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I recently installed a new bathroom exhaust fan, replacing a loud and underpowered one with a quieter and more powerful (110cfm) fan. Because of the existing framing and roof vent location, I was somewhat forced to install the fan in the particular orientation that you see (I'm sure with enough ingenuity I could have figured out a way to rotate the fan, but this is in a spot of my attic with very little room to work so I took the easy route). Because of this, I ended up with an unideal 180 degree bend in the ductwork. I am wondering if there is a way I can optimize this to make it "less unideal" if you will. I have considered a couple of solutions that I would appreciate some feedback on:

  1. Using 6" semi-rigid ducting (the fan exhaust port is only 4", so I would need an adapter/reducer of some sort) to reduce the strain caused by the bend
  2. Using two 90 degree rigid elbows to make the 180 degree bend (I've heard these are better to make bends with than semi-rigid duct)
  3. Combining 1 and 2: using two 90 degree 6" rigid elbows + some semi-rigid duct to get it to the roof.

As you can see, the total duct length is about 6 feet or so, so it's possible I am overthinking this and it would be fine as-is. The fan is at an acceptable noise level and does a good job of ventilating (it's "oversized" for the room it's in), so I'm mostly concerned about potentially burning out the motor by straining it too much.

P.S: I know it's recommended to insulate exhaust fan ductwork. I don't think that's common practice where I live because it never gets below freezing or cold enough for that to be a concern.

enter image description here

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  • To be clear, the fan has a 4" port and the roof a 6" port?
    – Machavity
    Commented Jan 30, 2023 at 18:18
  • If it's the type of duct I think it is, shortening it might be as simple as pushing on it, as it's sold as a compressed tube and then pulled out to length. So you could squeeze it shorter to suit without even cutting any off.
    – Ecnerwal
    Commented Jan 30, 2023 at 18:55
  • @Machavity The fan and roof collar are both 4". If I were to replace the duct with a 6" duct, I would replace the roof collar with a 6" one as well so I would only need to adapt the fan output.
    – Palin567
    Commented Jan 30, 2023 at 19:14
  • @Ecnerwal I actually intentionally stretched it out thinking that a wider radius turn would be better than a short 180 degree turn, even if it resulted in an overall longer duct. Is this incorrect thinking?
    – Palin567
    Commented Jan 30, 2023 at 19:15
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    You've got it going needlessly sideways, adding another bend that you can eliminate by just going directly at an angle to the roof exit. That is, you really have about 270 altogether, and can manage with only 180 total and less length. Less length is a good thing, and less bends are also a good thing. I don't think you have any need to redo the whole thing bigger or in smoothwall, from a practical point of view.
    – Ecnerwal
    Commented Jan 30, 2023 at 21:22

3 Answers 3

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Since I can't post a picture in a comment.

I'd suggest squeezing the excess length (and extra turn you're not counting) out of it. Might as well grab a roll of insulation and wrap the thing while you are there, too, though you seem opposed to the idea.

enter image description here

I suppose it might also be time to point out (yet again) that most fans don't suffer from "strain" when restricted. They move less air, so they do less work, and draw less power. You can see this with a kill-a-watt for plug-in fans, or by putting an ammeter on a hardwired fan, but folks tend to disbelieve it and claim the opposite, without evidence other than it's what they think going into the discussion and they have no intention of changing their minds...

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  • Thanks for the suggestion and for the information on fan strain. I don't know exactly where I heard that but it was probably reddit, which admittedly is not the most authoritative source. I also didn't realize until you said "grab a roll of insulation and wrap the thing" that they sell duct insulation separately. I figured I would have to replace the duct with pre-insulated duct which I am not a fan of because they are usually flex-duct instead of semi-rigid. I think I will go ahead and shorten it up and re-route it as you've described.
    – Palin567
    Commented Jan 30, 2023 at 21:55
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Honestly, it looks perfectly fine to me; looks like you did a great job.

My exhaust ducting is significantly longer than yours and I have not had any issues since I installed it over a year ago on either of my vents.

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    – FreeMan
    Commented Jan 31, 2023 at 14:39
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There's two major problems I see here

  1. Your duct is a bit long. The longer the air has to travel, the more it can condense. And that's a metal duct to boot, which means it could rust out. You probably don't want water dripping out of the fan in the winter.
  2. Your duct isn't insulated

If I were you, I'd go check out some directly insulated ducting (example). The duct itself is plastic, with metal wire embedded for stiffness. I'd also shorten the run as much as you can without putting any kinking bends in the duct. I had one duct I'd gotten lazy on and just used what I had left (about 6' long). In the winter, even insulated, the water would sometimes condense on the inside and drip back into the bathroom. Shortening it up prevented that from happening.

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  • Looks like an aluminum flex duct, so rust is probably not a big threat.
    – Ecnerwal
    Commented Jan 30, 2023 at 18:38
  • Thanks for the answer. Around here (desert southwest) it seems uncommon to insulate bathroom exhaust fans as it doesn't get very cold, certainly not below freezing. Would shortening the duct, even if it resulted in tighter radius turns, still be better than a longer duct with wider radius turns? What do you think about increasing the duct size to 6" to mitigate effects of the 180 degree turn?
    – Palin567
    Commented Jan 30, 2023 at 19:17
  • Where I live it doesn't get super cold either but you'd be amazed how little cold it takes to make it condense. 6" wouldn't hurt... but I don't see any reason to upsize. I have a similar setup over my master bath and it does fine now that I made it slope upward the entire way
    – Machavity
    Commented Jan 30, 2023 at 19:23

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