IMO yes, you don't need as powerful of a vent. With gas, something like 40% of the heat misses the pan and ends up in the room so in addition to getting the smells and steam out, you need to suck that waste heat out. With induction, you are using most of your "heat" to cook the food so you only need to deal with the smell and steam. We had a telescoping down draft before which was decently effective for the main center, and two back burners, but ineffective in the front. This go around we are doing an overhead because we didn't love the downdraft. Also, despite a more powerful overhead vent, I will likely continue to cook Chinese food outside.

I have induction and a 350 cfm
I wish I had gone wider than cooktop and a higher CFM as I do smell food the next morning.
I was trying to avoid MUA

You haven't overstayed. You're asking the questions that the silent majority who never post wants the answers to.

A larger fan on a lower setting will do a better job of venting than a smaller fan on a higher setting. I don't remember the physics, but Kas and others have provided the reasons, if you're interested in doing a search.

You don't need more power with induction because even if you didn't vent, you'd just have smells and cooking gunk. With gas you have potentially harmful stuff in the air. Given that, however, the recommendations are usually tied to what your stove will be doing on full tilt, regardless of fuel. That is, if you're grilling bacon and frying potatoes on two elements, boiling up a huge pot of bones in the middle, searing the meat you removed from the bones, and sauteing aromatics on the last element (i.e., starting stew and stock, and making breakfast), all at once, what do you need to pull from all that heat and effluent?

Even with induction, it's great if you have the space to have the canopy three inches bigger than the cooktop on all sides, and I wouldn't go under 600 cfm. For what I described above, you might want more like 1000. :) I don't think your 715 (not knowing the brand, just the number) is out of line at all.

You do need to be concerned about makeup air, though, especially if there's a gas appliance inside the house, or even a gas fireplace. In a well sealed house, a strong vent hood can pull out enough oxygen to snuff out a pilot light on a furnace, interfere with a dryer, etc., and can make a noticeable pressure difference between inside and outside. Some places require a specific ingress for make up air (MUA), while others let you put in what you want and expect you to open a window. If you have climate extremes, even if your house isn't sealed, you might want the makeup air to be cooled or heated. These are things to talk over with your contractor and/or building department.

Glad to know I haven't worn out my welcome and I very much appreciate the insights! (At least I haven't worn it out yet; there's probably still half a dozen questions on various kitchen topics that I have...)

I'm sorta thinking that if we go gas, we'd go with the 715 CFM vent, but that with induction we might be able to back off to 600 CFM. If so, I'm wondering if that makes a difference re MUA as it seems a bit murky to me as to at what CFM level you need it. Our home was built in 1948 and we live in Southern California near the beach so windows are almost always open. Given that, I'm also thinking it would make sense to have MUA for the 715 CFM vent, but possibly not at 600 CFM. (Interestingly, there are no MUA regulations in my city.)

We're looking at 36" inch wide vents for a 30" inch cooktop, but it's hard to find many that are very deep--the deepest I've found so far is 27". I'm also hoping to find an 8 inch duct as other posts here suggest that will be more efficient (and possibly quieter).

27" is deep enough if you have a standard installation in a standard 24-26" counter.

You know about the Kitchens forum, right? You can ask all kinds of non-appliance kitchen questions over there. Ask all the questions you want in any of the forums. That's what they're here for. And you're pleasant! Which means people will want to answer. If you think you're asking too many questions, try to find a few that you can answer and pay it forward. :)

I would be shivering near the beach with the windows always open, but given that, and the age of your house, you should be all right without a specific system for MUA, but try not to run the dryer while you have the hood on. :) And I'm assuming your water heater is outside and the furnace is either in the attic or, perhaps, you have lovely gravity heat from below? '48 seems early for electric. If you have one of those gas heaters in the middle of the wall, that would be another thing to make sure of while you're running the hood. I'm no expert! These are just the things that everybody warns about.

The answer to how much MUA is needed depends on what negative pressure the house (kitchen) gets down to when running the blower at some power level. The more negative you get, the less cfm are actually moved from the hood to the outside, the harder it is to capture and contain all of the cooking effluent, and the easier it is to backdraft any combustion appliances that depend on getting their effluent up a chimney using passive means.

Open windows (with screens) may be ok, depending on the area opened, the blower fan curve, and resistance of the screens (are they clean?). Window opening of the order of the area of the hood aperture might be needed, certainly half the aperture, to assure that the negative pressure is not a problem for the other combustion appliances (a mere 0.03 inches of water column lower pressure can backdraft some appliances.) .

If such appliances, where used, are not connected to the air in the kitchen, and no one is using a fireplace, then the pressure loss through a too-narrow window restriction can be tolerated, so long as the odor and grease continue to be evacuated for the blower setting in use.

In general, CFM for blowers is specified for zero static pressure, and without knowing all of the pressure losses, starting with the aire getting into the house and ending with getting out of the house, along with the fan curve of flow vs. pressure loss, and including the baffle/mesh pressure loss, one cannot possibly predict what CFM are actually being moved. In particular, wheither a 600 cfm rated unit actually moves more cfm than a 700 cfm rated unit cannot be determined without measurement.

All else being equal (which rarely is the case), the two Finnish papers noted in my Clippings have comparisons of upward velocity and volume rate from different cooking surfaces that will likely indicate a difference between gas and induction, but I wouldn't expect in most kitchen scenarios that the hot burnt gas that rises from around the pan being heated adds a lot the to hood requirements if the requirements are based on large-diameter hot cooking surfaces generating significant greasy effluent.

kas

Kas, I was hoping you would respond as I've been reading several of your past posts. Unfortunately, since I'm not very technical I only understand about 5% of what you write, but since I've read at least 20 of your posts, in aggregate I think I'm close to 100% comprehension. In particular, if I'm understanding your last sentence above, the requirements for venting induction aren't significantly different than venting gas. Instead, the venting is really mostly about the effluent. (Wow, first time I've ever typed that word.) And very useful info about MUA

Plllog, than you so much for your insights. Our gas furnace is in the attic and the water heater is outside. But I totally forgot about the dryer, which of course is gas!

Yes, much depends on the type of cooking being performed. I'm sure that a six-burner gas cooktop on full power searing six-pans of meat and Szechuan peppers would require a bit more air flow rate than six equally large pans performing the same task on some theoretical six-hob induction cooktop moving the same heat into the pans. But both would likely do fine if some general rules are applied to the ventilation setup that are intended to deal with unquantified general applications.

Lacking any technical information what-so-ever for this application, I would just suggest (a) making the actual flow rate at least 90 CFM per square foot of hood aperture (the area where the air enters the hood, hopefully never to escape), (b) making the duct size such that the total flow rate causes a velocity in the duct of at least 1000 feet per minute (this is a simple calculation left to the student), and (c) using a blower sized to be at least 1.5X the result found for (a). Then ensure that the flow rate of (a) can be met by the MUA source at low negative house pressure. This can be measured using modest cost devices that may be found on the Internet. (The sum of all the pressure losses in the path is what reduces the blower performance to less than rated CFM. )

(Note that other rules of thumb exist for (a); one such may be found in the Greenheck manual I link to in my Clippings. It is for commercial applications, however, and best applies to commercially configured cooking sources and hood designs. Instead of assuring that the velocity of the air at the baffles is a particular value related to effluent velocity, it tries to make the flow sufficient at the boundaries of the hood. Commercial hoods do not typically have low angle baffles filling most of the aperture area, but instead a narrower set of steep baffles higher up with angled hood surfaces rising to meet the baffles.)

I'm not convinced that rules of thumb based on BTU/hr burner capability are as appropriate as a rule based on air velocity at the hood aperture, as the latter allows for a range of hood aperture sizes.

kas

Thank you Kas for the detailed info (although you lost me at the "simple calculation" part). The 90 CFM per square foot of hood aperture sounds like a good data point (and one that even I can calculate). I think I'll use that as a starting point and then add a bit more oopmh to it to give us a margin of error.

I will also discuss MUA options with my GC, although at the same time it would be so easy with our set up (and weather) to broadly open a window (or two or three) or even open the french doors to the kitchen.