We don't know what your actual CFM will be due to pressure losses, both in the baffles and from the the undefined MUA system. Let's assume for comment sake that you actually achieve 800 CFM from the 1200 CFM blower. The goal for duct velocity is to operate in the 1000 to 2000 ft/min range, with the lower number quieter. This duct velocity range provides a balancing act between grease collection in the duct due to condensation and grease collection in the duct due to impingement.

800 CFM will need a velocity of ~2300 ft/min in an 8-inch duct, while 10 inches will require ~1500 ft/min. So 10-inch will be better. I would only expect 12-inch ducting to be needed for very high flow rates or very long ducts.

Note that a 42 x 27 hood will have an entry aperture of about 7.9 sq. ft., so 800 CFM is over 100 ft/min intake velocity. This should be adequate for most high temperature (high plume velocity) cooking.

Thank you Kaseki,

My GC has a HVAK guy. I would guess he should know this. I believe he is the one to install the hood.

Total BTU's are 189,000 assuming everything is smoking full boar. I seriously doubt All 6 burners will ever be running at the same time. I'm not sure I will have make up air. It is not required here.

Gas furnaces have their own fresh air intake and exit. Hot water heater as well. The only appliance that doesn't is a small gas fireplace. In addition the house is not tight (places you can look thru the cracks and see outside.) One of these days will have to address that.

Inga

The hood only removes as much air as that which gets to it through the cracks. You may not be able to achieve even 800 CFM without a means of MUA that has less pressure loss than your walls, unless they look like this:

well, they are pretty airy but not quite like that. :)) Maybe opening a window would improve the situation!

Yes, window screen pressure losses, while significant for passively vented combustion appliances, should be OK for your situation. Unless you live in a very temperate location, this approach may be a bit chilly in the winter. Also, a window by the cooktop would likely cause turbulence in the cooking plumes, so this outside air should flow toward the cooking zone from a less proximate window or windows.