How much CFM do I need?

eleenaApril 9, 2012

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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Here is what Blue Star says:

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

    Bookmark   April 9, 2012 at 8:22AM
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Thank you!

    Bookmark   April 9, 2012 at 10:37AM
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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.


    Bookmark   April 9, 2012 at 10:40AM
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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.

    Bookmark   April 9, 2012 at 11:31AM
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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.

    Bookmark   April 9, 2012 at 5:51PM
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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.

    Bookmark   April 9, 2012 at 9:07PM
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