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I am installing a kitchen fan about 9' away from the nearest external wall. The wall there is very narrow and all I can fit there is a dryer type vent that works with 4" round pipe. Also, my spacing behind the framing along those 9' allows me only about 4" of space.

Can I exhaust a kitchen fan through a 4" round pipe across 9' linear with only one 90° turn? I'm guessing kitchen fan ventilation is subject to more scrutiny because the fumes contain oils that settle along the pipe, presenting a cloggage (carbon monoxide) and fire hazards.

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Sure, you could exhaust a jet engine through a 4" pipe but the problem is the velocity of the air. As the size decreases the velocity increases for any given cubic feet per minute rate of exhaust.

So, what you have to do is calculate the velocity of the air through a 4" pipe given the CFM of the fan. And then will that velocity cause an objectionable noise. Most bathroom fans are connected through 4" round ducts and go up to 900 CFM or so?

What size fan do you want to use? What will the velocity of the air be? What is the recommended velocity?

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  • Just a plain vanilla hood fan
    – amphibient
    Commented Jan 30, 2016 at 2:12
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    I would think most of your oil and grease will deposit on the fan filters and the hood as they don't suspend very well. I wouldn't think there would be any problem doing what you described. If you are worried build it like plumbing with a capped tee that could be used to clean it out in the future. Dragging a rag soaked in ammonia through it once a year or two should clean out the residue.
    – ArchonOSX
    Commented Jan 30, 2016 at 2:17
  • All good points
    – amphibient
    Commented Jan 30, 2016 at 2:19

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