A hood aperture at the ceiling needs to be fairly large, ideally at seven feet extending about 20 inches beyond the locus of points that consititude the edges of pans on burners projected vertically to the ceiling. At 9 feet the overlap should be 25 inches larger. (These values are based on a 22.5-degree effluent expansion angle, but rising effluent angles have been measured at lower heights, so cooling effects might make the rising effluent shape less conical up high and allow some reduction in size.)

Flow rate (cfm) has to be larger proportionately to the larger aperature area of the high hood. Proportionately larger make-up air supply will be needed in many cases.

Design should approximate commercial hoods (without the fire extinguisher) which are typically mounted that high.

In any case, the higher the hood the more easily effluent can miss being captured and contained due to air turbulence from drafts, people moving, etc.

kas

Thank you for such a detailed (and a little overwhelming!) explanation.

This hood is 54" wide x 25" deep, and it's going over a 36" cooktop.

If the cooktop is 36" x 20", then there's 18" extra width and 5" extra depth on the hood - a little less than your calculations... but it's not like I'll be using all the burners all the time - maybe 2 or 3 max. For airflow, it's got 2 blowers with 940 cfm each. So, except for Christmas and Thanksgiving dinners, I feel that it should be adequate... what's your call?

Also, regarding mounting height - if I tell the contractor to make a soffit box maybe 8-9 down from the ceiling, will that make a major difference? I'd rather have it flush to the ceiling, but I can tolerate a soffit, IF there's a good reason for it.

I'm going to try to translate for kas, who, surprisingly, is being rather polite in his recommendations. I'm not so kind. I find this kind of concept is ridiculous, unless you have no interest in actually doing any useful ventilation. It's all for "show", not at all for "go."

I don't think you get the implications of kas's calculations. At nine feet ceiling height, the outer edges of the hood need to be 25'' bigger on all sides! That means, given a 36'' x 20'' cooktop that it would need to be roughly 86'' x 70''--that's right, 7 by 6 feet, to have adequate capture. Your proposed 4 x 2 foot unit would be an epic fail. As kas said, for this to work it would need to look like one of the huge hoods you see in a commercial kitchen. Nothing at all like the cute picture on the Futuro website. That's just useless.

Dropping the ceiling down with a soffit by 9 inches (assuming that's what you meant) reduces the dimensions needed by something around 4'' on each side--a meaningless difference.

You can install a hood for ventilation, or for style. If all you want is the latter, go for it. But don't expect any useful ventilation.

Sure, I understand your concerns, and if you can find someone who has a similar hood setup and finds that it works great, more power to you.

I agree totally with you that a 7x6' hood is ridiculous. That's really the point--that fundamentally, the concept of an "invisible" hood in the ceiling just doesn't work. It's simple physics, as kas was trying to point out. As the plume of effluent rises from your range, it inevitably spreads out laterally in all directions, in a pattern than can be roughly approximated as a cone with sides angled at 22.5 degrees. If your hood is not large enough to capture the effluent as it spreads laterally, it cannot contain it and then evacuate it through your ductwork. No amount of airflow, no matter how great, can compensate for lack of containment, short of flow that is so great that there is a true vacuuming effect. That is not achievable with conventional blowers--certainly not 1800 cfm. If you're talking about 10,000 cfm, maybe you start having a point. But then we're not talking hood vents, we're talking HVAC at that point.

I still disagree that all vents are ugly. For example, see the top right image here:

custom hoods from ModernAire

While this kind of unit is still substandard in performance, it's a heck of a lot better than trying to suck everything up at ceiling level!

It's beyond useless because it gives the illusion that the purchaser is actually managing to vent their kitchen properly. It's like trying to light your kitchen with a single 60 watt bulb from a 10' ceiling. It might be marginally better than nothing, but it's not ever going to be a substitute for the proper technology.

I suggest you visit some showrooms with island vent hoods installed. You will not find them all that obtrusive and you are sure to find something that works about 1000% better.

Those "hoods" are for people who only microwave leftovers. If you fit in that category, go for it. If you aren't, then you need to get serious about finding something that actually works first, and then secondarily satisfies an aesthetic.

Or move that cooktop to a wall where it's a heck of a lot easier (and cheaper) to vent it properly. You only spend 10% of your time in a kitchen applying fire to food. The vast majority of "cooking" involves prepping the food to be cooked. That takes up 70% of your time. Figure out a way to maximize the sociability of that task by maybe putting a prep sink on the island instead and relegating the fire to another location.

I'm definitely in hollysprings camp. I think islands are a poor place for a range. Conversely they are often great places to do prep, have seating areas for bar and food service, to place storage or appliances underneath, etc.

If you like aerosolized grease all over your ceiling and cabinets than go for this hood. I can't imagine this hood working any better than opening a window.
You need a real hood mounted 30-36" about the cooktop if you actually want it to work.

Also, unless you have a huge leaky house you WILL need makeup air with an 1880cfm vent hood code or not. Depending on where you live you will need to condition this make up air which will cost you at least 7,000-10,000.

I don't know how big your house is but lets assume it's 3,000 sq ft with 10 ft average ceiling height. You will evacuate all the air in your house every 16 minutes.
Good luck

Normally we speak of capture and containment as the primary duties (required capabilities) of a hood, and these usually require a modest sized hood 2.5 to 3 feet over the stove. There is a third capability of a hood, removal of air from the house. If the hood is too high for its size, then smoke, moisture, odor, and grease particles from cooking permeate the house, or at least the kitchen if the entry passages are small. Those portions that stay aerosolized will eventually be removed with all the air being moved. Some will condense on the walls, ceiling, floor, cabinets, curtains, etc. Thus, the hood will appear to remove it all, eventually, but surface cleaning will reveal why immediate removal is preferable.

kas

Clin: "surprisingly"? :)

Kas: I was struck by the remarkable restraint that you exhibited (and I lacked). Your post had the requisite information, but I could not resist pointing out the implications in a more direct way--a 7x6 foot hood!

To the OP: it is, unfortunately, an act of faith on the Appliance forum, that ventilation performance of a hood is ranked somewhere below godliness, but above most other attributes. This is due to a small, twisted, but very vocal group who truly obsess over capturing every molecule of effluent produced by their ranges. In that world, there is no question that a ceiling mount hood is blasphemy. And, we're right, such a hood will do a really lousy job of ventilating.

However, it should be pointed out that those outside the lunatic fringe may rightfully believe that aesthetics are as, or even more important than ventilation performance. A ceiling-mount hood is just fine as long as you have reasonable expectations--that it will wow the neighbors, but will also fail to prevent the smoke detectors from going off when you try to really sear a steak properly. That'a a tradeoff that may be just fine for you. Not for me, but possibly for you, as long as you know what you're getting. It's like those Gaggenau show kitchens--they are awesome as long as you don't actually cook in them. There are plenty of folks who would choose that over my now battle-scarred Lacanche range (with fabulous ModernAire hood). But I built my kitchen to cook first, impress neighbors second (though I still think it does the latter too!).

We have a flush ceiling vent and it clears the smoke and grease fine. No gunk on walls, cabinets or ceiling. It has been installed for 24 years. Our ceiling height is 8’.

Would you mind providing more data, @Paige King?

What is the capture aperture size of this ceiling vent? What is the rated CFM of the blower? What is the brand and model of this unit? What make-up air capability do you have?

Is the surface at the ceiling baffle like, mesh like, register like, or diffuser like? A photo may help. What vent cleaning regimen is necessary, or, alternatively, what is the condition of the duct inside surface?

What is your cooktop size and type?

@Paige King I would also love to hear more. I am struggling to find real reviews from people that have used these and only curmudgeons who only give theoretical reviews. I'm considering for my 8' ceilings over a 42" high island with Wolf gas range

Curmudgeons-R-Us

Please review the first dozen or so pages of the guide at the link below for insight into the methods used to meet the goal of proper kitchen ventilation. While theoretical for me (I don't have a commercial hood, only Wolf's largest Pro Island hood) it is not theoretical for Greenheck (or their now spun off ventilatiion division), or for the ventilation system of our forum's @opaone.

https://www.tagengineering.ca/wp-content/uploads/2015/02/KVSApplDesign_catalog.pdf

I think I should also supplement this thread with the thought that some recent ceiling vent schemes (not technically hoods) use slotted edges for their apparent air intake. While their noise behavior may exceed that of a large area of baffles, they may also provide an effective expansion of the capture area by pulling air across the ceiling -- if designed to do so. Now we have a larger effective capture aperture, as desired, but (!) if the ceiling is not warmer than the cooking effluent at that height there will still be condensation onto the ceiling.

It boils down to what you expect to be cooking. I would argue that wok cooking and steak searing will not be captured well, that bacon simmering may be only partially captured due to its low upward plume velocity causing spreading into the room air, but steam and steam embedded grease particles that don't attach to the surrounding ceiling will be captured.

And don't forget that the plume expansion angles that one can find in the Finnish cooking report do not account for air turbulence from people moving about in the room, or drafts due to heating or air conditioning. It is always best to capture as low as possible as limited by sight lines and head clearance.

In commercial settings where capture and containment is provided by ceiling systems, they cover the entire ceiling and use special techniques to disassociate grease in the plenums above the ceiling.

Heydal photo

Some quick points.

- We know a lot more about the harmful health effects of various things than we did 25 or 50 or even 5 years ago. And we'll know more next year and in 2025 than we do today. There is also some educated speculation about what will or will not prove harmful. There was educated speculation about the harmful effects of asbestos, formaldehyde, CO2 and other things years or decades before there was any data or proof (and similarly for things that turned out to not be so harmful).

- Houses today are much more sealed up than houses in the past. This has provided a lot of data on the need for ventilation (removal of harmful stuff that builds up in enclosed houses).

- Most harmful airborne stuff is invisible and odorless. You can't see it, taste it or smell it in the air (interesting note, the smell of NG is added because NG itself is odorless so without the smell people would never know if they have a gas leak in their home). Anyone who categoricallly states that their system removes everything is quite ignorant. Even with my access to test equipment I don't know. Brett Singer at Lawerence Berkely National Labs will say the same as will scientists at various labs in Europe.

- All I (or @kaseki or whomever) on here can do is provide what knowledge we have. As I've said many times - It's up to homeowners to take responsibility for their and their family's health.

Anyone recommend a self ventilating cooktop instead? I have an island as well with cooktop on it. I don't like the idea of dishes piling up at a sink where folks are seated and socializing.

There is not such thing that works. You need to have a proper exhaust hood above your cooktop.

Newcomers to this subject need to keep in mind that a hot cooking plume has upward momentum independent of whether there is a hood above or not. If there is a hood, its seemingly large CFM nonetheless induces rather low air velocity down at the cooking surface, particularly in an island configuration. Hence, the overhead hood captures effluent not by high air flow at the point of plume generation, but by lying in wait for the plume to rise to the hood. Containment depends on flow rate, and that happens within the hood to keep reflected plume components from escaping.

So it may be understood that ventilating via a cavity at the cooktop level requires moving enough air at a high enough velocity to divert the cooking plume from upward momentum to downward momentum. As seen in various "demos," low momentum steam can often be pulled to the side or even down, and this is also likely true of odor from simmering food, but hot searing plumes and wok cooking plumes require more airflow at higher velocity at the sidedraft or downdraft intake than anyone would want to tolerate, even with ear protection.

A downdraft would make the most sense surrounding a cutting board for capture of chopped onion mist, which may exhibit negative buoyancy in still air.