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Building Science

Four Components of Kitchen Exhaust Systems

Key conisderations for getting kitchen exhaust right

Range hoods form part of a home’s air quality control system. Some fashionable styles, such as bell-hoods over an island won’t work as well as the conventional, under cabinet models. The Heating and Ventilation Institute recommends multiplying the CFM requirement of a conventional hood by 1.5 when locating a hood over the kitchen island to make up for cross currents. Photo courtesy of Elica.

The modern range hood has become more than a mechanism for removing cooking odors. It is an integral part of a home’s indoor air quality (IAQ) system, serving various purposes including hygiene, safety, and decor. A high-quality hood will trap grease to keep the kitchen clean, exhaust excess heat, and keep indoor humidity levels in check while removing indoor air pollution caused by cooking, as well as those household cleaners commonly stored under the kitchen sink.

Outside vent vs. recirculation

An effective ventilation system exhausts stale indoor air to the outside (which means makeup air is critical but here the concentration is on the exhaust system only). A recirculation fan may filter grease and odors but not remove humidity and chemical pollutants. “If an outside ducted range hood is impractical, consider supplementing the range hood with a central ventilator similar to a bath fan that does vent to the outdoors,” advises James Lyon, PE.

The NuTone Mantra range hood product pic
The NuTone Mantra, which comes in 30-in. and 36-in. widths, is a good choice. With a 375 CFM blower, it’s still only rated at 1.5 sones (quiet). Broan’s top-of-the-line is an under-cabinet unit with both external and recirculating options. Featuring a highly efficient, multispeed fan and a combination of baffles and aluminum filters, the company guarantees the hood provides 98.1% removal of smoke and odor. Photo courtesy Broan-NuTone. Available at Home Depot for $280 for the 30-in. model.

When installing an outdoor-venting hood, ensure the ducting system uses a solid, galvanized steel pipe of a diameter equal to the hood port. A narrower duct will choke off the hood, reducing its aspiration capacity. A wider vent may seem like an upgrade but it may reduce air velocity, resulting in grease deposits along the pipe, says Lyon. The duct should follow a direct path to the exterior, avoiding…

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  1. GBA Editor
    Martin Holladay | | #1

    How is it possible to address this topic without mentioning that most houses will need a makeup air system?

  2. charlie_sullivan | | #2

    Shouldn't the guidelines for how much ventilation is needed be different for a gas stove vs. electric/induction? Not that any green builders should be installing gas stoves anyway, but explaining that need could be helpful for guiding people towards induction stoves (which have better cooking performance than gas as well as avoiding gas stoves' impact on the residents' health).

    1. tundracycle | | #13

      Not really. There are considerable VOC's, Carcinogens and PM in the effluent from cooking itself regardless of the heat source. For someone who only does boil-in-bag meals a lessor ventilation might be OK but for most people it's the same regardless of heat source.

      Induction manufacturers would like people to think otherwise though.

      1. charlie_sullivan | | #16

        Makes sense--it would be more the % of the time you need to use it than the capacity it needs. (It's not just boil-in-bag meals that don't need it--other boiling and steaming operations are, I assume, pretty safe)

  3. CramerSilkworth | | #3

    The 8th paragraph references hoods over 900 cfm, but code requires makeup air systems at 400 cfm if you have any combustion appliances that aren't direct-vent/sealed combustion or power-vented. And in a tight house MUA can be needed (not by code, but by reality) even on lower flow rates. As Martin mentions, no discussion of kitchen ventilation is complete without discussion of makeup air systems, which really are half the battle. Much more than half in my experience...

  4. Expert Member
  5. user-1147460 | | #5

    A minor threadjacking, but….
    Has anyone managed to develop a successful way to recover the heat from the exhaust of a commercial kitchen extraction system?
    I despair at just how much heat our hotel kitchen is blowing out into the universe.
    I’m no fool, I’m well aware of the fat in that airstream. But to somehow find a way to run that heat through a HWHP to return it back into the building via the hot taps….

    1. GBA Editor
      Martin Holladay | | #6

      You guessed it -- the problem is aerosolized fat (or grease, or oil, or whatever you choose to call it) along with particulates. When you cool the outgoing gases and solids, they accumulate on the sides of the duct as sludge. That's the basic problem.

      There's a similar problem with outdoor brick chimneys vs. indoor brick chimneys. The hotter the flue, the cleaner the flue.

      1. user-1147460 | | #17

        Fortunately, Martin, here in New Zealand we are required by law to get our commercial ductwork independently cleaned annually. In fact ours was done last week. I'd say that without that fire wouldn't just be a risk, it would become a certainty!

    2. charlie_sullivan | | #7

      There's a lot that can be done. Here's a presentation that talks more about it.

      1. Making sure you are effectively capturing the dirty air so you get the maximum effectiveness per CFM.

      2. Controlling the ventilation as needed, not just on full all day. DCKV = demand controlled kitchen ventilation (not dc kilovolts).

      3. Heat recovery, starting on p. 99. Not much details but examples showing it can be and is done at the commercial scale.

      The company that produced that presentation is a food service energy efficiency consultant.

      1. tundracycle | | #14

        @Charlie, great link from Fishnick.

      2. user-1147460 | | #15

        Ooh, yes! I have some homework to do, what a great resource!!

  6. user-7513218 | | #8

    I am sorry to not have tuned into the discussion earlier, I was at the builder's show. You are correct, many homes would require makeup air to keep other gas appliances from back-drafting. We dealt with this at our home and resolved it rather easily with a small duct connected to the hood that had an electronic damper. It was an accessory to the hood.

    1. tundracycle | | #11

      The MUA duct is connected to the hood? Which hood is this?

  7. Expert Member
    ARMANDO COBO | | #9

    Something to think about... I've learned thru the years, when designing homes for folks from other cultures (i.e. South Americans, Asians, Africans) who use a lot of strong spices, will benefit a lot form some type of MUA unit in their kitchens, regardless of Codes. Believe me, your clients will thank you for it.

  8. mikemorr | | #10

    Another option I'd like to see discussed are pop-up downdrafts that sit behind the range instead of being integral to it.

  9. tundracycle | | #12

    "Depth matters more than width"
    Not really. Air doesn't know depth from width. The open capture area of a residential hood should be 6" wider than the range and ideally 6" deeper (though this is difficult to find in consumer hoods). If you don't capture it then you won't exhaust it no matter how many CFM's you apply to it.

    You totally missed any discussion of containment volume which is critical to hood performance.

    "A hood set higher than 30 in. requires a stronger fan."
    Higher CFM's (stronger fan) can not usually make up for lack of capture. Rising hot effluent is difficult to 'bend' in to the hood. The higher the hood - the greater the open capture area needs to be. Again, if you don't capture it (and contain it) then you'll not exhaust it.

    "The next consideration is horsepower. In the world of hoods, you measure brawn by how many cubic feet of air the hood’s fan can exhaust per minute or CFM."
    Nope. This is consumer marketing plop, not physics. The two primary keys to hood performance are the size of the capture area and perhaps more importantly the size of the containment volume.
    Higher CFM's are easy to market to ignorant consumers but don't actually matter that much. And higher CFM's result in greater energy use (for the blower and for conditioning the make up air) and greater noise. AND, greater noise results in less frequent use of consumer hoods. That is the number one reason that consumers state for not using their hoods when they should.

    "In a downdraft, and sometimes freestanding hoods, such as those found over a kitchen island stovetop, the fan must pull significantly more air to compensate for cross currents to exhaust steam, odors, and grease through filters and ducts."
    Nope. Higher CFM's cannot pull rising hot effluent in to a downdraft. Downdraft generally do not work regardless of how many CFM's are used. They are next to useless.

    Overall the lack of knowledge and research in this article is disappointing to see on GBA. Good journalism looks beyond the marketing goop that for-profit manufacturers put out to find information that will benefit consumers. Unlike much of what is on GBA, this is not it.

    1. Armand_Magnelli | | #25

      W Ramsay's post is consistent with the research I have read from Lawrence Berkley National Lab (LBNL). The most important quote in my thinking is: "The two primary keys to hood performance are the size of the capture area and perhaps more importantly the size of the containment volume." I am surprised that no one has mentioned the LBNL research. Also surprising is the lack of a reference to ASTM E3087-18, Standard Test Method for Measuring Capture Efficiency of Domestic Range Hoods.

      1. tundracycle | | #27

        Also this:

  10. user-7513218 | | #18

    W Ramsey, the hood was a Thermador, but you can find MAU dampers from several brands of range hoods. Broan sells an 8" Universal Auto Make-Up Air Damper w/ Pressure Sensor Kit. The unit does not feed the hood, it simply opens and closes when you turn the hood on.

    There are many related considerations with any topic, but blogs generally focus on only a few. The intent is never to be exhaustive--pardon the pun. Depth is more important than width because if you cook on the front burners the smoke will rise in front of the fan and often not up into it. The same is true with width, to an extent, because usually, the range hood occupies the width of the stove below. I do agree with the comment that downdraft hoods are useless, but this type of categorical langue belongs in the comments and not the article -- thank you. And will deal with sones in a future post, as noise does discourage use in kitchens and bathrooms.

  11. tundracycle | | #19

    "The unit does feed the hood, it simply opens and closes when you turn the hood on."
    I'm quite confused here. The only time MUA should flow in to a hood is with an air curtain system and I've never seen that in any consumer hood. Air curtains also require powered MUA, not just a passive damper, as otherwise they are ineffective.

    1. user-7513218 | | #21

      The unit does not feed the hood.

  12. Expert Member
    ARMANDO COBO | | #20

    IMO, the exhaust fan should be 6” wider on both sides of the range, with side panels if you can, and the same hood depth as the range.

    We mostly install 2-MUA supplies on the ceilings about 6'-10' away from the exhaust fan, one on each side of the hood. If installing large hoods, we install 2 additional MUA supplies on the cabinet toe-kick or floor, if there is a crawl space or basement. I believe this strategy provides better flow rate and best effluent capture.

    I love Fantech...

    1. user-7513218 | | #22

      Thanks for the link to Fantech. Most range hoods occupy the width of the range below unless it's a house with a cook's kitchen. Should and do often collide. Armando, I'm curious about your experience with Latino clients. I also build for Latin American families, although generally very modest homes--which is what I do. Where do you build? Have you heard of the architect Guillermo Rojas in Los Angeles? He has a special take on the Mexican American home.

    2. Expert Member
      ARMANDO COBO | | #24

      Fernando – I design homes from coast to coast in CZ 2-5, but mostly in TX and the Southwest, where there are plenty of Hispanics from many nationalities, besides me 😜, plus we have large Asian communities as well, especially in DFW and Houston. I don’t know of Guillermo Rojas.

    3. doug_horgan | | #26

      We've stopped putting MUA in at floor level after enough "cold toes" complaints. Ceilings seem to work well but we don't usually try to get them especially close to the range hood, anywhere close to the kitchen where there are no doors to block flow seem to work fine.

    4. tundracycle | | #28

      Ceiling supplies that close likely neutralize what the hood is supposed to do and make it largely ineffective. Air will flow from the supplies to the hood and out - without taking any harmful effluent with it.

      Best is an air curtain built in to the hood. Second best is for MUA to come from some distance behind a cook standing at the range. If in the ceiling then it generally needs to be 2x or 3x the ceiling height IIRC so 30' for a 10' ceiling.

      Third best is to feed it in to the return duct prior to the filter & air handler with an interlock to insure that the blower (and appropriate zone dampers ?) turn on any time MUA is called for.

      1. AC200 | | #29

        I'm scheduled to discuss this with my HVAC designer next week. I have a couple options, neither perfect. I can have MUA packaged unit (Fantech or Electro) supply close to the ceiling with the ducting running through an unconditioned attic. The run would have to be about 30ft to a utility room so I can access the unit and change the filter, etc. A supplemental heater could temper the air in winter but it would dump into the eat in kitchen area hot and humid during the summer. I would also lose heating efficiency as it ran through attic and an area of low insulation where the vent is.

        Option two is the option 3 above, feed the packaged unit into the cold air return and have the furnace blower activate with the hood and MUA unit.

        Option 3 was having my wife crack open a window every time the vent hood was turned on past first fan setting. Having me complain every time she forgets to do it is not really a sustainable option.

        1. tundracycle | | #30

          I would feed it in to the return duct. That works reasonably well in the majority of cases.

  13. AC200 | | #23


    Electro makes packaged make up air units as well. I don't have experience with Electro or Fantech, but in my research, the Electro units have a few more features. The Broan unit is a simple motorized damper that opens on an interlock with the fan, but is far less costly than an electro or fantech unit. I have spoken with tech support from both Fantech and Electro and they are both knowledgeable solid companies.

  14. russell1313 | | #31

    Interesting discussions. As for location of the MUA vent, it seems one located in the floor level area of the island just across from the cooktop vent. Flow would come from behind and below, up and across into the vent hood area. As I am in new construction- with basement area below - I could install this type design with little trouble, 6 or 4 inches of duct with the damper interlocked to fan controls. How this works with the ERV system I am not sure - I doubt I would worry about it much. Any recommendations on vent hood fan/hoods.

    1. AC200 | | #33

      Much of it is covered in the Joe Lstiberek article that Armando posted below. It has even more of Joe's dry humor than most of his articles or talks. In my opinion, for many non-temperate climates, dumping unconditioned air under the range is a non-starter. No one wants to wear boots cooking in the middle of winter as 15 deg air gets drawn in right at your feet.

      In my case I passed on the 48" pro gas, kitchen jewelry, range which would have required a 1200 cfm 54" hood. A MUA nightmare. Going to install a 36" induction cooktop with a 42" hood and a package MUA system kinda like Figure 4 in the article.

      In your case a non powered 4" duct would have limited capacity for MUA. As the article states, size your hood for you cooking appliance and then design and size your MUA system.

      1. tundracycle | | #35

        Allan C, note that, contrary to marketing material, an induction range does not reduce the range hood ventilation needs. Cooking effluent often contains significant bad stuff that you want to exhaust. If all you do is boil water then Fig 4 is fine, otherwise not so much.

        1. AC200 | | #39

          Yes, but there is less heat and combustion gases to exhaust. Most sources tell you that 400 cfm is enough for an induction, but we will install a 600 to 900 cfm one and just run it lower speeds if not needed.

    2. Expert Member
      ARMANDO COBO | | #34

      Besides using induction cooking, the smart solution is designing a good MUA and ventilation. Here's a company that's well known for commercial ventilation systems, but they re coming strong on the residential side. You can specify the size of hood you would like to have and start from there to MUA and ventilation.
      Check this EEBA webinar with the folks at Greenheck:

      1. AC200 | | #36

        Hey Armando, I found my MUA unit at Greenheck, 64,000 cfm! The units are way overkill in safety interlocks for my needs and they won't pass any design criteria. I hope they can get more products that will be more widely accepted in residential where cost is a primary driver.

        For my case, I've specified a Vent-A-Hood insert. I have to concede some design concerns and have it wrapped in a quartz solid surface enclosure. It has a good deep capture area and a lower noise level. Not sure I believe their equivalent CFM claim. Hopefully final decision is MUA unit, Fantech or Electro and how much tempering.

        1. tundracycle | | #38

          I would avoid VAH. They are quite noisy per:

          Our was also a PITA to clean.

          1. AC200 | | #40

            Thanks for link to the article. I'm not totally thrilled about VAH either. I know they are pain to clean in spite of what they say. Will look at all the other alternatives. I was originally looking at an external blower for noise, but also need a good hood and baffle system. Hoods are afterthoughts for most, but the proper design of it and MUA is proving to require compromises all around.

  15. Expert Member
    ARMANDO COBO | | #32

    This is what I follow, and I recommend y'all reading this article by BSC. On the fast tract? Look at Figures 2, 3 & 4 towards the end. Those three figures will tell you how's done.

  16. tundracycle | | #37

    As @Allan C said "In my opinion, for many non-temperate climates, dumping unconditioned air under the range is a non-starter. No one wants to wear boots cooking in the middle of winter as 15 deg air gets drawn in right at your feet."

    In our case we have an Electro MUA that heats the air to about 40°f and then dumps it in to the return duct for the filter/furnace/AC/humidifier/dehumidifier to do the rest. Much more cost effective than alternatives to conditioning. The drawback is that the air is not necessarily delivered to the ideal places.

    I wonder if for Joe's Fig 2/3... You could do those off of the supply ducts of conditioned air but with duct blowers so that the proper amounts of conditioned air are delivered to the right places when the hood is running.

  17. AC200 | | #41

    "I wonder if for Joe's Fig 2/3... You could do those off of the supply ducts of conditioned air but with duct blowers so that the proper amounts of conditioned air are delivered to the right places when the hood is running."

    I was going to ask my HVAC designer about connecting the powered MUA unit to the kitchen supply vents. OK in winter as a heater will temper the air, but in summer hot and humid air gets dumped directly into the room. I suspect a higher rated system will be needed to account for larger line losses too.

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