Duct Microwave to Outside

[JIMMIEM]

Microwave is installed over the stove on an exterior wall. It is set up to be exhausted through the top. I want to run the vent through the wall. I have a 3 1/4 X 10 90 degree that will attach to microwave vent. I also have a 3 1/4 X 10 wall cap. I bought these at a sheet metal supply house....not a big box and am now wondering if I have the correct gauge. I have a potential problem with a wall stud that lines up with one edge of the vent. I was planning on reshaping the 2 pieces so that they clear the stud. I would have sistered 2 X 4's to the stud so that I could notch the stud but I don't want to cut any more of the cabinet than I have to.
My questions are is there a code requirement for the metal gauge I am using and is reshaping the ducts a good idea and will cutting down their volume be a problem?
I didn't transition to a 6 inch round in order not to waste cabinet space.

[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.

[JIMMIEM]

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.

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.

[wmckane]

You can use foil tape; I just prefer silicone where there may be moisture.

[JIMMIEM]

Originally Posted by wmckane

You can use foil tape; I just prefer silicone where there may be moisture.

I noticed some silicone brands in the sheet metal supply house that I have never seen in hardware stores or the big boxes. Are there different better grades of silicone that the pro supply places carry?

[wmckane]

There are different brands but as long as it's 100% silicone it will do what you need. Color doesn't matter.

[JIMMIEM]

I wound up using a 3.25 X 10 transition to a 6 inch round. I used foil tape to seal all joints and the transition to the top of the microwave around the damper. There is about 1 1/2" of space where the cabinet was cut for the microwave damper. I sealed the transition to the top of the microwave around the damper but I feel air around the space between the cutout and the damper/transition area. Is this normal? Should I somehow seal the top of the microwave where the air is coming from?