What should be the minimum CFM for a Wall Range Hood

skubaJune 20, 2013

I bought a 30" LG gas range and I need to get the hood. We are looking for a chimney wall hood. Kitchen is well ventilated with a window and skylight window. What should be the minimum CFM for the hood?

We plan to put a 36" hood.

Thanks so much

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skuba

And the duct is 8"

Thanks

    Bookmark   June 20, 2013 at 7:06PM
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Nunyabiz1

IMO the minimum for a hood would be 600cfm

    Bookmark   June 20, 2013 at 7:19PM
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skuba

Any other opinions? Also band suggestions? I am not finding many options with 600 cfm.

Thanks

    Bookmark   June 21, 2013 at 4:10PM
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kaseki

My suggestion is a minimum of around 90 cfm per square foot of aperture opening facing the cooking zone. This will provide a velocity over the opening of half the velocity of the uprising effluent, and probably equal it (as is desirable) at the baffle openings where the apparent aperture is smaller. If the aperture is too small, you may miss some rising and expanding effluent, but more cfm will not help suck the stuff that misses the hood into the hood (except marginally) because the air flow away from a hood aperture rapidly decreases with distance.

This flow rate is intended to be the actual flow rate. Do not expect a baffled hood or mesh hood with some ducting to actually flow the rated amount of the fan due to pressure losses. Best to err on the conservative side and assume only 2/3 of the rated amount is achieved. (This varies with a lot of things, including the pressure loss in providing make-up air.)

So given a 3 ft by 2 ft aperture as an example, the area is 6 square feet for which 360 actual cfm are recommended. Dividing by 2/3 yields a fan rating (at zero static pressure) of 540 cfm.

Find the aperture areas of candidate hoods and do the cfm calculations as above and find systems having a fan that approximately meets the cfm suggestion. Then select based on what you can stand aesthetically and financially. This is easier if the hood is selected based on size and character and the fan is mounted on the roof.

kas

    Bookmark   June 21, 2013 at 7:22PM
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attofarad

My wife has a 435CFM 30" over her 30" Viking range, which has four 15kBTU/hr burners. 8" duct, ~8 foot run, no bends. It is okay.

    Bookmark   June 21, 2013 at 9:36PM
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needinfo1

So, when making a choice between fairly similar appearance and style hoods, should I be looking for the one (whether with baffles or mesh) that has the largest percentage of the bottom collecting area covered by the filters? In other words, if one hood has a bottom that is 50% filter and 50% surrounding metal material it would not be as good a choice as one that is 70% filter area and 30% surrounding metal. Or, am I completely off base on this? Thanks.

    Bookmark   June 22, 2013 at 4:58PM
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kaseki

The short answers are yes and no, in that order.

If your hood has a flat bottom with filters in it, very unlike commercial hoods optimized for efficient grease collection, then only the mesh area or baffle area counts as aperture, and generally is the only area that effluent will be captured over. (Some smoke may move sideways along the metal and be captured at another mesh.)

Pyramidal hoods with baffles farther up inside them, are closer in configuration to commercial hoods, and all of the aperture at the bottom counts as aperture. Of course, design errors may make containment less than ideal, but usually given enough flow rate, all of the smoke captured will be contained.

I should note that hoods with horizontal baffles at the bottom may violate two design goals for baffles: The first is that the baffles need some angle to drain the grease to the side. Commercial hoods require an angle greater than 45 degrees, as I recall. Smaller angles should be fine for residential use, but zero is too small. The second is that if the hood height is short, then the airflow through the baffles farthest from the duct may be less. This will cause the grease collection to be less effective. Also, smoke capture will be potentially less effective with the lower air velocity at the baffles.

The second factor above can also apply to mesh filters, depending on how restrictive they are. More restrictive, the better the air flow is made uniform, but the lower the overall air flow.

kas

    Bookmark   June 22, 2013 at 5:28PM
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needinfo1

kas--

You are immensely helpful! Thanks so much!

After hearing all you have to say on the topic, since I am looking for a flat hood rather than a true pyramidal shape, I will try to find the hood that has the largest aperture area compared to other models. I will look for mesh filters rather than baffles since it seems that flat hoods are not where baffles really work very well.

    Bookmark   June 22, 2013 at 5:39PM
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kaseki

I think I would say that under those conditions baffles won't work better than meshes, assuming that the meshes are kept clean. If the meshes are half blocked with lint and grease, then the baffles are better. If the flow is marginal than meshes are probably better.

Also, the fan should be designed to go with whatever pressure loss results from whatever filter is used. One hopes that the manufacturer bothered to do so. No one (except commercial hood manufacturers) ever reports pressure loss as a function of flow, so it becomes a guess when trying to analyze a whole system.

kas

    Bookmark   June 23, 2013 at 9:37AM
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BBQ-Chef

The formula used for determining hood CFM is:
Total BTUs (add each burner BTU up) divided by 100
5 burner example 14000+12000+9500+9500+7000= 52000 BTU 52000/100= 520 CFM (min),

Next, if the hood should have open space inside to tap smoke and heat before it is exhausted, commercial style slats trap grease and do not reduce airflow as screen mesh does, as are easy to clean.

If the hood is over a island you will need to over size it due to cross breezes, Example 36" cook top would use a 40" hood, (not a 36").

As for noised reduction when operating may I suggest the following options: Oversize the cfm ( i.e. ranges calls for 520 CFM, use a 1000 or 1200 CFM) and run at a lower speed which is quieter. Use double squirrel cage fans abd not the flat blade fan.

Or, mount an external fan at the top of the stack on the roof or just under the roof inline, which will reduce noise. (be sure to have a damper that closes when off in the stack to prevent downdrafts when the fan is off. Note, if on a outside wall do mount to remote fan to close, as it will radiate the noise even loader. The farther away from the hood the better.

While I have not tried one, but the reviews on the "Z-Line" hoods have been the best I have read, and seems everyone who owns one is happy and feels it was a good value purchase. You can see their full line at
http://stores.zlinekitchen.com/StoreFront.bok
I hope this is useful to you.

Here is a link that might be useful: Z Line Hood

    Bookmark   June 23, 2013 at 10:38AM
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