No air flows out of the kitchen vent that hasn't flowed into the kitchen. In other words, there is always make-up air (MUA). Depending on how leaky or tight your house is, the MUA may be supplied with a small pressure drop, or a high pressure drop. Any MUA pressure drop, added to the pressure drop of the baffles/mesh filters and other hood and duct pressure losses causes the blower/fan of the ventilation system to flow less air. Hence, without dedicated MUA, your 400 CFM rated blower may flow as much as 300 CFM or as little as 100 CFM (or negligible CFM if your house is really tight). But it won't flow 400 CFM.

For a given hood intake aperture, a level of CFM lower than the recommended 90 CFM/square foot of aperture will only assure containment if the rising and expanding cooking plume is rising at a low enough rate, which for induction mainly depends on the pan temperature. Without grilling or wokking, your actual flow, whatever it is, may be adequate so long as frying is done at low heat. We have too little information to make further containment projections.

In addition, before containment can be achieved, capture must be achieved. The hood only captures the amount of the rising plume that it overlaps. Hoods do not materially pull effluent from the cooking surface level. The hood traps the unsuspecting plume as it reaches the hood. To evaluate the overlap, we will need the aperture dimensions of each candidate hood and the candidate hoods' intended aperture heights above the cooking surface.

kas

Interesting, thanks for the info. Didn't realize that a 400 cfm blower may actually be pulling far less than 300 cfm. My house was built in the 1930s so it's probably not very airtight.

The chimney hoods I am looking at are 30" wide and range from 20-22" deep.

To simplify and generalize, I guess my question is simply this: Given equal blower capacity (i.e., 400 cfm), is an outdoor-ducted standalone chimney hood TYPICALLY superior to a outdoor-ducted over-range microwave vent?

I occasionally fry things. My current outdoor-ducted over-range microwave vent doesn't do a very good job, and my house will smell like fried food for sometimes as much as 18-24 hours. If a chimney hood would be likely to resolve or reduce that problem, I would probably get one.

I think it is a valid statement that a chimney hood is closer to ideal for capture and containment than most microwave oven venting schemes. If you search for KVSApplDesign_catalog.pdf, a publication by Greenheck, and read it or even skim it, you will learn what is considered important for ventilating commercial cooking. From this base it is possible to "eyeball" residential devices and estimate how far they deviate from ideal and hence surmise whether or not they are the best solution for the space.

It is important to keep in mind that you can have only two out of three among performance, affordability, and aesthetics.

With respect to the association of "equal blower capacity" and the fact that two blowers are rated at equal CFM, I have to point out that neither blower will be operating in free air, so each has some pressure drop across it. For the blowers to be equal, their fan curves (flow vs. pressure drop) have to be equal. This is unlikely. Further, the microwave may be expected to have a more tortuous path for the air flow, causing it be operate farther to the left (lower flow rate, higher pressure drop) on its fan curve. This would be expected to further reduce its flow rate over that of the more conventional hood.

Your candidate 30 x 22-inch hood is (ignoring wall thicknesses that I don't know) 4.6 square feet in nominal aperture area. The actual desired flow would be 4.6 x 90 ~ 400 CFM. With a leaky house but no deliberate MUA you would want a blower rated for at least 600 CFM. We can cut that down somewhat by arguing that induction has a lower plume velocity, and the type of cooking is less extravagant. From page 9 of the Greenheck document take the velocity requirement to be 50 ft/min instead of 90, and this yields 230 CFM actual. A 400 CFM rated blower with leakage MUA may work for this.

However, 22 inches is not much frontal overlap, nor is a 30-inch wide hood much overlap for pans that may extend to the edges of a 30-inch induction cooktop. So, while what has been captured may be fully contained, unless your greasy cooking is done at the back, and a cabinet side partly blocks the cooking plume expansion toward the side of the hood, some odor and grease may be expected to still be present in the room.

For a higher level of capture, a 36 x 27 inch hood would be preferred. This would, however, require deliberate MUA, depending on how your code requirement is worded, what combustion appliances you have and their sources of MUA, and whether your AHJ understands what safety issues MUA is intended to avoid.

kas