How much CFM does one's ventilation hood really need?

7 years ago

Hi,

Posting again about ventilation hoods. What is a good CFM to be looking for in a ventilation/extraction hood over an island? Think: some Indian style cooking, ,occasional wok, mostly the usual frying, sauteing, cooking.

I thought I should be looking at 600 CFM but noise would be high running that on max and also it seems its not easy to find reasonably priced hoods with that high a CFM.

Please help.

Featured Answer

kaseki

7 years ago

For cooking plume containment by the hood from likely residential cooking, the specific air velocity should be around 90 ft/min at the hood aperture. This is equivalent to 90 CFM per square foot of hood aperture.

The resulting flow rate must be achieved in the presence of pressure losses. These are the pressure drops due to:

baffles
hood transitions
duct losses
exit cap transitions
make-up air losses

Blowers used to move the air through the ducting, whether in the hood, in the duct, or on the outside wall or roof have a flow rate that drops in some proportion to the increase in pressure loss in the air circuit, which in this case is stovetop-to-outside-to-stovetop. The achieved flow rate with pressure loss is given by the blower's "fan curve." Some examples may be found at Broan's web site.

If one doesn't know all of the above parameters, then a wild guess may suffice. I recommend choosing a blower with a zero static pressure flow rate of 1.5X times the required flow rate found by multiplying 90 CFM/sq. foot by the hood aperture area in square feet.

Please note that good ventilation requires both capture and containment. The former depends on the hood sufficiently overlapping the rising and expanding cooking plumes.

kas

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Comments (17)

James May
7 years ago
CFM <> DB. In other words the CFM the hood can move has nothing to do with the amount of noise the hood creates. It depends on many factors - fan design, fan size, hood type and design, etc.
transnationalq
Original Author
7 years ago
ok, going back to original question, what is a good CFM range to be looking for for the kind of cooking mentioned. we cook two to three times a day.

What factors would additionally impact an effective CFM without getting too caught up in finding perfection.
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There are multiple issues with hood cfm and MUA requirements that need to be addressed, including: Safety is dependent on whether the fan at full power with windows however you will open them without fail causes the house pressure to fall below about 0.03 inches of water; but(!), this is only an issue if there are combustion appliances that can be back-drafted by the pressure drop or if there is an operating open fireplace. A cracked window is going to provide only modest MUA cfm at low pressure drop. A wide open window (with screen) is needed for larger capacity ventilation if the pressure is to stay below the threshold of back-drafting. Restricted MUA causes a pressure drop which in turn reduces the capacity of the ventilation system. There is always MUA for whatever flow rate the hood actually achieves. For part of the year in most of the USA and Canada, the outside temperature is low enough that normal household room heating capacity (BTU/hr/room with window open) will not be sufficient for the larger ventilation fan capabilities. This means that it will get cold inside for the duration of grilling or wokking unless a blower supported MUA system is used with a heat exchanger. To keep rising effluent from curling out of the hood due to its momentum and "reflections" in the internal hood geometry, it is necessary for the air velocity established by the exhaust fan in the vicinity of the baffles (or in mesh hoods at the bottom) to equal the velocity of the rising cooking effluent. Because the fan air velocity will be about the same over the entire critical area, the flow rate (cfm) has to be about equal to the effluent velocity times the area. (There are fluid dynamics effects around baffles that make this kind of rule of thumb somewhat approximate.) People have different tolerances for how much odor they want in their houses, how much grease and moisture escape they will tolerate, and how cold they will allow their house to get (or their guests to get). This leads to a wide range of views on how well their venting system works. If limited by aesthetics, architecture, and cost, as most of us are, then one can only do his affordable best and accept less than perfect results. kas My Clippings has more on this subject
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Hi - this thread is a little old now so this might be too late, but I have some related Qs even after reading all this helpful advice. I'm starting gut reno of a 120+ year old townhouse in NYC and plan to install 36" bluestar with 2, 22K BTU burners (in addition to other smaller burners of course) which we'll use primarily for stir fry. We HATE grease smell in the house and cook all the time, so want to get what we need for the stove we have. So I'm fine getting major CFM, if that's what it takes, but my concern is the ducting. It seems we have 2 options - up an existing chimney (which we're having relined and is not used for fireplaces; fireplaces were all sealed up decades ago) - but that would be 3 stories or about 50 feet needed to get to the roof and I wonder if that's too far for it to function properly...2nd option would be to bring it down into a soffit on the floor below (a rental apartment that already has a dropped ceiling) and vent out of the exterior wall, but this would require making a new hole in our masonry, and would mean the venting would be just outside our kitchen, under a french door that we're likely to want to open often while we're cooking - so the smoke could in theory end up back in the kitchen, no? I am by no means an expert so forgive me if I'm totally off base here, but any thoughts/advice would be much appreciated. Thanks!
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The necessary information may be found by searching this forum. To summarize: Obtain a hood that overlaps the cooktop by enough to capture the rising and expanding effluent. Assume for this discussion a hood with a 42-inch wide by 27-inch deep aperture area. This is shy of 8 square feet. Desirable velocity at the baffles is 3 ft/s, leading to a need for 24 cu ft/s or around 1400 cu. ft./min. Experience shows that only half this is needed due to baffle behavior in directing the effluent flow close to the baffles. (Or to look at it another way, only half the baffle space is aperture.) This consideration yields a value of 700 cfm. To actually flow 700 cfm, the blower needs to do so at some pressure drop determined by the baffle loss, transition loss, duct loss, and MUA replacement air flow pressure loss. For likely conditions, a 1200 cfm rated blower would be desirable. At best, a VAH hood's rated actual flow (600 in Homechef59's message) only avoids part of the pressure loss. It is not equivalent to a 900 CFM blower with a baffled hood except when each is hanging in air without a house and ducting. While a 1200 cfm rated system is recommended in this example, stepping down to 900 rated cfm may be adequate depending on what plume velocity your cooking effluent actually achieves, and what area of aperture you end up with. Some effluent escape (capture but missed containment) or effluent missing the aperture (missed capture) can be expected at some level of flow and/or hood shrinkage. Note that this basis for CFM estimation does not depend on the number of burners that may be in use at one time, but on the pan temperature(s). Every plume rises and has to be captured and contained, and the other hood areas not directly impacted by a particular plume do not help capture and contain it. kas
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homepro01
7 years ago
Hello transnationalq,

What type of cooktop are you planning to ventilate?

Will you have the ability to add a makeup air system to your home? At 600cfm or greater, you will need this. They are an added expense by needed if you have gas fired appliances like a water heater in your home. Because you are in a region where you have cold weather, the makeup air should bring in warmed air rather than just cool air.

Is this in island installation? If so, island hoods tend to be more expensive because they are finished on all four sides. Also, island hoods may need to be sized slightly differently because they are working against physics. With a wall hood, the wall acts as a barrier to prevent the air from escaping on all three sides before the hood can effectively move the air.

Where will the vent be going out of the house from? The roof or a wall? Can you accommodate a roof mounted or wall mounted external motor? This could reduce noise in the kitchen and the vent hood in the kitchen is just a collection mechanism for the cooking smells being generated.

What is your target budget for the hood?
Fori
7 years ago
If you're planning on wokking, you might want to consider an oversized hood or a cooktop with a center burner that can put your wok entirely under the hood. I have a 36" hood over a 30" 4-burner range and my wok sticks out past the hood on the side. That reduces capture a bit. :)
transnationalq
Original Author
7 years ago
homepro01,

It would be an island mounted gas cook top- 30" Thermador 5 star burner total BTU output 52000 - and we were hoping to vent it outside via a duct going along the ceiling (inside/between) the joists. Is it a bad idea to build the duct for the cook top into the ceiling? I had been thinking of an external blower but they are pricey unless any blower can work with any hood that allows external blowers?
transnationalq
Original Author
7 years ago
Fori is not pleased, Do you have a four burner cook top? The largest burner on ours is in the centre and that's what would get used for the wok and high heat cooking. You think I should still consider a 36" hood?
PRO
Trevor Lawson (Eurostoves Inc)
7 years ago
To answer the question I think your looking based on your notes.

Island Location

Indian style cooking

Occasional wok,

Usual frying,

Sautéing,

Cooking 2 -3 times a day

I would suggest a minimum of 1200 cfm
transnationalq
Original Author
7 years ago
That sounds like I'd be stepping in to pro range style hood territory.
plllog
7 years ago
Look at ModernAire. They have all kinds of style hoods that you can get with 1200 cfm. (I have one--it's overkill for me most of the time, but it does a fab job on low, when I just have a pot of soup. :) )
Fori
7 years ago
In an island setting I'd go bigger for sure. My hood backs up against a wall with cabinets on either side. That helps a lot. All open on an island...you need moar!

If you use a large wok, be sure that putting it on the center burner allows you to use the other burners. (I have a 4 burner 30" range and maybe I just need a smaller wok.)
PRO
Trevor Lawson (Eurostoves Inc)
7 years ago
Size wise minimum 42" wide x 27" deep..... Deeper if possible. mounted as low as possible to the countertop.
User
7 years ago
Toldja not to put it on the island. Now you've got a minimum 5K expenditure. Add the piping, siding repair, and heated MUA, and it's a easily a 10K problem.
transnationalq
Original Author
7 years ago
plllog - How much did that hood cost you? The website says its 'trade only'. I guess I could call them too but thought I'd ask in case you remembered.
transnationalq
Original Author
7 years ago
Sophie Wheeler Its still a work in progress. Nothings been installed or fitted yet. We Um and Er but find ourselves going back to the same layout to avoid other compromises like shelf space for appliances, wall cabinets etc.
Russ Barnard
7 years ago
Might also look at these people, although I did not see a 27" deep model.. Modernaire would definitely have a solution for that.

http://www.therangehoodstore.com/island-range-hood/42-inch-island-range-hood.html

We went to the Range Hood store for our wall mount.

R
practigal
7 years ago
Are you going with gas? If so add up the BTU of all of the burners and divide by 90. If you're determined to have an island cooktop, don't do gas.

I don't know what your floor plan is like but it could be that you're making all of these compromises in order to have an island or some other feature in a space that would be better served without? Have you posted your floor plan on the kitchen forum, they generally have some good advice there.

How much CFM does one's ventilation hood really need?

kaseki

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