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Q&A Spotlight

Sizing a Kitchen Exhaust Fan

Answering the question: How much air should a kitchen exhaust fan move?

The most powerful exhaust appliance in most homes is the kitchen range-hood fan. Sizing the fan correctly is not the simplest of problems, but one rule always applies in a tight building: What goes out must come in. Image credit: Charles Miller

DCContrarian, a frequent contributor to GBA’s Q&A forum, now has a question of his own: How much air should a kitchen exhaust fan move?

“So how many cubic feet per minute should the fan that vents a cooktop be?” he asks in this recent post. “I’ve done an exhaustive search of the GBA archives, and there are 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 he’s been able to find online “lacks any sort of analytic rigor,” he continues. The rules of thumb he has found—1 cfm for every 10 Btus of output for a gas range, or 10 cfm for every inch of width for an electric range—suggest the capacity of the range hood should be about 300 cfm.

But what about a number that’s science-based? That’s the issue for this Q&A Spotlight.

The IRC addresses makeup air, not fan capacity

As Armando Cobo points out, a section of the International Residential Code (IRC) requires makeup air be provided for exhaust fans rated at more than 400 cfm. Each exhaust fan of that size must be “mechanically or passively” provided with makeup air at about the same rate that indoor air is being expelled.

“That says nothing about how big the exahust system should be,” DCContrarian says, “just the makeup air.” Plus, that code provision would not apply to an all-electric house—just to those with a fuel-burning appliance.

Where does that leave us? Picking a number out of the blue and letting GBA readers lob rotten tomatoes at it, suggests Malcolm Taylor.

“I’m pretty sure you just pick a size and we reply it’s either too…

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20 Comments

  1. REBECCA B | | #1

    So, anyone got some recs for off the shelf lower CFM (300-400) range hoods with good form factor for capture?

    1. Expert Member
      Malcolm Taylor | | #4

      Rebecca,

      One of the main criterion I'd use is how noisy the fan is. Most range hoods aren't used because people dislike having them on. A poorly performing range hood that is routinely turned on during cooking is still more effective than a better one that sits idle.

      1. CarsonB | | #12

        You can get external fans so the noise is outside. I’m getting one by fantech, which also has a hood liner to go with it.

        1. Expert Member
          Malcolm Taylor | | #13

          Carson,

          Outside or in another space? I thought the ones Fantech supplied were inline fans?

          1. CarsonB | | #14

            Outside. Maybe not the prettiest, but let it bother my neighbors;). https://shop.fantech.net/en-US/rvf--8xl--ec--ext--centrifugal--fan/p95294

          2. Expert Member
            Malcolm Taylor | | #15

            Carson,

            Perfect! Quiet trumps aesthetics on fans anyway. I don't mind how it looks though. I may retrofit one on our range hood. We use it but grit our teeth the whole time.

            Looking at those, maybe that's the way to go with bathroom fans too?

          3. CarsonB | | #16

            Sounds logical right? Why don’t you see more external fans in residential? Cost? I’m not actually sure how much quieter it will be, the only ones I’ve seen online also used silencers which I’m too cheap for. I’m also debating wether to figure out a venting for broiling in the wall oven. Maybe that will just be an open a window scenario.

          4. Expert Member
            Malcolm Taylor | | #17

            Carson,

            I never considered them because I didn't know they existed.

            I really dislike the way bathroom exhaust fans are installed. Their housing is hard to air-seal and the connections are not accessible once the drywall is on, making replacement difficult to do well. So I have started mounting the whole thing - fan and housing - in an airtight plywood box, where all the connections are from inside.

            But I'm reconsidering that approach since your post. What downsides can you think of? I guess you have to add an inline damper?

          5. Expert Member
            Michael Maines | | #18

            (Response to #17) Malcolm, that's an excellent idea!

          6. CarsonB | | #19

            I’ll try to do a post after my build with what worked and what didn’t in my somewhat unorthodox build. Downsides I can think of: 1. yes requires backdraft damper. 2. Cost, at least for a rangehood it’s cheaper to get the hood insert with a fan instead of a passive hood liner, external fan, and backdraft damper. Installation is likely more expensive as well. 3. Installation and maintenance may be more difficult depending on where on the exterior it’s mounted. 4. Increased noise and wall wart on exterior. I’m not sure aesthetically if it would be any worse than a regular vent though. 5. Temperature range of fan operation. I hope cold is fine, but I saw it shuts off at 104F. Fine where I am, but an issue for a place like Texas.

          7. Expert Member
            Malcolm Taylor | | #20

            Carson,

            If I switch mine out I'm going to use the existing hood and just remove the motor. Looking at the selection in the big box stores, there are a few quite nice ones that are probably cheap because of their poor controls and motors. If you only wanted the carcass they might be worth using rather than a dedicated hood from Fantech.

  2. Paul Pfeiffer | | #2

    The part of the question that still isn't addressed (at least not in the text--maybe it is in one of the cited sources?) is where in relation to the stovetop does the makeup air come from? Can you get adequate capture when it's mostly makeup air being exhausted?

    1. Expert Member
      Malcolm Taylor | | #3

      Paul,

      That's an interesting question. The two parts the discussion that dealt with that are fairly general:

      " ...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"

      But DC goes on to conclude that hood capture is what's most important:

      “...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.”

      If DC is right, the location of the makeup air shouldn't matter.

  3. Jenna Smith | | #5

    I don't understand this sentence: "When it’s 36 in. over the cooking surface, the hood should overhang 3 in. on each side; at 42 in., the overlap should be 6 in. on each side." Does this mean that if the hood is 36" above the range, it should overlap the left and right sides by 3", so a 36" range would have a 42" hood? What does "the overlap should be 6 in. on each side" mean?

    1. Jenna Smith | | #6

      Never mind, I just reread the sentence and understand what it's saying. Thanks anyway!

    2. Expert Member
      Malcolm Taylor | | #7

      Jenna,

      Yes you have it right. A hood 36" above the cooking surface should be 3" wider on each side. One that is 42" should be 6" wider on each side than the cook-top. This is fleshed out in the link provided in the article above: https://bamasotan.us/range-exhaust-hood-faq/

  4. Douglas Horgan | | #8

    For what it's worth, there is a code requirement for kitchen ventilation in the IRC.
    Section 1505.4.4 specifies minimum 100 cfm intermittent or 20 cfm continuous.

  5. Douglas Horgan | | #9

    I know of two good resources on this issue.
    The first is LBNL who have done extensive studies on range hood effectiveness. You can google and find videos of the investigators talking about it, a long-ish paper is here:
    https://www.aivc.org/sites/default/files/lbnl-5545e.pdf
    and a slide show with a fair amount of the same info is here: https://indoor.lbl.gov/sites/all/files/lbnl-6547e.pdf
    The second resource is the estimable ROCIS organization who have a general page here: https://rocis.org/kitchen-range-hoods
    and a guide to selecting hoods here: https://rocis.org/sites/default/files/user-files/ROCIS-Ducted-Range-Recommendations-Dec-2019-Update.pdf
    On page 6 is a list of recommendations, including at least 250-300 CFM with a high Capture Efficiency (CE).

  6. qofmiwok | | #10

    They keep pushing out the date, but ASTM E3087-17 is supposed to be implemented any time now, which creates a standard for range hood capture efficiency. I can't remember the organization doing the studies, but there is a big database of hoods. Currently that field is blank for all the hoods, and frankly the Sones ratings are totally inconsistent as well. So while it would be great, I wonder how long it will really be before we get good capture efficiency ratings.

    In the meantime someone referred me to Victory Range Hoods out of Canada. They seem to have very good products at good prices, and will put smaller fans in any of their hoods. That's a big benefit in my mind, because most decent hoods have huge cfm numbers. What I especially like is instead of those crappy mesh filters they have the baffle filters in many of their hoods. Most brands you have to spend thousands to get those.

  7. Adam Liberman | | #11

    I did extensive research when selecting a range hood and ended up getting a GE range hood insert with 610 cfm max rating. Generally running a larger capacity hood at lower speed is much quieter than running a smaller capacity hood at its maximum speed. Also, the higher capacity hoods have larger size ducting for lower static pressure. Many can be limited with a board jumper to 290 or 390 cfm to comply with code for no make-up air, which is what I did. However, some Best models comply with code by sticking a piece of metal in the fan exit port to block half the flow, which seems insane!

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