How much CFM do I need?

12 years ago

I could bet money I saw the number but I cannot find it now matter how much searching I do.

I am considering BS 24" range with two 22K BTU burners and one 15K burner. However, it turns out that I have problems with the vent and can have a maximum of 450 CFM (long story).

Another problem is that I can only use a custom-made in-line vent in the ceiling (long story, will post in a separate thread).

I can potentially reconfigure to have a vent come down close to the cooking surface.

How much CFM do I need for each scenario?

Thank you!

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Comments (6)

SparklingWater
12 years ago
Here is what Blue Star says:

"VENTILATION
Proper ventilation is highly important for
good operation. This appliance must be installed
under a properly designed canopy
hood. We recommend that the hood should
be six inches [150 mm] wider than the appliance,
however, it is not a requirement.
A strong exhaust fan can create a vacuum in
the room. For proper air balance, work with
your dealer and/or contactor to properly size
your hood and its exhaust fan. It is recommended
that your exhaust hood be powerful
enough to move a minimum of 300 cfm."

Each home ventilation need is unique to the house, as you've noted. Make up air (MUA) considerations with high BTU ranges/tops are an important consideration in addition to capture issues. If you feel rushed and are struggling with this important decision, perhaps forgoing range purchase until more specific details of your vent capabilities and home tightness are determined may be a consideration. Just a thought...

Close to the high combustion cooking surface if possible may be a strong consideration for capture, but I have no knowledge of those systems air flow etc.

Here is a link that might be useful: Blue Star installation instructions
eleena
Original Author
12 years ago
Thank you!
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Well, given a 2010 thread, start a new one. Also, reread your sentences before publishing to assure that they are as clear as possible. Since 2010 much information has been published here to answer such questions. In your particular case, HU-, an efficient response needs to be based on whether you want to make a modest corrective change, or redo whatever you have in place. First, to answer how much air is needed the following may help, given a conventional pseudo pro hood with baffles and large entry aperture: If your hood aperture is 4 ft x 2 ft for 8 square feet of area, then you need for most high heat residential cooking 90 CFM/sq. ft or 720 ACTUAL CFM. This flow rate can nominally be achieved using a blower rated at 1.5X that CFM or 1080 CFM at zero static pressure. This assumes a fully functional make-up air system providing 720 actual CFM, which itself may need a similar blower rating around 1000 CFM, or open windows (not usually code compliant), or living in Pago Pago where windows don't apply. Capturing oven smoke upon door opening is very difficult in any scenario other than where commercial large hoods overlapping are used with commensurately larger blowers and large entry aperture volumes. (See recent opaone thread.) In any case, given an actual flow rate of 720 CFM, then the duct should be sized for a flow velocity in the range of 1000 to 2000 feet per minute. Divide CFM by duct sectional area (in square feet) to evaluate a candidate duct diameter. (Area = pi times diameter-squared divided by 4).
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ditto weissman. -- I'll modify this statement: "... I can tell you anything under 400cfm is a waste no matter what you do otherwise..." Here goes. There are millions of cheap propellor blade fans installed, that function, in a sort of way, removing most of the smokey greasy air that some kinds of cooking generates. They do work. They are rated at various CFM numbers all less than 400 cfm. In reality, these blower motors are very weak if the air in the duct meets up against resistance. So, a short duct through the wall = they work OK, while a long duct up through the attic to the roof = they work erratically or poorly, and sometimes well. Some people have never seen these "hoods". Cheap rental apartments have them. You can see them in cheap hardware stores. The rated CFM is just a lie printed publicly which nobody can refute. It was the cfm produced in a resistance-less tube in a laboratory. In reality, something innocuous can make the fan perform poorly. Many things. Even something as dumb as "wind outside" i.e. your regular everyday weather. --- The next thing up the scale of goodness is a squirrel cage blower. You can see them in cheap hardware stores too. Some are weak, some are strong. Some can push air very well no matter how much resistance the duct length causes. They are all "rated" for a CFM in the hundreds. 300 CFM is fine if you truly get 300 CFM and if your cooking style compensates when the hood needs a 1/2 minute to catch up. Blowers inside "OTR" microwaves have to push air through tight spaces that bend and turn before the air even gets out of the microwave and into the house duct. I wonder if the blower motor inside an OTR microwave was "rated" only prior to it being installed inside the microwave. I suspect this is the case. Then, the OTR microwave can be sold as if it had a Large CFM while it might never ever reach that CFM under ANY circumstances. Let's get the Consumer Reports police to look into this. Although it is true that air can move around anything, it is also completely false when you want SPEED. Obstacles and resistance put a maximum cap on speed (of air flow). -- The chart they publish with the fan blower motor shows you how much degradation in performance was observed during the lab test, when a form of air resistance was created. But this too can lie, in a way, even while it remains true and accurate. -- Your duct diameter or duct dimensions are big factor. When you post these dimensions you will get more specific advice. Hth. Here is a link that might be useful: help make a hood decision (kitchen forum)
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The hood's width at 6" wider than the cooktop is ideal, but many people have the hood the same width as the cooktop, and it's fine. Often people don't want to give up the 6" of upper cabinets, or cannot, because the cabinets are already there. If you have the space for a 42" wide hood, it would be the better choice, but is not an absolute requirement. The thinking on hoods has moved somewhat from emphasizing the width of 6" wider, to more emphasis on the depth of the hood front-to-back. Many hoods sold these days are only 20" front-to-back, leaving the front burners uncovered. That is more of an issue than ensuring extra width on the hood. Find a hood that is close to 24" front-to-back if you can. The height of the hood is ideally 30-36" between the top of the cooktop and the bottom of the hood. If you install higher than that, you are allowing too much space for smoke and grease to escape and float around the kitchen. Go lower than that and your cooking area will feel cramped. I want to reiterate what Stan Z said about the correct ducting. You can have the best hood in the universe, but if your duct diameter is too small, your hood performance will be constrained. Most hoods require 8" or 10" diameter - the specs will say which. You can find hoods with specs for 6" diameter, but those hoods will be noisy, as air is pulled through a more narrow space. 600 cfms is usually enough for most people with a 36" width cooktop. Underpowering the hood is a mistake - you will have spent all that money on the hood, and it will be inadequate from the get-go. If you are thinking about going with more than 600 cfms, the advantage would be that you can run the hood on a lower speed setting most of the time, which will be quieter. The disadvantage of more than 600 cfms is that may require make-up air depending on your locality, building code, and how tight your house is.
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kaseki
12 years ago
Start with the uprising effluent velocity of around 3 ft/s or 180 ft/min, multiply by the hood aperture area in square feet. This is the air flow (cfm) required to capture and contain the cooking plume that is intercepted by the hood if the surface the rising plume meets is relatively flat and horizontal. With less velocity, some effluent may "reflect" (curl) out of the hood.

For pyramidal hoods, usually sold with baffles, the interior reflection angle is predominantly upward, and this allows the cfm to be less because some of the effluent momentum is still carried upward after the first bounce. One might get away with a factor of 0.5 on the product of aperture and plume speed.

For example, a 6 square foot baffled hood of the pyramidal style (e.g., commercial, Independent, or ModernAire baffled hoods), would require around 540 cfm (actual).

Please consider that due to various duct pressure losses, hood pressure losses, and house negative pressure, the actual flow rate that a particular blower can move will be significantly less than its rated (at zero static pressure) flow rate.

So, a four square foot hood would require 2/3 of the above calculated value, or 360 cfm. But to make up for the losses, the blower should be rated perhaps as much as 1.5 times larger to actually move 360 cfm. C.f. Blue Star's choice of words: "... move a minimum of 300 cfm."

Please consider all these values nominal, and dependent not only on many factors not accounted for here and rarely specified so that they can be accounted for, but also on user tolerance for odor, type of cooking, etc.

kas
docsknotinn
12 years ago
The bottom line is that no one can give a totally accurate response with out seeing your home. Determining CFM is far more complicated than simply calculating mass. There are a number of other factors to consider. What is the diameter of the exhaust? How long is the run? How many turns in the run?
There is a significant loss of CFM for every 90 degree turn or even a bend in the exhaust.
A qualified installer should be able to help you factor your CFM requirement after they inspect your home.
brickeyee
12 years ago
The CFM 'game' is played by having zero loss in the duct coming from the hood.

It is measured wide open, zero length duct many times.

This produces large numbers that are very unlikely to be achieved in practice.

It dos not take much duct to significantly reduce flow rate.

Add to that the baffle effectiveness will decreased if the flow velocity is not high enough.

The typical baffle filter relies on the particles in the air stream having enough momentum to not be able to follow the quick turns needed to pass thorough the baffle.

At low velocity they do not have enough momentum and can make the turns.

A high capacity blower with a variable speed is a good compromise.
SparklingWater
12 years ago
I'm sure eleena and others are appreciative as I (a ventilation novice) of all the expert comments offered here. Thank you all very much for your help and insight.

How much CFM do I need?

Comments (6)

SparklingWater

eleena
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How much CFM do I need?

Comments (6)

SparklingWater

eleenaOriginal Author

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eleena
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