Why wasn't rigid duct used? Thats a lot of flex duct that can get dirty with grease and other stuff that can get trapped.
The damper is basically a flapper door that comes with the microwave that you can put on the back or the top , depending on how you are venting the microwave. It prevents outside air coming in. Sometimes if you have a good damper outside, you really don't need the microwave damper as this is just another load that the fan has to push to get the air out.
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