The goal of such codes is to keep the house pressure from falling too much (relative to the outside) for proper combustion appliance exhaust flow. For some such appliances, the pressure drop that causes backdrafting is quite small. The Broan unit may meet code because such codes are generally written without addressing the actual issue nor considering all of the factors involved in meeting the goal. One could, for example, have no combustion appliances, say by using electric heat and dryer. Then the Broan meets code at a very low cost and is effectively irrelevant to the backdraft issue. One could also have combustion appliances in a sealed room with its own source of air from the outside. MUA is again irrelevant to the backdraft issue. It would potentially be relevant to avoiding MUA by wall leakage. A passive system will have a pressure drop. Whether this drop has any backdrafting effect on combustion appliances has to be evaluated. In general, filtering the MUA will cause a pressure drop that would, by itself, cause backdrafting in some combustion appliances. However, blower augmented oil burner furnaces can perhaps operate at this pressure loss if not too large (hood cfm is modest). Even oil furnaces will backdraft with passive MUA at higher hood cfm levels. Where complexity (and cost) really become manifest is where the hood cfm is large and variable, there may be a second exhaust that is large and variable, some number of bathroom exhaust fans that may be on or off, window opening variability, fireplace in use or not, etc. Such conditions make the correspondence of primary hood motor drive and actual MUA required relatively weakly correlated. Then closed-loop active control of MUA is needed via electrical or pneumatic components. kas

Thanks for the comment. It's a 10" duct and the blower is inline remote with a muffler. It's all on the exterior of the house past 10" thick ICF walls. The blower itself will be suspended by porch rafters to reduce vibration transmission and the porch rafters are tied to the house with a ledger board on the exterior of the ICF so there's little to no vibration transmission into the house. The inspector is requiring a 10" MUA duct. I know that at full blast, it's not going to be whisper quiet, and I'm only expecting to use it full blast in rare occasions, but we do want the option for that many CFMs. As I mentioned above, the range top is 48" with four burners, a char-broiler, and a griddle for a total output of 111000 btu. I doubt we'll ever use everything full blast at one time but my biggest concern is smoke from the char-broiler and grease/oils from the Chinese cooking I frequently do. The MUA will come through the toe kick of the range top base cabinet so it helps create an air curtain to pull out smoke/grease/oil. Believe me, I have the same concerns about cold/hot air in the MUA, but to achieve the worst case scenario for cooking containment, we need to look at corresponding CFMs. I'll have to see how the controls open the MUA damper. I wonder if it's the case where the damper only opens over 500 CFMs, but I would imagine that with our tight house construction, we might not want that.

"And that would be super-awesome if #5 applied. Read again." I just did. And I read it a few times before I posted. The section is "Fresh Air Openings for Confined Spaces." The subsection (B) is "All Air from Outdoors." #5 says: "5. Alternatively a single permanent opening may be used when communicating directly with the outdoors, or with spaces that freely communicate with the outdoors. The opening shall have a minimum free area of 1 square inch per 3,000 Btu/ hr of total input rating of all equipment in enclosure (see Figure 20)." Maybe you are focused on "all equipment in enclosure?" The only thing in the enclosure would be the WH. It would be helpful if you gave me a hint as to why #5 doesnt apply. Anyway, I think what I will do is ... nothing. Before I tear out a perfectly good WH that I haven't even made a mortgage payment on yet, I should probably wait and see whether I actually have a problem. I have CO detectors throughout the house so presumably I will know if there is a back draft situation. Thanks a lot for all the replies. They were very helpful in helping me think this through.

I'd suggest reposting in the Appliances forum. That's where lots of ventilation discussions take place--and also where downdraft vents are widely viewed as not very effective, so beware. :)