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Our over-the-range microwave is currently configured for recirculating ventilation, which is less than ideal for many reasons (one of which being that it's over a gas range..). We discovered that there is ductwork behind the microwave that vents outside but the microwave was never connected, and the ductwork was not installed high enough for a typical rear-vented microwave exhaust (for reference, the house was built 2008). After pulling the microwave down, we found the first picture: looks like someone found there is ductwork and didn't explore any further? I decided to cut out more drywall to see what is going on. Turns out there is what appears to be a roughly 30x3.25" rectangular duct that extends down the wall about 2ft, then connects to an oval duct on the left side that makes its way out of the house. This interior wall is framed with 2x6 and OSB on the opposite side, most likely because it's a large vaulted ceiling behind here. They notched out the 2x6 to accommodate the rectangular ductwork. So, I'm looking for advice on the best way to vent my microwave here. I'd prefer to avoid cutting into the upper cabinet, so I'm leaning towards continuing the notch of the 2x6 all the way up to the cabinet (the 2x6 is not centered and would block one of the rear exhaust vents of the microwave), cutting out a 10x3.25" hole in the existing ductwork, and finding some way to attach and seal something like this. Does that make sense? Any alternative suggestions?

original

after removing some drywall

exterior vent

Dan
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  • What does the exterior vent look like? (BTW, here's a non-FUBAR link: https://www.lowes.com/pd/Lambro-10-in-x-24-in-x-3-25-in-Galvanized-Steel-Stack-Duct/3624356) – Huesmann Oct 11 '23 at 13:53
  • @Huesmann added a picture of the exterior vent (which has a mediocre damper). Also fixed the link, doh! – Dan Oct 11 '23 at 17:49
  • I think your "snorkel" plan is reasonable. I sure hope that stud is non-structural (even though it's too late to do anything about it!) and that there's enough slack in the wires to push them to the side. That duct is clearly venting *something*, though. You may want to install a baffle(s) to prevent greasy air from "backflushing" to the duct's actual source. – Huesmann Oct 12 '23 at 12:59
  • I agree; I plan on taking out as little of the stud as possible, and the wires are actually back far enough that I probably don't even need to remove their staples. I was also planning on getting the proper damper for our microwave (https://www.amazon.com/Frigidaire-5304464343-Vent-Damper-Unit/dp/B00LXAS8YG) and giving some attention to the flap on the exterior. I forgot to mention that we validated this exhaust is connected to the ductwork using a leafblower :-D Thanks for the input! – Dan Oct 12 '23 at 13:11
  • But what's at the *other* end of the duct—the not-wall-exit end? – Huesmann Oct 12 '23 at 13:16
  • One problem you may run into is sealing that snorkel, though. – Huesmann Oct 12 '23 at 13:16
  • As in what's on the exterior wall? A damper flap as well, although I'm not sure how great it is. As for sealing the snorkel, I was planning on taking an approach like this: https://www.finehomebuilding.com/2017/05/01/range-exhaust – Dan Oct 12 '23 at 13:30
  • No, that duct *goes* to the wall. But where does it *come from?* – Huesmann Oct 12 '23 at 13:32
  • I plan on sending a borescope in through the exterior vent later today to validate that it's a straight shot that's connecting the rectangular ductwork behind the microwave all the way to the vent on the exterior wall. – Dan Oct 12 '23 at 13:47
  • Are you saying that it's not currently venting *anything?* – Huesmann Oct 12 '23 at 13:48
  • Correct. As far as I can tell, that exterior vent is connected to that sealed ductwork and is effectively a closed system with nothing _actually_ using it at the moment. – Dan Oct 12 '23 at 14:10
  • Have you been able to trace the duct to its other end? – Huesmann Oct 13 '23 at 15:17
  • My borescope is pretty crappy but from what I can tell, that exterior vent is leading straight up to the ductwork that was behind the microwave. I didn't see any other blatant junctions along the way. – Dan Oct 13 '23 at 19:57
  • Sure, but (assuming the exterior vent is to the left) where does the duct go to the right? – Huesmann Oct 14 '23 at 13:27
  • Sorry for the horrible depiction here, but hopefully this will help explain: https://imgur.com/a/C1eYWrT There's a roughly square section of 3.25" deep ductwork that spans two stud cavities (30" wide, light blue). They notched out the 2x6 to fit it. In the bottom left, they connected oval ductwork (green) straight down to the floor, where it then meets up with 6" round ductwork (red) that goes straight to the exterior wall vent (dark blue). I confirmed this is all that is connected by drilling a hole in the big square and sending in the borescope from the inside. – Dan Oct 15 '23 at 22:37
  • That is Fing peculiar! – Huesmann Oct 16 '23 at 22:12
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    Indeed. I ended up going forward with the "snorkel" last night and it worked! Thanks for the vote of confidence :-) – Dan Oct 17 '23 at 12:56

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