return but is can't get in to the radiator.

    When the radiator is off the pressure in the return can compress the air in the radiator enough to allow the steam from the return in to the radiator where it condenses and pulls more steam in.
    bburd