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Radiator vents hiss when system shuts down

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bwroga
bwroga Member Posts: 44
The steam system in my house seems to work well. All the radiators warm up and only one of them makes a clanking sound. :blush:

In "We Got Steam Heat", it says that if you can hear your radiators venting, it is your system crying for help. I don't hear them venting when the system is on, but when the system shuts off, I can hear air getting sucked in through them. Is this "my system crying for help", or is the book referring to an audible hiss when when the system is on and air is coming out of the vents?

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  • Jamie Hall
    Jamie Hall Member Posts: 23,313
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    Hearing air sucked in at a radiator when the system shuts off isn't really a problem... but it may indicate that the main venting isn't adequate, or is located in such a way that it cools less rapidly, and the air pulled in by the condensing steam has to come from the radiator vents instead.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    bwrogacross_skier
  • JUGHNE
    JUGHNE Member Posts: 11,062
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    A flame over the vent hole will show you in what direction the air is flowing.
    Probably vacuum formed on shut down.
    If your main vents cool and open that hissing would be at that location.
    bwroga
  • cross_skier
    cross_skier Member Posts: 201
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    Been there.  I had a Hoffman 75 on each of my 3 mains.  Dan suggested I add a 1/2" check valve on the header and that worked.  Soon after I added Gorton 1's and D's to antlers on the mains and found I no longer needed the check valve
    bwroga
  • bwroga
    bwroga Member Posts: 44
    edited January 2022
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    I turned the thermostat up a few degrees and went into the basement to look at the main vent. I was hesitant to hold a flame up to the vent, just because it is so close to the ceiling joists. The main vent was silent at first, but towards the end of the heating cycle it was making a loud hissing noise.





  • Jamie Hall
    Jamie Hall Member Posts: 23,313
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    Poor little thing. I'm sure it's doing the best it can, but... just isn't big enough. Also, what pressure is it being run at?
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • bwroga
    bwroga Member Posts: 44
    edited January 2022
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    :D
    The pressuretrol is set to 0.5 with a 1.0 differential. The pressure gauge says 0, but it's a 0 - 30 psi gauge.

    What type of vent would be a good replacement?
  • cross_skier
    cross_skier Member Posts: 201
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    Usually mains complain when pressure hits 2-3 psi.  
  • ethicalpaul
    ethicalpaul Member Posts: 5,702
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    When you say "at the end of the heating cycle" do you mean after the firing has stopped? Because in that case, there will be a large "inhale" of air into the system that is going to happen somewhere and wherever that "somewhere" is, it will usually be heard.

    That's not to say your main vent isn't undersized...that can't be judged by the restoration of air into the system after it's done making steam.
    NJ Steam Homeowner. See my sight glass boiler videos: https://bit.ly/3sZW1el
  • bwroga
    bwroga Member Posts: 44
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    Sorry, what I meant was towards the end of the cycle, after it has been running for awhile, before the system shuts off.
    ethicalpaul
  • cross_skier
    cross_skier Member Posts: 201
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    Then pressure is likely too high.  Adjust pressuretrol or vaporstat down.
    ethicalpaul
  • bwroga
    bwroga Member Posts: 44
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    I think the pressuretrol is set as low is it can go (0.5). Am I wrong? Should I lower the differential?
  • cross_skier
    cross_skier Member Posts: 201
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    If it's on a pigtail first check that it isn't clogged
  • Hap_Hazzard
    Hap_Hazzard Member Posts: 2,846
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    bwroga said:

    I was hesitant to hold a flame up to the vent, just because it is so close to the ceiling joists.

    I use a stick of incense instead. Great for finding drafts too.

    Not only is the main vent too small, but the location of the drip it's mounted on is baffling. It should be at the end of the main, but I'm seeing what looks like steam pipes coming from two directions. I'm not sure I can figure out what's going on from the pictures. You might want to get a pro out to your house and have them look over the piping at some point, but for now, get a main vent, like a Gorton #2, and mount it up on a pipe nipple as high as you can so it won't get clobbered by steam hammer.
    Just another DIYer | King of Prussia, PA
    1983(?) Peerless G-561-W-S | 3" drop header, CG400-1090, VXT-24
  • JUGHNE
    JUGHNE Member Posts: 11,062
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    Possibly contributing to your odd venting of radiators;

    Your main vent looks to be connected where the end of mains (EOM)
    join together.

    Steam travel in each main will never be equal. The shorter main will get steam to the EOM before the longer one does.
    When that happens the main vent will close.
    The longer main will still have air that will probably get vented out the rad vents.
    And on shut down the vacuum will create some hissing at rad vents.

    Each EOM pipe needs to drop down to the floor separately into the wet return pipe at the boiler. Each EOM needs it's own air vent.
    The 2 EOM's must be separated by their own water loop.
    This is created when each drop down to the wet pipe.

    Your near boiler piping is quite interesting.
    Amazing what can work fairly well.
    cross_skier
  • bwroga
    bwroga Member Posts: 44
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    If it's on a pigtail first check that it isn't clogged

    Is this something that is safe to try to do myself?
  • bwroga
    bwroga Member Posts: 44
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    Here is a map of the steam pipes in the basement:



  • bwroga
    bwroga Member Posts: 44
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    JUGHNE said:

    Possibly contributing to your odd venting of radiators;
    Steam travel in each main will never be equal. The shorter main will get steam to the EOM before the longer one does.
    When that happens the main vent will close.
    The longer main will still have air that will probably get vented out the rad vents.
    And on shut down the vacuum will create some hissing at rad vents.

    Does the fact that I heard such a loud hissing coming from the main vent indicate that it isn't closing when it should?

    Is it possible that since the main vent won't close, the pressure can't get high enough to trigger the pressuretrol?