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Air vent on two pipe steam

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Hi,

I have a two pipe steam system.  There is a large, 6 foot long radiator in my living room.  This radiator has an air vent on it (C).  The vent is placed just above the return pipe (not at the top of the radiator). 

I'm assuming someone put this vent on because the steam trap went bad and they didn't know what they were doing.  Two pipe systems are not common in my area.  All the literature I have read says there should never be radiator vents on a two pipe system.  Has anyone found a reason to put one on?

The only thing I can think of is it may have been put on to heat the radiator faster (again, it is a large radiator) which would more be to cover up a pipe sizing issue than anything.

Nick

Comments

  • Jamie Hall
    Jamie Hall Member Posts: 23,323
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    It would seem to me that if the trap were actually failed closed that condensate would not drain. Which suggests that it may have been in an effort (probably futile) to get it to heat faster. It is also possible that there is a problem with the return line, however, which prevents air from leaving. Can you locate and follow the return line to see if there is no low point -- even a sag -- which might create a trap effect?
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    nlongworth2
  • nlongworth2
    nlongworth2 Member Posts: 11
    edited January 2022
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    Hey @Jamie Hall,

    Thanks for the quick response.

    Return line is good.  It is short, boiler is very close to this radiator. 

    About the steam trap, interestingly I did have a trap in this house which actually broke (piece snapped off inside) and a piece of the trap was blocking the line.  It slowed everything down but didn't actually stop anything.  That was a much older trap though, it was hidden in the ceiling of the basement and it looked like they missed it when they put new traps at the radiators at some point in the lifespan of this system.  There were two traps on that line before I removed that old one.

    So no reason in your mind either to have the vent if the trap is working correctly?

    Nick
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 15,540
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    @nlongworth2

    There was only one type of two pipe system that used air vents and that system had no traps on the rads.

    It was known as a "Two pipe air vent system" and was pretty old and rare. On trapped systems the supply pipe and the return pipe sizes were quite different maybe 3/4 and larger supply and the returns are usually 1/2" at the trap at least.

    Two pipe air vents were usually like an 1 1/2 supply and 1 1/4 returns
    nlongworth2
  • nlongworth2
    nlongworth2 Member Posts: 11
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    Hey @EBEBRATT-Ed,

    I definitely have a more modern system.  3/4" feed and 1/2" return.  I've heard of those older systems though.  I don't think my house ever had that.

    Nick
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 15,540
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    @nlongworth2

    then you shouldn't have an air vent there
    nlongworth2
  • Steamhead
    Steamhead Member Posts: 16,842
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    @nlongworth2 , can you post some pics of that radiator, the traps you found in the basement, and any devices in the piping around the boiler, and a view of the boiler itself? This will help us ID your system. Also, where are you located?
    All Steamed Up, Inc.
    Towson, MD, USA
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
    Consulting