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Community and Q&A

Sizing a Range Exhaust Vent Fan

NICK KEENAN | Posted in General Questions on

So how many CFM should the fan that vents a cooktop be?  I’ve done an exhaustive search of the GBA archives, and there’s lots of articles for sizing the makeup air to the vent fan, but nothing I’ve been able to find that helps you decide how big the fan itself should be. Most of what I’ve found on the Internet lacks any sort of analytic rigor, and just depends on rules of thumb, like 1 CFM for every 10 BTU’s for gas, or 10 CFM for every inch of width for electric.  (Which incidentally both drive you toward around 300 CFM).

Is there anything more science-based?

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Replies

  1. Expert Member
    ARMANDO COBO | | #1

    2021 IRC M1503.6 - "Where one or more gas, liquid or solid fuel-burning appliance that is neither direct-vent nor uses a mechanical draft venting system is located within a dwelling unit’s air barrier, each exhaust system capable of exhausting in excess of 400 cubic feet per minute (0.19 m3/s) shall be mechanically or passively provided with makeup air at a rate approximately equal to the exhaust air rate....." You'll find almost word for word on the 2015 IRC M1503.4

    1. Expert Member
      NICK KEENAN | | #4

      That says nothing about how big the exhaust system should be, just the makeup air. And it only applies if you have combustion appliances, this house is all-electric.

  2. Expert Member
    MALCOLM TAYLOR | | #2

    DC,

    I'm pretty sure you just pick a size and we reply it's either too small and you will die of particulate poisoning, or too big and will require an elaborate powered make-up air system.

    1. Expert Member
      NICK KEENAN | | #5

      That's the feeling I'm getting. Reading the requirements for makeup air it seems like when you go to a big fan the complexity and expense quickly escalate.

      There's also the issue of energy consumption. If it's 20F outside and you're venting 900 CFM that's 50K BTU/hr. This is a house where the total Manual J load is 29K per hour. Now some people say you should put the makeup air close to the vent so you get a strong cross-draft which then wouldn't be pulling heat from the rest of the house. But others say that the makeup inlet should be far away because the cooking by-products will diffuse and you want to be venting the whole house. In that case you'll be pulling heat out of the whole house.

      Some writers talk about conditioning the makeup air. Hence my earlier comment about increasing complexity and cost.

      1. tundracycle | | #9

        We have an electro industries 1400 CFM MUA for our range hood (and an 800 for fireplace). The MUA's are set to 45°f output temp for heating and both feed in to the return duct just prior to the furnace so the majority of conditioning is done by our primary HVAC system. The hood MUA is balanced to 3 speeds of 460 / 600 / 940 actual measured CFM of the hood.

        This is not as ideal is feeding the MUA directly to one or two specific places for better air flow but the cost for that (ducting + dedicated furnace/AC) wasn't in the cards. What we have works well.

  3. tundracycle | | #3

    Some info here: https://bamasotan.us/range-exhaust-hood-faq/

    What sticks in my head is 90 CFM / sq ft of opening.

    1. Expert Member
      NICK KEENAN | | #8

      I found that article very helpful. Key takeaways:
      * The hood is important. It should overlap the cooktop by a minimum of 3" and ideally 6" on each side.
      * Hood volume is important. It should be at least 18" deep, 24" is better. It should be unobstructed for that height.
      * Recommended is 90 CFM per square foot of hood. I have to wonder about that a bit. That means an airspeed of 90 feet per minute at the entry to the hood, or about 1mph. It also means an 18" hood is emptied 60 times a minute.

      So the strategy isn't to suck the contaminants out of the air. It's to catch them in the hood, and then empty the hood frequently.

      He also notes that both CFM and noise ratings for hoods are highly speculative, you're lucky to get half the advertised rating in an actual installation.

  4. Expert Member
    AKOS TOTH | | #6

    I would not get too focused on CFM as it matter much less than the design of the hood itself. Get a hood or fan cabinet with a large aperture and good capture volume and it will work much better than many of those "professional" 1200CFM hoods.

    I run an 400CFM unit at home and until I fabbed some stainless baffle extensions, it barely did anything. It still sucks (more precisely sucks but not that well) but the best that could be done after the fact.

    Noise is also very important, you are much more likely to use a quiet hood on max than a loud one on low.

  5. tundracycle | | #7

    @Akos, wish there was a way to give you a thumbs up.

  6. JC72 | | #10

    You won't find anything that's "science based" because of variances in range hood design/size, location and duct design (ie. flue diameter, length and # of bends).

    1. tundracycle | | #13

      But that's all science though right? If you have X range then a hood of Y dimensions and a blower of Z CFM's with a duct and MUA system static pressure of SP should exhaust some relatively predictable percent of effluent assuming C air currents through the kitchen? You should be able to design for all or most of the variables?

      1. JC72 | | #14

        The current system does design for that. However the other variables are really big in terms of required fan capacity.

        Location matters: Wall mount is less drafty vs island mount.
        Run time.
        Capture volume is unique to every hood. Deeper hoods capture more.
        Filter method is also unique. (mesh or baffles)
        Distance from the cooking surface. (24" or 36" from cooking surface)
        Finally you have to determine the amount of heat coming off the range (Fuel type, number of burners, capacity of burners).

        The current method probably leads to some oversizing but that's not a bad thing. It only becomes an issue when homeowners have competing priorities and don't wan to spend the $$. Such has having passive house level of air tightness with a 36" 6-burner "professional" gas range.

  7. Expert Member
    KYLE WINSTON BENTLEY | | #11

    I wonder if it couldn't be crudely calculated, based on a few assumptions / ideas

    1) Volume of water that is turned into steam from liquid
    2) Volume of CO2 / steam resulting from gas combustion if not electric
    3) Differential volume of air resulting in temperature change

    I've always though an efficient design might be one that uses some type of forced air, directed at the hood from the sides of the range, to shroud the byproducts into a central collection / elimination port. I don't know that any type of system exists, but there would be little penalty on bringing in outside air for it, assuming it was going to be exhausted again. All of the energy lost would be a byproduct of cooking, and not really an energy that was cared about in the first place.

    1. Expert Member
      NICK KEENAN | | #15

      I suspect that's where you land with rules of thumb like 1 cfm for every 10 BTU with a gas cooktop.

      The rule of thumb for electric is 10 CFM for every inch of width, which works out to 60 CFM per square foot (assuming a 24" depth). Which is the same order of magnitude as the 90 CFM per square foot mentioned above.

      1. tundracycle | | #16

        I think those specific rules of thumb aren't useful and that correct design goes something like: Hood size is based on height above the cooking surface (plume expands so higher = larger (36" above surface is approx 3" overhang on each side, 42" is 6" overhang, etc. (36" above island surface is 12" on each side, etc.). Then the CFM's are based on that size (90 CFM / sq ft for normal cooking, 120 CFM for high heat cooking, etc). The blower then sized to achieve the desired CFM's given the static pressure components in the exhaust and MUA systems.

        There shouldn't be any difference between gas, induction or whatever.

        1. JC72 | | #17

          Remember hoods are supposed to evacuate heat and moisture both of which gas produces a lot of on its own.

          1. tundracycle | | #18

            Yes. But so does cooking itself. I think the induction folks tried to imply that their ranges don't need ventilation or as much ventilation but the reality is that they need just as much as gas. Both result in a lot of stuff we don't want to breath.

  8. tundracycle | | #12

    For science-based the best may be within: https://www.greenheck.com/resources.

    Such as: https://greenheck-cms-prod.azureedge.net/atg-cms-prod/docs/default-source/pdf-downloads/application-articles/fanselections.pdf

    I think Halton has also published some technical articles.

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