Quote:
Originally Posted by wmckane
What is the microwave fan's output in cfm? This will tell you what size duct you have to use and will tell you round and rectangular equivalents. If you are making a 90 degree turn you will restrict the airflow by some amount. I don't have an easy way to calculate that but in a short distance to the exterior it probably won't matter too much. If you are intending to modify rectangular duct to avoid a stud you need to make sure that you aren't creating another restriction by reducing the internal area of the duct. If your wall vent is round remember that it has to be sized to the microwave's output and realize that you need additional linear room to convert from rectangular to round to (again) avoid causing a restriction. I don't think metal gauge is an issue but I would seal every connection with silicone. And of course, no duct run going uphill.
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The installation manual gives the equivalents for the various components that could be used. I'll be using a 3 1/4 X 10 with the 90 degree turn that will connect to the damper and a straight 3 1/4 X 10 wall cap that will connect to the 90. The 2 parts add up to 65 equivalent feet. The total allowable equivalent feet is 140. My modification would reduce the volume by approximately 15% which would leave me below the 140. Can I seal with aluminum tape? The only vertical is when it comes off the top of the microwave...the rest is horizontal.