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The bathroom is 112x80x96, the duct to outside will be 3-4' at max This is a 100CFM 25W, 4" fan

The only reason why I want to buy this one is because it has humidity sensor and timer for under 90CAD... I could use an adapter installed near the opening in the wall since the ceiling is open but not sure if that will diminish the side effects of the diameter mismatch between fan and duct

I wouldn't like to increase that opening in the wall

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    Do put it as far along the duct as possible. The less 3" duct you have the better, performance-wise.
    – Ecnerwal
    Aug 27 '20 at 0:30
  • I am just wondering if that might kill the motor sooner
    – MiniMe
    Aug 27 '20 at 0:59
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    Won't kill the motor. Will move less air than it would with 4" duct the whole way. It's common to misunderstand how fans behave with and without restrictions - in most cases, rather than "working harder" to "push through a smaller duct" the motor is actually working less, because it's moving less air due to the restriction, and thus doing less work as less mass of air is getting moved per unit time.
    – Ecnerwal
    Aug 27 '20 at 1:18
  • I am not sure if the mottor can sense the opposing force and slow down (reduce the power) in order to decrease the torque and the useless power consumption (trying to pump out more air than the duct can take at a normal speed/power/duct size)
    – MiniMe
    Aug 27 '20 at 1:56
  • As stated, common to misunderstand. The fan will in fact spin faster (or the same speed, depending on type of motor) but draw less power. Physics is phun.
    – Ecnerwal
    Aug 27 '20 at 11:14
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Yes it will. There are many different types of 4" to 3" reducers that will work with your fan. You will have to match one to the material of your vent and the duct. Below is a picture of one sold by Lowes.

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