I am currently in the process of renovating a duplex. It is an upper/lower that I plan to live in. The units more or less mirror one another, however there are some nuances in the kitchen.

I have over-the-range microwaves and would like to vent them each through the wall cavities and through the attic/roof. I have finished the lower kitchen but have access to behind the microwave with the hallway/staircase behind it. It is a 1920s house and there is an existing cavity for an old cast iron stoker pipe that has been removed that I can make larger if necessary. My upper kitchen walls are still open. I have done some research but I haven't found tons of clear answers on to what size pipes are permissible/etc.

Here is my original plan vs. what I am now considering:

Original plan:

Vent each microwave using 4" round pipe through the wall cavities. These would need to be slightly compressed obviously as studs are only 3.5" deep. I would need a couple 90" elbows to make this work, but can once I am in the wall I have a fairly clear path to the roof. HOWEVER, the more research I do suggests that 4" round pipe is not sufficient for microwave/fume hood fans.

New plan:

Lower unit: vent microwave with oval pipe that should fit inside the wall cavity (to be honest I'm not exactly where to find this). I would love to go straight out the back of the microwave vs. the top of the microwave and back into the wall to reduce using cabinet space. If anybody has links to where to find the right materials, I'm all ears.

Upper unit: vent microwave straight vertically using 6" round pipe and a rectangular to round convertor. This will occupy useful cabinet space, but it is what it is. I can go straight into the attic and continue through to the roof.

I have a 300 CFM microwave FWIW. Samsung 30 in. W 1.7 cu. ft. Over the Range Microwave in Fingerprint Resistant Stainless Steel ME17R7021ES - The Home Depot I am always interested in the path of least resistance, however I want to set things up for years to come if possible as well in the event I ever get larger fans in place. I am wondering if 6" round pipe really necessary for a 300 CFM fan, or If I can get away with 4" pipe.

If anyone has any other considerations, please let me know. I will post pictures momentarily.