20 October 2009

How many CFMs do you need for your cubic feet?



Many thanks to the gang at Faber Hoods for this very helpful guide on the technical side of kitchen ventilation.  True kitchen ventilation (rather than the cheap and usually ineffective method of hanging a vented microwave over your cooking surface) is an idea that's catching on again. All ventilation uses a measure called Cubic Feet per Minute to indicate how effective the blower motor in a ventilation system is at circulating air. Few topics can confuse people as quickly as CFM ratings. There is a mistaken belief, that like most everything else in appliances, bigger is better. Not necessarily.

Using a hood with higher CFM (above what you need for your cooking surface) means more air is being pulled out of your kitchen and your home than needed. Therefore a lot of cooled or heated air is being pulled out your home, which would lead to higher heating and cooling bills.

Also, a situation of negative pressure can also occur when too much air is being pulled out of the home and isn't being replaced by air from the outside. Homes built today are increasingly air tight and when too much air is pulled out of a home, you need to sometimes make up for that lost air by pumping outside air into the home. There are all kinds of rules of thumb regarding make up air and it's best to consult with an HVAC specialist before you install a high-powered ventilation system in a newer home.

When you're choosing a hood for your cooking surface, one that has too many CFMs won't be energy efficient and too few CFMs won't provide adequate ventilation. The more CFMs, the more energy they use and the more noise they make. The key is to buy the right hood for the job at hand. Somewhere there's an ideal CFM count to match your needs.




So buy a hood that can remove the heat, steam, odor, smoke and grease produced by your cooking properly while at the same time not overdoing it. This diagram below shows a good way to estimate how many CFMs you need for your kitchen. In this kitchen, the ceilings are ten feet tall (Z). The walls are 10 feet (X) by ten feet (Y). So Z x X x Y = 1,000 cubic feet. If you have a 500 CFM rangehood in this kitchen, in 2 minutes you will have completely exchanged all the air out of the kitchen (or 30 exchanges in an hour). The National Kitchen and Bath Association recommends 8 to 15 air exchanges in an hour for proper ventilation, so in this example, we're at double the recommended level. Rules of thumb like this can get you started but the amount of heat generated by some cooking appliances throws a wrench into the works. Heat is measured in British Thermal Units, or BTUs. There are additional calculations that need to be worked out when it comes to using professional-style ranges so be sure to consult with a professional kitchen designer before you commit to buying anything.



So even though the example above has us at twice the recommended CFM, using a four-burner gas cooktop will put you 100 CFMs under the required 600 CFMs for use over gas. If you're upgrading to something more substantial, a 48" range top for example, you're going to need at least 1,000 CFM. In the opposite direction, because induction cooktops generate so little radiant heat, a 300 CFM ventilation hood over it would work out perfectly. Confused? Don't be.

Calculating the volume of your room is helpful and knowing the heat output of your cooking surface is helpful too. Combining the two and coming up with a satisfactory CFM takes a bit of judgement and experience, but that's why I'm here. Me and a whole bunch of compatriots who like nothing better than to figure stuff like this out.

15 comments:

  1. Great article, great leads! I've lived in 12 different houses in 20 years, so 12 different kitchens, and real ventilation is the first luxury/necessity that I look for. I really don't get the microwave/hood venting back into the room, it's so sad. I hear that's filtered, but still. I must cook smellier food that looks worse on walls than other people!

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  2. It's all about getting the right tool for the job Johnna.

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  3. I am getting a 36" capital range. what kind of hood would be appropriate for that?

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  4. Julie: You need to talk to an appliance specialist. The best way to determine how powerful a hood you need is to have someone calculate the CFMs you need based on the BTUs generated by that range. That information should be available on Capital's website too.

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  5. Hi Paul. I keep getting different info from different appliance places! So, I've been trying to get some guidance online. Basically, it has 6 power flo sealed burners - 19,000 btus each and each goes down to 140 degree simmer. the range also has an infrared glass broiler if that needs to be considered. there is no griddle or grill. does that help?

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  6. Julie,

    You need a hood at least 36" wide and it should have a blower motor capacity between 600 and 1200CFM. I would not go below 900 if it were going in my house and if you're planning to have all six burners fired up at once I'd go for 1200CFM. If you're not likely to use ALL of the burners at one time very often, then 900 would work fine. Just remember that the more powerful the blower motor, the more expensive the hood will be. The more powerful too, the louder it will be.

    Does that help?

    I'll send you the pages from the spec book if you shoot me an e-mail at p.anater@gmail.com.

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  7. I am getting a 36 inch 6 burner Thermador pro range. The wood hood I have from my cabinets does not fit the thermador blower motor. I found a Faber 900 cfm blower motor that seems to fit my specs and was just wondering if this is a good quality reliable solution?

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  8. Faber makes a good, reliable blower motor and liner. The cfm requirements for that range will be in the set-up guide that came with it and any decent appliance sales person will be able to tell you the appropriate cfm rating you need.

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  9. we have purchased a Sears Pro gas cooktop. Six burners,1 16K BTU, 1 17.2K, 2 9.5K and one 675-5K. We have gotten various answers regarding the number of CFM's we need. The hood will be made of cabinetry material, and so we will use an insert which will be ducted outside( the cooktop is on an exterior wall. I have heard everything from 600 cfm to 1200. The kitchen has a slanted ceiling which starts at 9', the kitchen walls are 12x14. the kitchen is open to a family room.I don't want to get too large of a unit, but neither do I want one too small.
    What should we consider?

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  10. Anonymous, one of the problems with buying a major appliance at Sears is that the appliance is sold to you by someone who doesn't know what he or she is doing. A competent appliance salesperson would have worked all of this out for you.

    Depending on how much you plan to cook on that cooktop, 900 cfm ought to do the trick. If you decide you want to go higher than that, go to a remote blower and please don't buy any of this ventilation equipment at Sears.

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  11. I have an approx. 200 sq ft kitchen and I'm planning to get a range hood with 300 cfm. Will this be an issue for the pressurization of the house? Or cause some kind of back draft of carbon monoxide into the house?

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  12. You don't run into pressure problems until you start using 1000 cfm blowers. What kind of cook top are you using 300 cfm for?

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  13. I have a JennAir over the counter microwave which I believe has a 300 CFM rating. I'm thinking of getting a gas range - 4 burner - perhaps a Jenn Air or Kitchen Aid. Would the microwave vent be sufficient? Thanks!

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  14. You will probably need a more powerful motor over a gas range. Gas cookers send out more radiant heat than electrics do and you'll need a more powerful motor to get the heat out of the kitchen.

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  15. Hi Paul,

    My house has a gas cooktop. The most powerful burner has 11,000 BTU. We stir fry a lot.

    The house has an open floor plan. Do I take into account the whole open area when calculating the needed CFMs?

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