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I am replacing the original kitchen exhaust fan on my 1950s house. The old one vented sideways to an immediate 90-degree bend (to 8-inch rigid ducting) and then straight up about 24~36 inches to the roof cap.

Existing and new

I purchased a new unit with the same sideways exhaust but after reading some interwebs resources (for example) it sounds like avoiding 90-degree bends is best practice. I didn't see anything in the new unit's installation instructions about 90-degree bends or really any kind of ducting recommendations.

If I were to switch to a unit with a vertical vent I would need to make up the displacement by using flexible ducting instead of rigid, which I hear is also non-ideal. (Unless there are bends available at less than 90 degrees for 8-inch ducting; didn't see any at HomeD.)

Proposed vertical

Which would be the best solution? Or is there a better one yet?

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See sketch below.

A possibility of using a adjustable 90° fitting would do you right in this scenario and you could get the identical roof pitch in that fitting. So this would be a compromise, being less than that stated "stay away from 90 bends".

These would let you make an angle equal to your roof's pitch. 0° to 90°

sketch showing vertical flue bending towards roof with adjustable bend

  • Note that depending on where the roof vent actually is, a single bend may not be enough to reach it. – Martin Bonner supports Monica Aug 27 at 10:06
  • I actually just bought two of those adjustable fittings last night -- had no idea they existed (though that's not saying much). They may just do the trick! – calcium3000 Aug 27 at 11:42
  • This is a much better solution than the original even. Bends and elbows are pressure drops, the less of either the better but 90 << 30 in term of impact. – Stian Yttervik Sep 20 at 13:25
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Redoing the roof penetration is a considerable hassle. I would stick with the current route.

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    I don't think either would require redoing the roof penetration -- just the ducting running from the exhaust unit to the roof. – calcium3000 Aug 26 at 14:55
  • I agree with Jim. Your fan's probably about 210 CFM and you won't loose much with a 24 to 36 inch run. – JACK Aug 26 at 16:17
  • My fault, for some reason when I looked at the diagram I thought it indicated that the roof penetration was to be moved. – Jim Stewart Aug 26 at 18:40
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Your "vertical" diagram actually appears to show two 90° elbows. It would be much better with two 45° elbows, and searching for "8 ducting 45 degree elbow" finds lots of hits.

The other important question is "what does the ducting inside the HVAC unit look like?" My guess is that internally there is a duct going vertically or horizontally, and to get an outlet in the other direction, they add a 90° elbow. If the elbow gets added to create a vertical outlet, there's no point.

Certainly two gentle bends are better than a sharp bends, and smooth interiors are better than corrugated - but it's not that critical unless you are planning on long runs (and your diagram appears to show a short run).

I would just use whatever is most convenient to install.

  • Yeah, my MS Paint skills aren't what they used to be -- the ~8 inch delta is more important to the drawing. And I agree that it doesn't make sense to use a side-vented unit just to add an elbow to make it vent vertically, but it's certainly a bigger operation to fit new ducting and exchange the (special-ordered) unit for a new (special-order) one. I'm between getting it done quickly and getting it done best. – calcium3000 Aug 26 at 14:54
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I didn't see anything in the new unit's installation instructions about 90-degree bends or really any kind of ducting recommendations.

Figures 1 thru 4 in your installation instructions are the ducting recommendations from the manufacturer. They show 90° bends...

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