Bath Fan to Weak?

Discussion in 'Plumbing Forum, Professional & DIY Advice' started by molo, Jul 17, 2007.

  1. molo

    molo Member

    Sep 23, 2006
    Cold New York
    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: Jul 17, 2007
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Sep 2, 2004
    Retired Systems engineer for defense industry.
    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.
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  4. jimbo

    jimbo Plumber

    Aug 31, 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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