And you might also need make up air, otherwise your fancy hood isn't going to get anywhere close to the advertised cfms.

I fear the OP's research may be incomplete. Generally, it is best to determine requirements first and select appropriate components second. Here are some highlights of the requirements process.

1) Capture. The hood aperture must overlap the cooking zone to capture the rising and expanding cooking plumes carrying grease, water vapor, and odor. The OP's 42-inch selection is generally wide enough for the described conditions, but a similar overlap at the front is necessary. So a front-to-back depth must be selected.

2) Containment. The hood has to move the plume effluent from its entry aperture to the other side of the filtration (baffles or mesh) and thence out of the house. This typically requires an actual flow rate around 90 CFM per square foot of aperture area for the hottest grease or oil vapor to ensure that the plume momentum will not overpower the hood flow and reflect out of the hood.

3) Overcoming pressure losses. No blower will meet its rated flow rate except when operating in free air. With filter, duct, and MUA pressure losses, the actual flow may be only 2/3 of the rated flow, so the blower should be selected to have a rated flow rate about 1.5X the required flow rate.

4) Duct velocity. Once the actual flow rate is determined, size the duct so that the velocity of the air and effluent in the duct is around 1000 ft/min.

5. Make-up air. There is always MUA to balance the flow from the hood to the outdoors. They have to be the same. If not provided by deliberate means, the MUA comes from house leakage. In doing so, it makes the house pressure negative, which has negative consequences, including safety consequences and hood performance consequences. Deliberate MUA is an entire topic in itself, but unless over boosted with a blower, will still have some pressure loss that the hood blower has to make up for. Successful MUA design and integration into the residence architecture may be the most difficult of the requirements tasks.

After all of this is evaluated, then the HVAC components can be selected, while remaining aware that only two of aesthetics, affordability, and performance can be maximized. Else, they have to be balanced in some way.

kas

Thanks all for setting it straight. If MUA is required I can discuss that with the contractor. Everything I had read mentioned significantly less flow is needed when compared to gas. I have only had OTR microwaves before so this would be my first hood.

With induction, one doesn't have the gas combustion adding heat (and energy) into the rising plume. The maximum uprising velocity will be less for modest cooking on induction -- about half the velocity in one experimental situation I have a report on. I suspect that cooking conditions where the cooking plume greatly dominates the gas plume will cause the induction and gas plume velocities to be similar. In other words, the induction sear or induction wok cooking will (or at least can) have plume velocities approaching those with gas cooking.

I have a 36-inch induction cooktop and a Cooktek induction wok hob under one long hood. Both can generate a lot of plume effluent. My hood is running roughly at 90 CFM/sq. ft. at full power. For searing or wokking, I run full power and it is successful at capture and containment. I haven't tried to reduce power under those conditions to qualitatively see where there is noticeable failure to contain. I can say that even with this flow rate, it is easy for drafts to disturb the plumes and cause lost capture. This would be worsened by lower air velocity at the aperture.

However, even if you embrace half recommended velocity and take 45 CFM per sq. ft. of hood aperture (say, 42 x 24 = 7 sq. ft.) as the requirement, you should be looking at a blower rated at 500 CFM with good MUA. If the MUA will be filtered without a blower, or some the MUA path has some bare minimum diameter to get past the code enforcement officer, then a 600 CFM rated blower may be needed. Pressure analysis is needed to know for sure how close you can operate to any minimum requirement.

Hood ventilation is one of those areas where one needs to accommodate the worst case, else the worst case, no matter how rare, will load the house walls and furniture with grease and odor.

For additional perspective, read the first dozen or so pages of:

http://www.greenheck.com/media/pdf/otherinfo/KVSApplDesign_catalog.pdf

Everything I had read mentioned significantly less flow is needed when compared to gas.

While induction won't have the emissions that a gas flame does, the determining factor for cfms should be what you intend to cook. A vegetarian family won't need as much exhaust power as a family that eats a lot of meat. Think of it this way - will a juicy hamburger cooking in a pan throw off less grease and smoke when cooking on an induction cooktop vs. on a gas cooktop? The amount of grease and smoke will be the same on both.

You can get away with 400 cfms, and for most locales you won't need MUA at that level. I would never choose 300 cfms. Think about a typical bathroom exhaust which is 85-120 cfms, and that is just for shower steam in a small contained room.

Whether you will get the amount of exhaust you want at 400 cfms depends on your expectations and what you cook.

You also need to know the size of your exhaust duct diameter that goes from the hood to the outside. Hoods will specify what size exhaust duct they require, so you need that information. If you are still in the build stage of your house, I advise you to make sure an 8" diameter exhaust duct is installed. Don't let your builder tell you "a 6" duct is fine". Sure, a 6" duct might be easier for the builder, but you are the one who has to live with it.

Depends on what you plan to cook. For boiling water you'll be OK so long as you don't mind a bit of steamy air, much more and you'll want a bit more exhaust.

More: https://www.gardenweb.com/discussions/5161173/hood-faq