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Can a bathroom exhaust fan be oversized?

entomodonata | Posted in General Questions on

Remodeling the 2nd story of a 1.5 story Cape, and building a ~110 sqft. master bath. I plan on installing a remote bath exhaust fan (Panasonic Whisperline Remote FV series) and need to choose between the 100 CFM and 200 CFM models. The plan is to have two intakes — one over shower/tub, and an additional about 8′ away above the toilet, and then exhaust out my gable vent about 15′ away to avoid punching more holes in my new roof.  I have more than adequate soffit and ridge vent ventilation, so I can afford to sacrifice one gable vent for this. 

The 100CFM unit uses 4″ ducting, and I think it will be too restricted pulling from two intakes. 200 CFM unit uses 6″ ducting, so I think I’ll be ok pulling two 4″ intakes, and exhausting through a straight run of 6″ out the gable vent.

Rule of thumb is 1 CFM per square foot, but is there any harm in oversizing? With it being a remote fan, noise won’t be an issue.

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  1. Jon_R | | #1

    What is the path for the makeup air for this exhaust?

    1. entomodonata | | #2

      My generally leaky 1940’s board sheathed house? In all seriousness, I’ve been air sealing as much as possible as I work my way through this house (rim joist, door sweeps, 2nd floor remodel will be pretty airtight), but it’ll never be perfect.

      There’s no current plan for dedicated makeup air. My stove and downstairs bath fan vent well without them.

      Is this something I should consider?

      1. Jon_R | | #3

        If it's 200 CFM pulled from a closed door bathroom, then yes.

        1. entomodonata | | #4

          Hi Jon, I appreciate your replies. Would a 3/4” gap under the bathroom door (and/or a sliding barn door) which opens to a 400 sqft space make any difference?

          My concern with the 100 cfm model of the fan in question would be efficiency losses from two 4” ducts running to the main fan. I think I need a ‘real’ 100 cfm for this space, which led to the thought of upsizing to the next unit.

          Perhaps I’m better off using the 100 cfm unit with just one centrally placed intake.

          There’s a skylight in the room and I’m worried about interior condensation, but perhaps I can place the intake pretty close to the skylight.

          Appreciate any further opinions you may have.

          1. Jon_R | | #6

            A rough analysis without the details - 3/4" under the door would be about right. But maybe someone else has a door undercut vs bathroom exhaust CFM rule of thumb.

            IMO, up-sizing a variable speed fan is a good idea - you can always turn it down.

            Another opinion - locate all fan intakes up high - where they will maximize steam removal.

  2. Expert Member
    AKOS TOTH | | #5

    If you look at the flow curve of this unit:

    It has a vertical slope up to a certain pressure drop. That means as long as you keep your ducting reasonable, it will deliver 100CFM without issues. This looks like it can handle about 70' equivalent length of 4" rigid (so about 90s and 20' of pipe).

    If you do need a lot of ducting, you can always step it up to 5" or 6" out of the unit, that will significantly reduce the restriction.

    You don't need a larger unit, just need to take a bit of care with the piping.

  3. pjpfeiff | | #7

    I have a small (42 sq ft) bathroom with a Panasonic fan with selectable exhaust rates. I have found that at 80 CFM I can feel an uncomfortable draft in the shower, but 50 CFM is fine.

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