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Green Building Blog

How to Provide Makeup Air for Range Hoods

If your kitchen has a powerful exhaust fan, it may be making your indoor air worse

The most powerful exhaust appliance in most homes is the kitchen range-hood fan.
Image Credit: Charles Miller

When Cheryl Morris moved into her new home, she realized that the kitchen exhaust fan was probably too powerful. Whenever she turned on the 1,200-cfm fan, strange things happened. “It pulled the ashes out of the fireplace, halfway across the room, right up to my husband’s chair,” she says. Those dancing ashes demonstrate an important principle: Large exhaust fans need makeup air.

The air that fans remove has to come from somewhere

Most homes have several exhaust appliances. They can include a bathroom fan (40 cfm to 200 cfm), a clothes dryer (100 cfm to 225 cfm), a power-vented water heater (50 cfm), a woodstove (30 cfm to 50 cfm), and a central vacuum-cleaning system (100 cfm to 200 cfm). The most powerful exhaust appliance in most homes, however, is the kitchen range-hood fan (160 cfm to 1200 cfm).

Although tightening up homes is a good way to make them more energy-efficient, builders need to remember that plugging air leaks makes it harder for air to enter a home. Every time an exhaust fan removes air from your house, an equal volume of air must enter. If a house doesn’t have enough random air leaks around windows, doors, and mudsills, makeup air can be pulled through water-heater flues or down wood-burning chimneys, a phenomenon called backdrafting. Because the flue gases of combustion appliances can include carbon monoxide, backdrafting can be dangerous.

One important way to limit backdrafting problems is to avoid installing a wood-burning fireplace or any atmospherically vented combustion appliance. Appliances in this category include gas-fired or oil-fired water heaters, furnaces, and boilers connected to old-fashioned vertical chimneys or flues. Instead, install sealed-combustion appliances with fresh-air ducts that bring combustion air directly to the burner. Most sealed-combustion appliances are practically immune to backdrafting problems.

Small exhaust fans—those rated at 300 cfm or less—usually don’t…

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13 Comments

  1. Malcolm Taylor | | #1

    Location of make-up air
    If you are going to supply un-tempered make-up air, I wonder what difference there is in where it is located? A motorized damper and grill in the backsplash of the range might mitigate comfort and heat-loss concerns. Does anyone know if there has been any studies showing whether the proportion of air exhausted that comes from the vent, not the room, varies significantly based on it's location?

  2. User avater GBA Editor
    Martin Holladay | | #2

    Response to Malcolm Taylor
    Malcolm,
    Ideally, the range hood exhaust fan will pull away the smoke and odors emanating from the stove. The problem with your backsplash idea, which has been tried, is short-circuiting: the exhaust fan often ends up pulling most of its air from the makeup air grille, allow the smoke from the roasting meat to drift into the kitchen.

  3. Malcolm Taylor | | #3

    Good Point
    I still can't but think that a directed supply air could somehow be more efficient. Years ago I worked on a couple of high-tech manufacturing facility designs. Their air supply and exhaust was situated to take the contaminants produced directly out of the rooms without affecting the products being produced. The same is true of hospital operating rooms with their vertical air flows.
    Someone with more ingenuity than me might come up with a solution.

  4. User avater
    Carl Seville | | #4

    Makeup air design
    Martin - Although this is about commercial equipment, http://www.fishnick.com/ventilation/ventilationlab/ has some really good info, including some downloadable PDFs on this link on how to design effective makeup air systems. It addresses the short circuiting you mention as well. It would be nice to see this level of research on residential systems.

  5. Kohta Ueno | | #5

    Providing Makeup Air (CEC and BSC Resources)
    As a general warning on providing makeup air--in general, you want to let the warm plume of air rising off the cooking surface rise **into the hood** to be exhausted. Introducing lots of makeup air too close to this "plume" will disrupt it, and often result in worse capture efficiency. Even concepts that seem to make sense--like a "curtain" of air at the front of the hood--can result in disrupted airflow and worse capture efficiency. The less you disrupt the plume, the better it works.

    There's a great report from the California Energy Commission using schlieren (flow visualization) images to show what's going on. It looks like it's the same (or similar) material that's in ASHRAE HVAC Applications.

    http://www.energy.ca.gov/reports/2003-06-13_500-03-034F.PDF

    Also, for those who want the entertaining version of kitchen exhaust a la Joe:

    BSI-070: First Deal with the Manure and Then Don't Suck
    http://www.buildingscience.com/documents/insights/bsi-070-first-deal-with-the-manure/

  6. Kohta Ueno | | #6

    Why can’t range-hood exhaust ducts be designed to recover heat?
    Just for folks' reference--in **commercial** kitchens, range hood heat recovery does exist--per the Environmental Building News cover story from February 2010. But I've never heard of it for residential. The story emphasizes just how hard of a problem it is to deal with cooking grease, per Martin's column.

    Capturing the heat from CKV seems like common sense, and the technology has been around for years, but major manufacturers no longer sell systems, mainly because of one big problem: grease.

    Grease in cooking effluent accumulates on hoods, ducts, fans, and even building roofs, and it poses a serious fire hazard. Heat exchangers provide an ideal spot for this grease to accumulate. The typical method for removing the grease from these systems is through an automatic washdown system, but these can be expensive to buy and maintain, and they consume a lot of water. Despite these challenges, heat-exchange technology is poised for a comeback. Innovent (www.innoventair.com) is selling a flat-plate heat exchanger for CKV that includes automatic washdown. The company does not recommend the unit for use with broilers or other heavy-duty equipment, but it could be cost-effective in an institutional setting or school cafeteria.

  7. User avater GBA Editor
    Martin Holladay | | #7

    Response to Kohta Ueno
    Kohta,
    Thanks for the useful links. And thanks for backing up my analyses of the problems associated with exhaust fan short-circuiting and gummed-up heat-exchange systems.

  8. Brian Just | | #8

    "Dynamic Relationships" = depressurization
    Martin, your "dynamic relationships" seem to be a euphemism for, or maybe a distraction from, the depressurization that accompanies large exhaust and inadequate make up air. The code requires make up air for large exhaust to minimize depressurization. Designing for reduced exhaust due to small make up air openings seems to embrace depressurization as a solution to its own problem.
    I like Broan's approach: give us really big (passive) openings. Once a designer contemplates the opening required to match the range exhaust, perhaps they'll consider reducing the size of the range fan.

  9. User avater GBA Editor
    Martin Holladay | | #9

    Response to Brian Just
    Brian,
    My use of the phrase "dynamic relationship" was neither a euphemism nor a distraction. I am well aware of the problem of depressurization. Although the magazine editors at Taunton chose to omit most references to depressurization in this article, referring instead to backdrafting problems, there is no doubt that the entire focus of the article is on the problem of depressurization.

    For a more extensive discussion of these issues, you can read the online article on which this magazine article was based: Makeup Air for Range Hoods. In that article, I wrote, "Since most residential kitchens are adequately served by a 150-cfm or 250-cfm range hood, it comes as no surprise that a 1,200-cfm range hood can cause depressurization and backdrafting problems."

    I certainly don't recommend that readers design insufficient makeup air systems to "embrace depressurization," as you put it.

    The dynamic relationship I discussed is a fact. It is based on physics; it is not a description of desirable outcome. As an exhaust fan ramps up, infiltration through cracks in the envelope increases. The change in the infiltration rate happens as a direct result of the depressurization caused by the exhaust fan -- not because a designer wants it to happen.

  10. Peter Whitman | | #10

    Gas cook tops vs. induction
    It seems like one of the contributing factors in the make-up air issue is the choice of cook top. The commercial gas ranges that are all the rage produce huge amounts of heat and off-gas, and also have high ventilation requirements.
    In the highly insulated home we are building, our HRV contractor recommends an induction cook top with small hood. When we need to turn on the ventilation system, we switch the boost on the HRV.
    Any comments?
    As usual, thank you in advance.

  11. User avater GBA Editor
    Martin Holladay | | #11

    Response to Peter Whitman
    Peter,
    Like you, I prefer range hood fans to be small. As I wrote in the article, a range hood fan rated at 160 cfm to 200 cfm is a good way to go.

  12. Malcolm Taylor | | #12

    Peter
    Another GBA article you may find interesting:
    https://www.greenbuildingadvisor.com/blogs/dept/green-building-news/hazards-cooking-gas

  13. Charlie Sullivan | | #13

    Gas vs. induction
    Peter, yes, it is a good idea from an air-quality perspective to run a hood at least on low whenever you have a gas burner on. That probably negates the efficiency advantage of gas vs. electric, and now that we have the induction option, the combination of induction with a small (200 cfm or less) hood seems good. Then you only run the hood if the cooking is generating stuff you want to vent.

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