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venting microwave exhaust. 4", 6", plenum?

I want to install an undercabinet microwave vent hood combo over kitchen range. the current range hood (no microwave) exhausts into cabinet above, 4x10 outlet adapter, , 2 6" 90's, into wall, about 10" right of center, and up to roof.

the new microwave will require raising the cabinet about 6". I don't think that the current 6" rd 90's will make the curve into adapter any longer. Possibly, using 4" 90's may work. The other option is to have a custom plenum made so that exhaust hood goes up into plenum and plenum out into wall (6" duct to roof). two questions:
1. would it be ok to use 4" out of the exhaust then into the existing 6" to roof?
2. anything wrong with the plenum idea? better, worse, no difference, than using 4" duct (assuming 4" duct will accommodate the curves)?

A. Spruce
Re: venting microwave exhaust. 4", 6", plenum?

Go with the plenum method, as this will maintain maximum air flow capacity which will help the fan draft smoke and odor more efficiently.

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