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Thread: Bathroom fan vent diameter

  1. #1
    DIY Junior Member DIYJared's Avatar
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    Smile Bathroom fan vent diameter

    Hi All,
    Been using this forum for a while and it has been very helpful!

    I'm remodeling our two bathrooms and had the plumbing inspection today (all good). One thing he did mention though was that the two bathroom fan exhaust vents were too small (4" flexible). He said Oregon code was 5" rigid (preferred) or flexible and all less than 25' in length. I found a site for Oregon codes (http://www.cbs.state.or.us/bcd/progr...ine_codes.html), but I can't seen to find reference to 5" diameter pipe. It's not that I don't believe him, but I'd like to know how to look these things up in the future.

    Thanks for any hints or info...
    Jared

  2. #2

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    I found it in the oregon residential Specialty Code Chapter 16 Table M1507.4 perspective exhaust duct sizing.

    Some inspectors think they can make their own rules instead of following the written law.

  3. #3
    DIY Junior Member DIYJared's Avatar
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    Ah, cool...thanks! I see it now. Looks like you can use 4" (rigid) with a 80cfm fan, but only if it is 20' or less.

    Thanks again - appreciate the guidance...

  4. #4
    Moderator & Master Plumber hj's Avatar
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    Some inspectors use the "rule of thumb" they are used to instead of referencing the code as it is written. The size/capacity of the fan and the length should dictate the pipe size.
    Licensed residential and commercial plumber

  5. #5
    Retired Defense Industry Engineer jadnashua's Avatar
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    Smooth ducting has much less backpressure than corrugated stuff, and larger diameter stuff flows better than smaller ducts (might seem obvious, but it all factors in).

    FWIW, the instructions on my bathroom vent called for 6" ducts. Larger ducts can allow a bigger diameter fan, which can move as much air, but do it more quietly than a smaller one running faster and with more turbulence.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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