How much makeup air does a second kitchen or high-CFM hood need in an Arizona home?

A second kitchen or high-CFM hood needs makeup air at a rate roughly equal to what the hood exhausts. Under IRC Section M1503.6, when a fuel-burning appliance is inside the sealed house, "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." So if your scullery hood moves 600 CFM (cubic feet per minute, the measure of how much air a hood pulls), it needs about 600 CFM of fresh air coming back in. The new wrinkle with a second kitchen is that you now have two cooking zones, and each hood that clears 400 CFM is its own trigger that has to be solved on its own.

This page covers how the 400 CFM rule applies to a back kitchen, how much makeup air to plan for, and how Arizona builders supply it. If you want the rule for the main kitchen hood by itself, see our page on makeup air for a high-CFM range hood.

The 400 CFM trigger applies to every hood, not the house total

The 400 CFM threshold is checked per hood, so a second kitchen does not add its hood to the main one to reach the trigger. Each exhaust hood system is judged on what it alone can move. A main kitchen hood rated at 600 CFM is over the line. A scullery hood rated at 500 CFM is also over the line. Both need their own makeup air, sized to their own rating.

The word "capable" in the code matters. The rule looks at the hood's rated capacity, not how you usually run it. A 900 CFM blower on the lowest speed is still a 900 CFM hood in the eyes of the plan reviewer. That is why a scullery or back kitchen with a real cooktop changes the math. The point of a scullery is to do the heavy searing and frying out of the great room, so its hood is often a true pro unit at 600, 900, or 1,200 CFM. Once that number clears 400, makeup air is on the plans for the scullery, separate from whatever the show kitchen needs.

There is one condition built into the current code worth understanding. The 2021 and 2024 IRC tie the makeup air rule to having a "gas, liquid or solid fuel-burning appliance that is neither direct-vent nor uses a mechanical draft venting system" inside the home's air barrier. In plain terms, the rule fires when you have an atmospherically vented gas appliance, like an older-style gas water heater or furnace, plus a hood over 400 CFM. Most Arizona custom homes have gas somewhere, so the safe planning assumption is that any hood over 400 CFM needs makeup air. Earlier IRC editions, such as the 2018 code many Arizona jurisdictions still enforce, applied the 400 CFM rule without that fuel-appliance condition. Confirm the adopted edition with your city, because the code edition varies by jurisdiction across the metro.

How much makeup air to plan for

Plan for makeup air at about the same rate as the hood, so match the duct and the supply to each hood's rated CFM. The code language is "approximately equal to the exhaust air rate," which means a 600 CFM hood gets roughly 600 CFM of makeup air, and a 1,200 CFM hood gets roughly 1,200 CFM. You do not get to supply less and hope the house makes up the rest, because in a tight modern home there are few leaks for that air to sneak through.

With a second kitchen, run the numbers for each space:

• Main kitchen hood. Size makeup air to its rated CFM. A 48-inch pro hood at 900 CFM needs about 900 CFM back.
• Scullery or second-kitchen hood. Size its own makeup air to its own rating. A back-kitchen cooktop hood at 600 CFM needs about 600 CFM back.
• Do not assume they run at once, but design for it. Two cooks during a holiday party may run both hoods. Each makeup air path is independent, so each works whether one hood or both are on.
• Mind the total when both run. If both hoods can fire together, the house can be moving 1,500 CFM out and 1,500 CFM in during a big event. That total shapes duct sizing and whether you temper the incoming air.

The makeup air system also needs at least one damper, a movable flap that opens when the hood runs and closes when it stops. The code requires "a gravity damper or an electrically operated damper that automatically opens when the exhaust system operates." In Arizona, where you pay to cool air for half the year, a tight automatic damper is not just a code item. Leaving an open hole to the outside would bleed conditioned air and run up your power bill all summer.

How Arizona builders supply makeup air to two kitchens

Builders run a dedicated outside-air duct and automatic damper to each kitchen, and on luxury homes they often temper the incoming desert air. The standard setup is a makeup air duct from an outdoor intake to a register near the cooktop, with an electrical interlock that opens the damper when that hood switches on. Two kitchens usually means two of these paths, one for the show kitchen and one for the scullery, each interlocked to its own hood.

The Arizona wrinkle is temperature. Outside air in Phoenix can top 110 degrees in summer and drop near freezing on a January morning, and dumping that raw air straight into a kitchen is uncomfortable. The U.S. Department of Energy notes that energy-efficient homes "require mechanical ventilation to maintain indoor air quality," and a second kitchen adds to that load. Higher-end builds add a makeup air unit (MAU) that gently warms the incoming air in winter and sometimes pre-cools it in summer. With two high-output hoods, some homes share a single larger tempered unit across both kitchens, while others give each its own. Your mechanical designer sizes that during design.

A few practical notes for a two-kitchen plan set:

• Place each supply away from its burners. Fresh air should enter the room, not blow across the flame and scatter heat or kill the burner.
• Locate the outdoor intakes well. Keep them away from a dryer vent, a flue, the pool deck, or a trash area, so you pull clean air and not dust or odors every time a hood runs.
• Coordinate with gas appliances. A gas furnace, gas water heater, or vented fireplace plus a strong hood is the exact mix the rule guards against, because negative pressure can backdraft those appliances and pull combustion gases indoors.
• Account for the electrical load. A tempered MAU is itself a circuit, and two kitchens may mean two of them, which folds into your electrical service size.

The takeaway for an Arizona custom home with a second kitchen is simple. Check the rated CFM of each hood. Any hood over 400 CFM gets its own makeup air at about the same rate, with an automatic damper, under IRC M1503.6. Plan both kitchens together during design, because adding makeup air after the ductwork is roughed in means opening walls. Confirm the hood ratings and the adopted code edition with your builder, and see our companion pages on the scullery or back kitchen, the butler's pantry, and the makeup-air glossary entry.

The Jematell Homes approach

We build for the Sonoran Desert, not a generic climate. We would rather answer your questions before you build than after, so get in touch any time.

Sources

• ICC 2021 International Residential Code, Section M1503.6 (Makeup air required)
• ICC 2021 International Residential Code, Section M1503.4 (Makeup air required, prior numbering / 2018 edition)
• U.S. Department of Energy, Energy Saver: Whole-House Ventilation