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Radiator Hissing

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HotRod1975
HotRod1975 Member Posts: 16
in Strictly Steam

When I turn my boiler off I have one radiator on the second floor that hisses very loud like an engine for 10-15 seconds. Then goes silent is there an issue? Or what I can do to fix

Comments

  • dabrakeman
    dabrakeman Member Posts: 819

    Moments after the boiler shuts off vacuum is created in the system from rapidly collapsing steam. This is normal. Air is going to reenter replacing the steam in the system through the easiest paths available which tend to be the largest venting orifices in your system. You would like to think this would be the main vents in the basement but I too have the same situation where because the main vents have not reopened I get the post cycle hiss from radiator vents. I can say it would normally be from the largest radiator vents that you have installed that you would get the majority of the hiss. If it is occurring in a bedroom or somewhere where it is more annoying you could consider downsizing the vent in that room which will often move the issue elsewhere. Smaller radiator vents in general I think would help mitigate the issue. Perhaps others on this forum might have some further insight.

  • ethicalpaul
    ethicalpaul Member Posts: 7,981
    edited November 11

    I think your insights are right on the money @dabrakeman. I would say check the main vent (in the basement) to ensure it is operational and not failed closed.

    NJ Steam Homeowner.
    Free NJ and remote steam advice: https://heatinghelp.com/find-a-contractor/detail/new-jersey-steam-help/
    See my sight glass boiler videos: https://bit.ly/3sZW1el

  • HotRod1975
    HotRod1975 Member Posts: 16

    it’s a maid o mist 6 on a second floor.

    The Gorton 1 on long and short mains hiss at the end also but I thought that was normal. This second floor one is the only one that hisses loud on shutdown pretty quiet when heat is on

  • dabrakeman
    dabrakeman Member Posts: 819

    I have two radiators in my home that I purposely vent faster than the others. One in my kitchen because the room is just under radiated and the other in my family room because that is where in this big house my wife and I hang out every evening… Consequently the kitchen radiator has a MoM #6 and the family room a Hoffman 1A. When I had the 1A set on near max it would often whistle on some end of cycles, sometimes along with the kitchen radiator, sometimes in place of. When I set the family room lower (between 3 an 4) it never whistled anymore, only the kitchen none does which was more tolerable for me so that is how it has stayed. I could probably knock that kitchen down to a MoM #5 and the problem may go away entirely but for the conditions in my house I need more heat than I can get in that kitchen. The longer the cycle the more likely the hiss as well although I never see pressure above 0.5psi. I'm sure you are probably also monitoring the other discussion on this same topic:

    https://forum.heatinghelp.com/discussion/202606/radiator-vents-sucking-in-air-at-end-of-cycle

    I would say consider whether you can reduce the vent on that second floor radiator or look into adding a vacuum breaker as is being discussed on the other thread.

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