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The microwave over my stove expired. I pulled it down, and discovered that the ductwork to attach the fan to the outside world was misplaced: it's about two inches to the right of where it should be. The builders applied tape and sheet metal screws to attach the flap and duct to the old microwave out of line with the fans.

I'd rather not produce a repeat of that charming bit of 'craftsmanship'd. Can someone help me plan a proper repair? There's a round duct that goes out through the wall of the house, and it's bonded to the rectangular duct that you can see in the picture. I've pulled all the sheet-metal screws and tape to get a look at what's going on. Hypothetically, I'd want to somehow detach this 'box' from the 'pipe', and mount a new one with a suitably asymmetric hole for the pipe to compensate for the failure of the center of the pipe to match the center of the stove. I don't know how the two pieces of ductwork were connected to start with, and I'm loath to attack what I've pictured here with a sawzall (or equivalent) without a clear plan of what I'll do thereafter.

duct

  • The fact of the matter is that you've overestimated the importance of alignment here. Simply seal well underneath the cabinet, using foam tape or whatever, and move on with life. It's not that critical. As long as there's a clear airflow path (and the offset doesn't really affect that), it'll work fine. – isherwood Jul 30 at 15:45
  • There is a damper / flapper that fits into brackets in the microwave. It can't be mounted where its supposed to go and also fit into the duct in its current location. Its flange will block some airflow if I just float it and tape it. Mayhap still not significant. – bmargulies Jul 30 at 21:54
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I expect that you'll find a tabbed collar formed into a hole cut into the rectangular duct with either a fly cutter (the right way) or roughly with sheetmetal snips (the common way) - but if you look up the duct from below with the microwave removed, you'll be able to see that.

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You'd be able to reach in and bend the tabs back straight and separate the two. You'll need a replacement length of rectangular duct that you could cut an offset hole in, and you'd likely be able to reuse the collar and the cap at the top. That is a cap? or is it a wall of the duct that has been folded down?

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The best possible idea is to find a good sheet metal place and have them build you something beautiful and custom. It wouldn't be cheap, but shouldn't be outrageous, unless they have to come and visit (which they shouldn't, if you can make a good drawing).

Alternately, get about double the amount of tin ducting that would ordinarily go into such a project and start hacking away with abandon. Use good metal tape (not duct tape -- eww!), screws where appropriate, and wear gloves. Then -- and this is where I give away all my trade secrets -- hide that mess with a 3 sided box made out of plywood or something nice you've got lying around.

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