6" duct on 4" fan?

Discussion in 'Remodel Forum & Blog' started by qwertyjjj, Jun 25, 2020.

  1. qwertyjjj

    qwertyjjj Member

    Joined:
    Jul 11, 2014
    I have to install a Panasonic whisper thin as there is plumbing above where I want the fan and not much space. The fan accepts 4" duct but I have to run 32 feet to outside.
    Can I just increase to 6" duct or will the fan not cope? Doesn't this decrease airflow?
    If the fan is 4, why isn't the airflow limited to that of a 4"?
     
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Joined:
    Sep 2, 2004
    Occupation:
    Retired Systems engineer for defense industry.
    Location:
    New England
    If yours is like the last Panasonic one I installed, it has a chart in it that lists the recommended duct size based on the effective distance (this include the actual distance and an addition for things like elbows). A larger duct will have less resistance, and allow the fan to maintain the velocity with less back-pressure and can also save on some electricity since it won't have to push as hard. The one I had had a 4" opening with a larger section that would accept a 6" duct over all of that...I'd probably use the 6" duct for a run that long and keep in mind, solid duct has less restrictions than plastic with a spring reinforcement. You do probably want to put some insulation over the solid duct and it's a good idea to get it to slope slightly outside so any condensation doesn't come back and drip down from the fan if it's cold out. The insulation helps, but may not prevent it, so letting it drip outside is better than back into the room. It's hard to keep a flexible duct from having some dips in it, and those can accumulate moisture.
     
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  4. qwertyjjj

    qwertyjjj Member

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    Jul 11, 2014
    I guess I was thinking like water pipe. If you incorrectly have a 1/2" pipe pushing water in a 3/4" pipe it reduces flow.
    So, I'm having trouble understanding how a 4" pipe can push 6" air in the duct fast enough.
    I have both been types the Panasonic choice with 4/6" even though the hole is 4 and I have a whisper thin which is 4" only but I could add a reducer to make it 6
     
    Last edited: Jun 25, 2020
  5. Reach4

    Reach4 Well-Known Member

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    Sep 25, 2013
    Location:
    IL
    Nothing wrong with a pipe getting bigger.
    It reduces velocity (ft/sec) but does not reduce flow rate (cfm). It will increase flow due to having less backpressure.
    Not a problem. Not an exact analogy, but imagine 2 northbound traffic lanes feeding into 3 traffic lanes. It does not reduce traffic flow. The analogy to air or water flow breaks down in that with traffic, the velocity (mph) is not going to drop as a consequence of switching to more lanes.
     
  6. qwertyjjj

    qwertyjjj Member

    Joined:
    Jul 11, 2014
    It's 32ft but I have to go across the ceiling then down a wall 10ft then horizontally 20ft to exit.
    Going to get some water somewhere in that, no?

    Instead of round 6 could I use 5x6 rectangular?
     
    Last edited: Jun 25, 2020
  7. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Joined:
    Sep 2, 2004
    Occupation:
    Retired Systems engineer for defense industry.
    Location:
    New England
    It depends on how cold your attic gets, how much moisture there is in the air, and how well the duct is insulated whether or where things will start to condense.

    A 6" round duct has an area about 6% less than a 5x6" duct. So, in that view, the rectangular duct should flow the air better. It's easier to find adjustable elbows for round duct than rectangular.

    Ducts get sized by their effective length. That's the actual linear length PLUS additional effective lengths for each change of direction that slows down the air flow. A round duct, even with changes of direction probably has less restriction in it than rectangular ones partly because they tend to be a smoother rather than square.

    Whatever duct you put in, insulate it. Ideally, the whole thing would slope towards the outside, but if you can keep it from condensing until it gets over the peak, it should still work. It's very disconcerting to have cold condensation dripping down out of the fan assembly.

    I think you'll find that the instructions for the fan say that for a 32' actual length (probably closer to 38' or so with the angles in effective length) that you should use the 6" round...that's mostly because it fits onto the fan easier than adding another fitting to change the outlet to rectangular. Depending on how it is sized, you might get enough air flow with just a 4" round, but it won't let the fan maximize its output.
     
  8. wwhitney

    wwhitney Active Member

    Joined:
    Mar 17, 2019
    Location:
    Berkeley, CA
    That makes sense as a first approximation, but apparently it's not just a question of area. A circular duct will have less pressure drop than any other duct shape of the same cross sectional area. There's a standard way to convert rectangular ducts to equivalent circular diameters, and here is a purported calculator for doing that:

    https://www.engineeringtoolbox.com/equivalent-diameter-d_205.html

    It says a 5" x 6" duct is equivalent to a 5.98" duct, rather than the 6.18" duct that would have an equal area. So not a large effect, although the effect increases as the aspect ratio increases. E.g. a standard 3.25" x 10" is equivalent to a 6.0" duct, but an area match would be a 6.43" duct.

    Cheers, Wayne
     
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