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  1. #1
    Join Date
    Nov 2009
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    16

    One-pipe system, steam radiator air vent question

    Sorry for a question that will seem very basic to many...

    I have one-pipe steam heating system. Should a radiator air vent ever let out very hot air (or steam)? It happens like this: shortly after the boiler starts, air vent lets some air out, radiator gets warm, air vent closes. If the boiler keeps working for a longer time (~30 minutes) and the radiator gets really hot all the way, the air vent seems to open again and starts letting out very hot air until the boiler shuts off. Is the air vent malfunctioning?

    Thank you

  2. #2
    Join Date
    Jul 2008
    Location
    Poestenkill, NY
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    769
    I would say that it is, especially if you ever get actual steam coming out.

  3. #3
    Join Date
    Nov 2009
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    16
    larobj63, thanks for your reply. Just for my education, can you give me an idea of what kind of malfunction might that be? I mean, if the vent was clogged, it wouldn't close at all, or would close inconsistently, but this one closes promptly when steam hits it. It seems to open again when the temperature inside radiator gets really high (or stays high for a relatively long time).

  4. #4
    Join Date
    Jul 2008
    Location
    Poestenkill, NY
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    769
    Well... It would be pure speculation, but I'll give you a thought at least, based on what I know about steam traps. And, we'll talk in general terms because there is more than one type of trap...

    Steam traps generally work in such a way that when heat hits the trap, something expands and pushes a little steel piece into an orifice, closing it. This design exhausts warm air, but keeps hot steam. The something that expands can either be a bellows, or a parafin wax.

    Now, this something that expands may only be really effective when it is hotter than it's surroundings, because the expanded medium is pushing something closed, but working in an environment that needs to stay the same size, or it won't push anymore. In other words, I suspect that when the entire trap is hot, like after a 1/2 hour of full heat, the expandable medium is no longer effectively pushing the oriface closed, because whatever the expandable medium is incased in is also really hot, and slightly expanded.

    Now, when the bellows or wax (or trap in general) was new, it might have worked. But after hundreds of heat cycles, it is failing when the whole trap gets hot.

    Food for thought at least. Whatever the reasoning, the symptom of a failed trap is letting steam go by, so....

  5. #5
    Join Date
    Nov 2009
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    16
    Quote Originally Posted by larobj63 View Post
    Now, this something that expands may only be really effective when it is hotter than it's surroundings, because the expanded medium is pushing something closed, but working in an environment that needs to stay the same size, or it won't push anymore.
    Yes, this sounds reasonable, thanks.

  6. #6
    Join Date
    Nov 2009
    Posts
    16
    Out of curiosity, bought a new vent, same brand and size, and it did exactly the same thing. Wondering if this can be caused by the size of the vent hole being too big for the radiator, but don't see how this works...

  7. #7
    Join Date
    Sep 2002
    Location
    Northeast Ohio
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    4,842
    Time ton stop guessing and call a steam hydronics expert before you scald yourself or loved ones
    A good HVAC tech knows how, an educated HVAC tech knows why!

    DEM


  8. #8
    Join Date
    Nov 2009
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    16
    Quote Originally Posted by heaterman View Post
    Time to stop guessing and call a steam hydronics expert before you scald yourself or loved ones
    Thanks for the advise, I will do that.

    However, can you explain how and under what circumstances a new air vent can work like that? I searched the Internet, and it's nothing, like that can never ever happen.

  9. #9
    Join Date
    Oct 2004
    Location
    Harrisburg, Pa
    Posts
    56

    Sounds like you have a problem.

    Quote Originally Posted by ALex265 View Post
    Sorry for a question that will seem very basic to many...

    I have one-pipe steam heating system. Should a radiator air vent ever let out very hot air (or steam)? It happens like this: shortly after the boiler starts, air vent lets some air out, radiator gets warm, air vent closes. If the boiler keeps working for a longer time (~30 minutes) and the radiator gets really hot all the way, the air vent seems to open again and starts letting out very hot air until the boiler shuts off. Is the air vent malfunctioning?

    Thank you
    On one pipe steam systems the radiator vents open to release cold air from the radiators and close when hot steam makes it into the vent. When the steam condenses in the radiator, it cools. The cooler air enters the vent and the vent opens allowing air to exit and fresh steam to enter the radiator. This open and close process takes place many times throughout the normal heating cycle.
    If your vents make loud noises (whistling or sounds like the radiator is panting) or steam is found coming from the vent you have a problem. The issue could be a bad radiator vent ,a main line vent failure, piping issue, steam pressure set too high, or to an over sized boiler. In any case you need a qualified Tech to look at your situation. Poorly operating steam systems can raise fuel consumption considerably. Excess steam pressure can be down right dangerous.
    Steam systems are remarkably simple but extremely finiky.
    Dont mess with this yourself.

    Good luck

  10. #10
    Join Date
    Nov 2009
    Location
    Saskatchewan
    Posts
    492
    You can buy them with a dial to set the room temp. They have a spring that works against the temp regulator. They do work.

    The odd hiss on a standard steam vent, on the first start of the heating season, is ok. Not if the boiler has ran for a few days though.

    They are bellows / heat active. Dirt really messes them up. So does carry over from high TDS. You did check TDS? The hole size has nothing to do with it.

  11. #11
    Join Date
    Jun 2006
    Location
    New Jersey
    Posts
    4,330
    Quote Originally Posted by ALex265 View Post
    Sorry for a question that will seem very basic to many...

    I have one-pipe steam heating system. Should a radiator air vent ever let out very hot air (or steam)? It happens like this: shortly after the boiler starts, air vent lets some air out, radiator gets warm, air vent closes. If the boiler keeps working for a longer time (~30 minutes) and the radiator gets really hot all the way, the air vent seems to open again and starts letting out very hot air until the boiler shuts off. Is the air vent malfunctioning?

    Thank you
    This normally caused by the steam pressure being over the rated PSI of the vent. Have your boiler checked out by a qualified contractor. THAT KNOWS STEAM.

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