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Thread: Vent Terminal Size

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  1. #1
    DIY Senior Member seaneys's Avatar
    Join Date
    May 2007
    Chicago Suburbs

    Default Vent Terminal Size


    I live near Chicago and am subject to the Illinois Plumbing Code

    For some reason, I thought that I needed to expand to a 4" vent terminal poking through my roof. Section 890.1440 of the Illinois Plumbing Code says that I only need a 3" vent terminal.

    I have two vent terminals covering about 3.5 baths. Is there something I am missing? I was thinking it would be a good idea to do the roof work before the snow hits tomorrow.

    Any ideas why I thought I needed to jump to 4"?


  2. #2
    Moderator & Master Plumber hj's Avatar
    Join Date
    Aug 2004
    Cave Creek, Arizona

    Default vent

    The Chicago code used to require a one size increase with 4" minimum. The purpose of the increaser is to minimize the possibility of closure due to hoarfrost. A 4" increaser is almost twice the size of a 3" one so it would give a greater degree of protection.


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