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Green Building Blog

Makeup Air for Kitchen Exhaust

What the code does and doesn't say about kitchen exhaust and makeup air and how to prevent backdrafting of certain combustion appliances

In homes with naturally drafted fuel-fired appliances like a boiler inside the envelope, kitchen range hoods that exhaust more than 400 cu. ft. of air per minute require makeup air so they don’t depressurize the home and draw combustion gases from the appliance into the living space.

Building codes are chock full of instances in which requirements for a certain assembly or feature are laid out in great detail, but the thing itself isn’t actually required. It’s usually not explicitly stated that the feature isn’t mandatory, just noted that it has to be done a certain way when it’s included. Not surprisingly, this approach leads to occasional misunderstandings—some widespread.

A great example of this is the domestic kitchen range exhaust, more commonly known as a range hood. While some believe differently, there has never been a model code requirement to provide a kitchen range exhaust in any American dwelling. There are, however, plenty of code requirements that address kitchen exhaust when it’s installed—so many requirements, in fact, that I’m going to focus on just one: makeup air.

Makeup air has gotten some attention in recent editions of the International Residential Code (IRC), but interestingly, the provisions about kitchen exhaust are pretty similar to the 1971 Council of American Building Officials (CABO) code where they first appeared. Code writers back then realized that mechanically exhausting air from a kitchen could compete with the natural venting of fuel-burning appliances. An exhaust fan that depressurizes a building can cause conventional fuel-burning appliances to backdraft into the house, filling it with harmful gases. This concern hasn’t changed, as these code snippets indicate:

1971 CABO, M-1210—Exhaust and ventilation systems: Air requirements for the operation of exhaust fans, kitchen ventilation systems, clothes dryers and fireplaces shall be considered in determining the adequacy of a space to provide combustion air requirements.

2018 IRC, G2407.5—Makeup air provisions: “Where exhaust fans, clothes dryers and kitchen ventilation systems interfere with the operation of appliances, makeup air shall be provided.”

But these excerpts still don’t provide any insight into when makeup air is required and how to provide it.

Because kitchen makeup air is related to kitchen exhaust air, it’s appropriate to address kitchen exhaust and how it relates to the misunderstanding that opened this article. Chapter 3 of the International Residential code, “Building Planning,” contains the majority of the code’s “you shall provide this” requirements, and there are none in there—or anywhere else in the IRC—that say you must have a kitchen exhaust, and there never have been. (This isn’t to say there aren’t jurisdictions that require range hoods; states and municipalities can and do tack things like that onto the model code.)

This holds true even for gas ranges, which confuses many people. Why does a 40,000-Btu-per-hr. gas-fired water heater require venting, but a 60,000-Btu-per-hr. range doesn’t? After all, both can produce carbon monoxide. The reason they’re treated differently is because the occupant controls and thus should be aware of when the range is in use; a water heater operates automatically and without the occupants’ involvement. A failing water heater could poison you while you sleep, but if toxic gases begin to overwhelm you while you’re boiling noodles, you have a good chance of recognizing what’s happening and stopping it. The requirements for carbon monoxide alarms only in proximity to bedrooms reinforce the nature of the concern: fuel that burns while you sleep.

Other notable makeup-air provisions

There are other, less controversial provisions related to makeup air that are also worth looking at. Here’s just a summary; to see them in their entirety, read them for free at iccsafe.org

R303.5.1—Intake openings: A 10-ft. separation is required between the inlet of the makeup-air duct and any noxious contaminants, such as vents, chimneys, plumbing vents, streets, alleys, parking lots, and loading docks. Interestingly, exhaust from toilet rooms, bathrooms, and kitchens is not considered hazardous or noxious, and the IRC explicitly allows makeup-air intakes to be located near these vents (in other words, your kitchen makeup-air intake can be right next to your kitchen exhaust-vent termination).

R303.6—Outside opening protection: Both exhaust and intake openings that terminate outdoors have to be protected by corrosion-resistant louvers, screens, or grilles. The openings in these covers can’t be less than 1/4 in. or bigger than 1/2 in. in any dimension, and must be protected from the weather.

M1503.6.2—Makeup air dampers: Makeup-air systems require dampers that automatically open when the exhaust system is in use.

With this understood, let’s look at the requirements related to exhaust—which, again, only apply once the choice to install it has been made. Early in Section M1503 of the 2018 IRC, “Domestic cooking Exhaust Equipment,” the text reads, “Where domestic cooking exhaust equipment is provided…,” reminding us that installation of this equipment is not required; rather, if it is installed, these are the requirements.

There are many provisions in this section that I’d recommend reading as a follow-up to this article, but I’m going to jump to the part about makeup air. These guidelines were added to the 2009 IRC, to the dismay of many homeowners. They require that all kitchen exhaust hood systems capable of exhausting more than 400 cu. ft. per minute (cfm) be provided makeup air at a rate approximately equal to exhaust. This air has to either be discharged into the same room as the range hood, “or into rooms or duct systems that communicate through one or more permanent openings with the room” where the range hood is installed.

There was no mention of why this was required. Homeowners who dreamed of a large overhead exhaust in their kitchen remodels weren’t happy with the added expense and additional breach of their thermal envelope, especially since the reason for it wasn’t explained. Nonetheless, the requirement held through the 2012 and 2015 editions; only new requirements for dampers were added.

Then, in the 2018 IRC, the makeup-air provisions were tweaked. The hazard first spelled out in 1971—the presence of fuel-fired appliances inside the home—reemerged. The requirement to provide makeup air is now tied to the actual danger. But while the risk identified all those years ago hasn’t changed, the available appliances have. Appliances that draft naturally are still around, but are incredibly inefficient because they use heat generated by the appliance as the mechanism to carry the combustion products to the atmosphere. If you have one of these appliances—be it a furnace, a boiler, a water heater, or something else—and a range hood capable of exhausting more than 400 cfm, you need makeup air.

But with greater attention to energy conservation, direct-vent and mechanical-exhaust appliances are increasingly common in homes. Direct-vent appliances pull their own combustion air from the exterior through a dedicated pipe and have a closed combustion chamber; they do not communicate with the air in the home and aren’t affected by the depressurizing effects of a ripping kitchen exhaust. A mechanical venting system may share the same air as the rest of the house, but it has a fan and can fight back against the range hood to force the exhaust out of the house. As long as fuel- burning appliances inside a home’s thermal envelope are either direct-vent or mechanically exhausted, a kitchen exhaust can be any size and no makeup air is required.

Not all new code provisions are more restrictive than previous versions. Some are meant to correct the over-steering of past editions that sent us fishtailing down the road. For now, the IRC seems to have it right—changes to the mechanical chapters of the 2021 IRC were completed in 2018, and no modifications were made to the requirements for makeup air.

-Glenn Mathewson is a consultant and educator with buildingcodecollege.com. He is a regular contributor to Fine Homebuilding magazine.

8 Comments

  1. Doug McEvers | | #1

    This is important information for more reasons than range hoods. With more and more direct vent furnaces being installed, standard water heaters are sometimes left as the only appliance on the gas flue. Many older homes have no provision for makeup air and got by because of high infiltration rates and typical stack effect principles. As we move towards energy improvements (tighter building envelopes), replacing the 3" bath fans with higher CFM 4" fans the orphaned water heater in the basement must be considered for safety.

    1. User avater GBA Editor
      Brian Pontolilo | | #3

      Hi Doug,

      I agree and I think it is particularly important because replacing bath fans and upgrading kitchen exhaust are often DIY/handyman projects and as the article says, you don't need makeup air until you do. When you do, it's a serious issue.

  2. Chris Charron | | #2

    So what does this mean if local code is 2015 IRC?
    Currently building a house, only combustion appliance is a wood stove with outside air kit. Have only been looking at 390-400CFM range hoods because I don't want to add makeup air.

    Can I make the argument that because it's allowed under 2018 IRC I should allowed to install a 600CFM vent hood? Does the zehnder ERV mean I won't depressurize the house?

    1. Yupster | | #4

      You can make all the arguments you want, it completely depends on your local building official.

      An ERV provides balanced ventilation. It supplies as much as it exhausts. It is completely disconnected from the range hood and cannot balance the range hood. For one thing, that would mean the ERV would have to supply 600 cfm. ERV's operate in the 100 cfm range. Two, it would have to be able to shut off the exhaust side of the ERV whenever the range hood runs. Controls do not allow this. Third, the makeup air needs to be mostly delivered to the kitchen. ERVs deliver air throughout the entire house.

  3. Steve Hoge | | #5

    I have often wondered if air-gapped double-wall duct of sufficient diameter could be used to supply makeup air that is naturally heat-exchanged across the length of the galvanized duct and thus somewhat conditioned by the outgoing exhaust air. The duct connections might be a bit tricky but being able to construct a single run to the kitchen that provided both exhaust and makeup air could be advantageous. (Handling of condensation might also have to be provided for.)

    I've also wondered when the makeup air can feasibly be introduced by a natural draft if the static pressure of the makeup ducting was low enough, and when an actual makeup supply fan was required. I have seen both kinds of systems proposed.

    1. Matt F | | #6

      At a commercial level, here is an example of exhaust air to hot water preheat: https://www.youtube.com/watch?v=x5HyR1Nrc6E

      You wouldn't want the heat exchange to the makeup air during the summer.

      I wish range manufactures would incorporate makeup air in an updraft configuration to improve capture efficiency. Something like a wide, thin supply against the back wall, washing air up would be interesting.

      Edit: I also want a standard high output burner in the central location for the best capture efficiency.

      1. Steve Hoge | | #7

        I had assumed that if a passive makeup air supply was sufficient that it could be implemented in the summertime by simply opening adjacent windows.

        I had also been attracted to the idea of providing makeup air at the back wall behind the range, but it turns out this location is prone to short-circuiting if the makeup air plenum does not have a very large cross section and is located well below the cooking surface. See the attached 2004 paper for some graphic Schlieren visualizations of the airflow and recommendations for best practices in commercial environments based on different locations of the makeup air supply.

        Also another version of this paper can be found at: https://fishnick.com/publications/ventilation/makeup_air.pdf

  4. Dan B | | #8

    Hi all,

    If we want to bypass the HVAC system altogether and create a makeup vent directly into the kitchen like your illustration above, does it make a difference where the makeup air register is located? This particular builder installed the makeup air right into the hood (https://www.youtube.com/watch?v=u2uVaTMXd_8), which seems great but very costly. The makeup air ceiling register illustrated above seems more cost effective, but does it provide sufficient makeup air to replace the displaced air from the hood? We're looking to remodel our kitchen, and local building code requires us to provide makeup air for hoods that are 400 cfm or above. I've spoken to several contractors in town, and they all suggest attaching the makeup vent to the return air duct connected to the HVAC system, but I'm concerned about whether this will ultimately damage our HVAC unit.

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