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4″ or 6″ bath fan duct?

user-1105327 | Posted in General Questions on

Hi all. Sadly, i have another project so i’m going to be pestering you with dumb questions again…

We are installing two panasonic whispergreen, .3 sone, 80 cfm bath fans. they both have relatively short duct runs that exit through the wall. the inlet on the fan can be either 4″ or 6″ diameter. i understand the fan will operate at 80cfm regardless of the length or diameter of the duct but which diameter is more energy efficient..?

thanks again

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  1. user-626934 | | #1


    Side note - the fan will NOT operate at 80cfm regardless of diameter and length...but it does have the ability to ramp up/down the power input to the motor in order to deliver the 80cfm...up to a point....add enough bends/fittings, make the duct diameter small enough and/or duct length long enough, and it will not be able to move 80cfm.

  2. GBA Editor
    Martin Holladay | | #2

    John is correct. The answer is 6-inch duct. For more information on this topic, see A Failure That Stalls the Certification of Many Energy Star Homes.

    By the way, the duct you are talking about is not an "inlet." It's an exhaust duct.

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