You have come to the right place to find disagreement with sales persons. :)

Due to the different construction styles of residential hoods relative to commercial hoods ventilating relatively standardized commercial cooking appliances, some of the historically used rules of thumb are not well aligned to the actual physics of cooking plumes vs. residential hood designs. Hence, the BTU rule of thumb is not suitable, in my opinion. Even for commercial conditions, the former ventilation company Greenheck had a similar opinion. I use a modified version of the Greenheck method.

Still available here, I think: https://www.tagengineering.ca/wp-content/uploads/2015/02/KVSApplDesign_catalog.pdf

For full capture of rising and expanding cooking plumes, the hood must overlap them. Nominally, a 36-inch cooktop should use a 42-inch wide hood. Front to back distance should overlap the front burners by a similar amount.

Containment in a residential hood with low reservoir volume below the baffles requires immediate entrainment into the baffle gaps. I believe the flow velocity of the baffle gaps should be about twice the velocity of the rising plume when the baffle gaps are roughly 50% of the baffle space.

Here is where cooking style comes into play. If you never (or hardly ever) cook hot enough to cause vaporization of oils and greases, then the peak plume velocity will likely be less than the 1.2 m/s that has been documented in a series of experiments I have read about. How much less is of course relative to how much one is cooking closer to a simmer than a sear. For searing and wok cooking, I recommend 90 ft/min for a hood entry aperture velocity. You can try to get away with less if you have a feeling from past experience how your normal cooking compares with searing.

Hood system CFM has to be the product of the velocity one attempts to achieve and the hood entry area in square feet. So for the searing case, we have 90 ft/min x 3.5 ft x 2 ft (say). This equals.630 CFM. But wait, there's more! Blowers are rated at zero static pressure (hanging in free air). With a hood there is a pressure loss at the baffles that is significant. There is a slight (if the ducting is large enough and short enough and free of bends enough) loss in the ducting. There is an undefined loss in the make-up air conditions (see below). I recommend using a factor of 1.5 to account for these losses, leading to a 900 - 1000 CFM blower rating.

If you are confident about the limited plume velocities of your cooking plumes, then you can drop this value to whatever your risk perception will allow.

No air goes out the hood that isn't replaced into the house by some means. One source is house leakage, but this is not really good for you or the house. It is a hazard if there are any combustion appliances that can be back-drafted at the house pressure that will result. (See many "MUA" threads here.) Also, in many municipalities, more than a rated 400 CFM will require an MUA system of some type. And we must not forget that the blower flow rate drops with the house negative pressure, so without sufficient MUA the hood will be partially gagged.

So plan on having to work out a means of bringing exterior air into the house. The path should have a filter, and in some locales a heater unless you live in an open window climate. There are many ways of achieving systems of this type. In some cases, a supplemental MUA blower will be needed. Relatively turnkey systems can be purchased. The variations on this theme are so extensive that reading threads here where attempts are made to find a realizable approach may prove enlightening.

Thanks for taking the time for this response! To recap so I'm understanding correctly, that I'd need a 42" hood with around a 900-1000 CFM rating, and an MUA blower (with a heater because I live in MA). I appreciate you sharing your vast knowledge!

MUA lives in the world of "it depends." We need to know your combustion appliance situation. If you don't operate a gas hot water heater, for example, or you do but it is in a room that is sealed from the house with its own source of fresh air, then you may not need an MUA blower to closely balance inside air pressure with outside air pressure. You may be able to make do with a filtered duct to outside matching the size of your hood duct (8 to 10 inches) with a heater in the path. Or, you might be able to duct into a basement or mud room where you added a heater outside of the duct that could handle the needed room heating.

We can quantify the needed heating.

For example, in the graph above if you look at the value 600 CFM on the abscissa and find the yellow line above it, which could deal with zero F outside temperature, and then look across to the ordinate, the required heating power is about 35k BTUh. This can be achieved taking a loop from a hydronic heating furnace, or via other schemes such as a 10 kW electric heater.

It would be best to first determine just what the local code calls for.

thank you. Yeah, the it depends is tricky! I do have a gas dryer pretty close by the kitchen, and a gas fireplace in the living room which is not too far away either. So based on those things it seems like a pretty solid MUA setup would be necessary.

my appliance salesperson recommended that I get a zephyr titan hood that is 750cfm and a second blower that would bring it up to 1350cfm. that seems super high to me, but I don’t know. It seems like with that much power than a proper MUA setup makes sense.

Im working on getting a contractor opinion on my particular options but gosh is this confusing!

Check your duct size - the duct that goes from the hood to the outside. It needs to be either 8” or 10“ diameter. Don’t let your contractor say “6“ is fine”. If the duct is too small, you will have increased noise and decreased efficiency, and that would be the case for any hood exhaust that you install.

That’s a good point. There isn’t a duct there currently so we’ll be starting from scratch.

New FAQ with some info that might help: https://bamasotan.us/range-exhaust-hood-faq/

For indoor air quality you will definitely want a proper MUA.

Am I understanding correctly that your duct will go up in to the ceiling, make a 90° bend and then straight shot to the exterior? How long of a run?

Kaseki and Opaone I'm in a similar boat to Melissa, and after reading the Bamasotan FAQ article, I'm even more confused about what to go with. What hoods do you have and why did you choose what you did?

From the FAQ:

SO, to cut to the chase. The quietest (and at the same time best performance) system will be a commercial system from Accurex, CaptiveAire, Hoodmart or similar with a proper sized hood with proper containment and proper baffles + well designed duct + duct silencer + a remote inline or external blower (that’s designed and balance for quieter and more efficient operation). Second best is the same system but with a consumer hood such as Wolf or Modernaire.

The first, Accurex, is what I have and I believe the second, Wolf + Silencer + Fantech blower? is what @kaseki has. The Accurex system is both the quietest and highest performing and easily worth the minimal extra cost for us. I think what @kaseki did is the best option w/ consumer grade components.

I think I'll create a section just for this with Good, Better, Best recommendations.

For the exhaust hood system: Wolf Pro Island hood (largest size to fit 36-inch induction cooktop plus adjacent Cooktek induction wok hob; Fantech silencer (10-inch); and Wolf (Broan) 1500 CFM (rated) 'down-roof' blower. Not enough ceiling height in my case for a commercial hood. Limitations on my time and the duration of the already wildly extended reno scope limited the possible 'creativeness' I might have been able to embrace if the only thing to address was the ventilation. In particular, I did not have time to deal with interfacing non compatible hood controls with blowers. A degree of style was needed where the hood also supported the visual divide between kitchen and reduced living room area of the existing house.

@opaone - my appliance sales person (after extensive conversations with his boss) is adamant that for my 1987 built house, MUA is not necessary. Not sure how much I believe that, especially given the info here.

ideally the duct wouldn’t go through the ceiling at all, just as high as the ceiling with enough clearance for a 90* bend and through the wall. Our ceilings are a bit less than 8’ so I’m hoping that’s a possibility, the duct will be very short.

@Melissa, Yeah, your appliance sales person and their boss are ignorantly wrong, especially w/ the CFM's they've recommended. In most parts of the U.S. it is also illegal, for very good reasons, to install a hood over 400-600 CFM without MUA. Lack of proper mechanically powered MUA will reduce the performance of your hood, increase the noise, create potential health problem in your house and possibly create mold/rot problems in your walls. You don't want any of that.

It might be difficult to reduce the noise very much with a very short duct run. You should use an external blower if at all possible, NO internal blowers. If you've a long enough straight run then definitely install a Fantech silencer. After the 90° bend above the hood then how long of a distance to the outside wall?

Yeah, I've gathered as much re: MUA. I haven't been able to find much info about code in my town but I need to probably reach out to a professional for that. Google searches aren't the thing for small-town bureaucracy, turns out.

The hood/range is on an external wall, so it's literally just right outside.

The MUA issue boils down to what will pass inspection vs what you really need. You need deliberate air replacement or else have to allow for the limited flow rate from a gagged hood blower. You need low pressure drop air replacement if you have combustion appliances working from the same internal air pressure. In between situations may vary, such that powered MUA may not be needed. Persons without combustion appliances can test this by operating the hood on full power with windows closed and feeling around windows and light switches for air leaks. 'Tis better to put a duct through a wall into a basement, say, than have all the leaks moving dust into the house from the walls and moisture into the walls from outside.

That makes sense. Would it have to be a duct from the kitchen/hood to the basement?

Gotcha, but due to aesthetics of it being on the center wall of my kitchen, would there be any issue with treating it like a hood liner, and covering it with something? Or would this affect the function of it? We have 10 foot ceilings....

If your interface with the kitchen is the ceiling, I would use a diffuser (see Hart & Cooley) with vanes directing the flow away from the hood. Various other ways of hiding the air duct can be imagined, and so long as they don't interfere with the flow, or create turbulence at the cooking zone, they should be fine.

Gotcha, but due to aesthetics of it being on the center wall of my kitchen, would there be any issue with treating it like a hood liner, and covering it with something? Or would this affect the function of it?

What is 'it'? Hood or MUA?

Sorry, should have been clearer.....the hood. I was thinking that we could go with the Accurex, but with the look in our kitchen, I could cover it with a Range Hood cover potentially. I'm trying to get an HVAC consult for the needed MUA. Which Accurex did you select? We are purchasing a 36" BlueStar Platinum series range.