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Thread: Bath Fan to Weak?

  1. #1
    DIY Senior Member molo's Avatar
    Join Date
    Sep 2006
    cold new york

    Default Bath Fan to Weak?

    Hello All,

    Photo 2 shows the damper that comes with the vent kit (it's hanging down). My fan only lifts it 1.5", and not the full 3.5" it would go to fully open the front of the vent. Is the plastic too heavy or the vent to weak? It is a 50cfm fan for a 35sq' bath. The duct length is very short (3' max with no turns other than where it comes out of the side of the unit and does a slow bend to vertical to hit the roof) Should I try a 70cfm, do something to the damper, or is this amount of opening acceptable?

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    Last edited by molo; 07-17-2007 at 06:20 PM.
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  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
    Join Date
    Sep 2004
    New England


    50cmf is a fairly weak fan. You want a fair number of air turnovers in a room to extract the moisture. Calculate your room size and see how many per hour you're likely to get and come back with what you have. Note, the air must come from somewhere, so depending on the gap around the door, leaks, or a window, that can affect its operation, too.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  3. #3
    Plumber jimbo's Avatar
    Join Date
    Aug 2004
    San Diego


    Your bathroom is 240 cu.ft. A 50 CFM fan is 3000 cubic feet per hour, or 10 air changes per hour, which is plenty. It is a weak fan, and depending on the length of the duct work, apparently it does not have enough duct pressure to open the slots all the way, but even partly open should be OK. If the duct length is too long, that will affect your total air flow.


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