I am installing my OTR microwave. The venting looks like it will definitely not align (thanks HVAC guy).

In any case, can I simply use flexible venting to connect the microwave flange to the roof venting?

Also, I need to extend the boot to allow for the damper to close (nobody does this on the YouTube videos). Do I need to create this, or can this be found in stores? I could not find anything like it a Home Depot the other day. I guess I could use a longer boot (say 4" instead of the "required" 3.25").

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I would find a boot that covers the cabinet opening and install that, then see where you stand. A bit of flex duct may be a good solution (the semi-rigid kind, not the cheap foil junk), but a side-offset boot may line up perfectly. This one looks like what I have in mind, though the offset may be an illusion:

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Image courtesy of Home Depot

Be aware that such a boot should slip over the flange on the microwave, eliminating contact between the exhaust and the wood of the cabinet.

  • I think the offset is correct, 'cause you need to go from the back of the cabinet (the microwave exhaust) to the center-top of the cabinet where the round duct comes down. See my top picture here: diy.stackexchange.com/questions/209345/…
    – SteveSh
    Nov 23 '20 at 20:35
  • Agreed; or start with a couple round sections of the type which rotate one end relative to the other, then finish with a round-to-rect adapter, assuming there's room for all that. Otherwise, it's a minor sheet-metal job to build a custom offset. Nov 24 '20 at 16:22
  • I have never worked with sheet metal! I also need a minimum 3" extension piece to allow for the damper to close. I guess those are not off the shelf? Thought this would be a simple problem.
    – Shinobii
    Nov 27 '20 at 16:25
  • 1
    Yeah, that's why most such ducts are rectangular all the way up to the ceiling.
    – isherwood
    Nov 27 '20 at 16:37

I had the exact same problem. I used a 3.5" x 10" rectangular duct to go up through the cabinet into the attic. In the attic, I used a rectangular to 6" round adapter, then 2 adjustable elbows to get the right offset and angle to line up with the roof vent, and a straight pipe from the elbows to go up through the roof. Bonus... I still have enough space in the cabinet to use it for storage.

I wouldn't recommend flex duct because all the ridges will collect airborne grease being sucked up from the stove. enter image description here

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