Bathroom exhaust fan, vent pipe diameter changes

Discussion in 'Shower & bathtub Forum & Blog' started by msimon, Apr 2, 2008.

  1. msimon

    msimon New Member

    Messages:
    8
    Location:
    Los Angeles, CA
    I am remodeling our hall bathroom (8 ft x 5 ft) and am replacing the 40+ year old inline GE vent (about 1,000 sones) with a new Panasonic fan.

    The room already has a vent going straight up and through the roof, which amounts to about 5 feet through the attic. We live in Los Angeles, CA, so ice and snow is not a problem for us.

    I would like to use the vent that's already in place in the roof. I plan to use a metal vent pipe as recommended and I know I'll need a 90 degree elbow plus some other elbow to line up the vent with the roof opening.

    The interesting part: the new fan has a 4 inch vent opening, while the old inline fan has a huge 8 or 9 inch pipe.

    My question is, should I add connectors to decrease the 9 inch pipe to 4 inches way up high near the roof, or down low near the fan?

    My thoughts are: do I want more of the large diameter pipe with a large volume of air at a lower pressure, or more of the smaller diameter pipe with the smaller volume of air but higher pressure? Which would make the fan happier?

    An alternate idea is to just run a 4 inch pipe all the way up and into the current (roof jacket?) and fasten the new pipe to something in there.

    Thank you in advance for your feedback.
  2. msimon

    msimon New Member

    Messages:
    8
    Location:
    Los Angeles, CA
    No ideas out there? Even the great Jim (jadnashua) and HJ don't have an opinion this time? :)
  3. Cass

    Cass Plumber

    Messages:
    5,980
    Location:
    Ohio
    The literature should tell you what size vent to run and the Max. length of the run including length subtraction for 90s and 45s.
  4. msimon

    msimon New Member

    Messages:
    8
    Location:
    Los Angeles, CA
    The manual says to use 4" pipes, or 3" with the included reducer. The 4" hole out the side of the fan casing is a good clue, also. Nothing is mentioned about increasing the vent pipe's diameter, though.

    I'm wondering, should I increase from 4" to 9" as soon as I can above the fan, or do it close to the opening in the roof?
  5. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,917
    Location:
    New England
    One issue may be if the roof jack has a built-in damper. If it does, the output from the smaller fan may not open it. If the damper is in the fan, and there isn't one in the roof jack, it probably doesn't matter...it would appear to the fan to be exhausting to the outside, the air will still move. Since it will be moving slower in the larger pipe, it might cool off more and condense which could drip back into the room. If you can seal it, you may want to keep the smaller size as far as you can, and still ensure it goes outside, and not into the attic. I wouldn't reduce it to 3", I think I'd keep it 4" as far as I can, then reduce it at the roof jack.
  6. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,503
    Location:
    Cave Creek, Arizona
    pipe

    If the pipe has no restriction in it, just increase the 4" at the closest, easiest point. All you worry about is the CFM, not the reduced velocity.
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