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Thread: Increasing Exhaust Fan CFM

  1. #1

    Increasing Exhaust Fan CFM

    I have a 12" exhaust fan in my shop that exits to the outside. Would love to increase the current CFM of that fan. Any ideas or suggestions on how I might do that?

  2. #2
    Join Date
    Sep 2015
    San Diego, Ca
    I have a 24 or 30 inch whole house fan in my shop. It works well.

    So, to increase the CFM (working from logic), you could increase the motor speed (but if it is a already a 3450 rpm motor there isn't much you can do). Or you could (maybe) modify the fan assembly to have a slightly bigger or more efficient fan.

    I guess that my recommendation is to go with a bigger exhaust fan.

  3. #3
    Join Date
    Sep 2008
    Gassaway, WV
    I replaced a 16" exhaust fan with a 24" , think the CFM is 4600 in my shop. It really helped with dust, not so much for chips. I have an 18" fan that is on a stand and swivels that I can move around, keep it up wind when using the lathe. It really helps keep my face mask from fogging up. I also have a DC with 6" plastic pipe. Sometimes I run them all. There still is quite a bit dust that settles in the shop, but it really helps my sinus. The down side: noisy, doors are hard to open, in the winter I have to wear arctic clothing.

  4. #4
    Join Date
    Mar 2003
    SE PA - Central Bucks County
    Increasing fan speed can help a little, but only so much air can transit through that 12" space. The optimal operational solution is to up-size the fan in all honesty.

    The most expensive tool is the one you buy "cheaply" and often...

  5. #5
    Quote Originally Posted by Steve Doerr View Post
    I have a 12" exhaust fan in my shop that exits to the outside. Would love to increase the current CFM of that fan. Any ideas or suggestions on how I might do that?
    How big is your air inlet? Your fan will move more air if it has sufficient inlet into the space. You need at least 12"diameter inlet to the room for the fan to reach full capacity.
    Lee Schierer - McKean, PA
    USNA- '71
    Captain USN(Ret)

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  6. #6
    Thanks everyone for your input. My problem is that my shop is in the basement and my exhaust fan had to fit in-between the floor joist in order to to get it to vent outside. One of my problems may be that I need to duct my fan so that the opening to the fan is not tucked back up in the floor joist.

  7. #7
    Join Date
    Sep 2013
    Wayland, MA
    Window fans are pretty inefficient, If you are space constrained you might look at an industrial blower (like a furnace blower, but heftier), they can push a lot of CFM through a 12" pipe. Not cheap unless you can find one used.

  8. #8
    Join Date
    Nov 2003
    Central North Carolina
    Most commercially made propeller type fans are designed to fully load their motor to just shy of it's maximum capability. Changing or modifying the fan blades to increase the cfm will overload the motor and it will fail, hopefully NOT burning up your shop and home in the process. It would be much better, and safer to find a fan with increased capacity.

    Propeller type fans are not that efficient for moving a lot of air in a small footprint. You might consider replacing it with a squirrel cage type fan like is used in a warm air furnace. Mobile home furnaces have squirrel cage type fans that require an opening about the size of your present fan. Maybe you can find a used one some place, and put it to use. Keep in mind also that when you remove a significant amount of air from a space, you will need to let air in through a different location to replace that air. If you don't let replacement air in the fan, no matter which type, will not be very efficient at removing air from your shop. Just opening a window on the opposite side of your shop might increase the efficiency of your present fan enough that you won't need a larger fan.


  9. #9
    I agree with the camp that says the fan is designed to do what it is doing. Modifications may provide a small benefit but, at the eventual cost of the whole unit failing. Best result will come from a replacement that does what you are looking for.
    Happy family, pale applause, each to his revolving doors.

  10. #10
    Join Date
    Sep 2016
    Modesto, CA, USA
    Best bet would to to add another fan in the next joist bay over and wire them together. Note that a squirrel cage fan will be quieter for the same CFM.

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