Makeup Air Requirements: What the 400 CFM Code Trigger Means

If your kitchen hood's listed flow capacity exceeds 400 CFM, IRC and IMC rules require you to provide makeup air that is approximately balanced with the hood exhaust, and that supply has to run automatically whenever the hood runs. In practice, that means a motorized damper, a tie-in to your HVAC supply, or a dedicated tempered makeup air unit, chosen based on flow volume and climate.

TL;DR:

• Use the hood’s listed maximum CFM, not its usual setting; a 600 CFM hood still triggers the requirement when operated at lower speeds.
• Passive grilles suit modest exhaust; HVAC integration may need conditioning above roughly 10% of system airflow, while dedicated tempered units suit flows or cold climates.
• Inspectors check hood specifications, intake placement, duct routing, and wiring that proves the damper or fan opens automatically with the hood.
• A compensating hood may avoid the mandate when documented net exhaust falls below 400 CFM, but reviewers still require its listed rating.
• Simple damper and duct work typically costs $300 to $800; tempered units cost $1,500 to $4,000 or more, with state rules potentially adding verification.

Table of Contents

• Why IRC and IMC Require Makeup Air Above 400 CFM
• Exceptions, Local Overlays, and What Inspectors Check
• Three Ways to Deliver Makeup Air and How to Pick One
• Controls, Interlocks, and Keeping Makeup Air Comfortable
• Sizing, Placement, and Avoiding Backdraft Problems
• What Makeup Air Systems Typically Cost
• A Design Checklist and When to Bring in an Engineer
• What to Do in the Next Few Days
• How We Help You Design and Permit Makeup Air Systems
• FAQ
• Sources

Why IRC and IMC Require Makeup Air Above 400 CFM

IRC 2024 Section M1503.6 sets the residential trigger: when a kitchen exhaust hood's listed flow capacity exceeds 400 CFM, the dwelling needs a makeup air system sized to replace what the hood removes. On the commercial side, IMC Section 508.1 requires makeup air whenever kitchen exhaust systems operate, with supply approximately matching exhaust and without degrading hood performance.

The trigger point that trips up a lot of plan reviews is the word "listed." It's the hood's rated maximum capacity on its label, not whatever low speed the cook actually uses day to day. A 600 CFM hood needs makeup air even if the homeowner never touches anything past speed two.

The rule exists for real reasons:

• Strong exhaust without replacement air depressurizes the house, which can pull combustion gases back down chimneys and flues.
• Depressurization also weakens the hood's own capture performance, letting smoke and grease aerosols escape into the room instead of up the duct.

Exceptions, Local Overlays, and What Inspectors Check

Some compensating hood designs reduce the net exhaust volume enough to fall under 400 CFM, which can exempt a project from the makeup air mandate, though plan reviewers will want documentation of the hood's actual listed rating either way. A handful of states layer stricter requirements on top of the base code: some energy-code jurisdictions require tempering and third-party verification for larger intake volumes, so a design that's compliant in one state can need extra equipment in another.

At plan review and rough-in inspection, expect scrutiny of:

• The hood cut sheet showing listed CFM, cross-referenced against the drawn makeup air path.
• The motorized damper or MUA unit location and duct routing back to an appropriate outdoor intake.
• Control wiring documentation proving the damper or fan is interlocked to the hood, not independently switched.
• Tempering coil specifications where local rules or climate conditions call for conditioned supply air.

Three Ways to Deliver Makeup Air and How to Pick One

Most residential and light commercial projects land on one of three approaches, and the right pick depends mostly on CFM and climate rather than preference.

1. Engineered openings or transfer grilles. A passive duct or grille sized for the flow works for smaller volumes, but performance is sensitive to wind and stack-effect pressure differences, so it's a fit for modest exhaust loads only, not high-capacity hoods.
2. HVAC-integrated supply. Routing makeup air through the central air handler keeps equipment count down, but guidance from the Building America measure guideline cautions that adding outdoor air beyond roughly 10% of the system's design airflow can shift entering-air conditions enough to require supplementary conditioning or equipment changes.
3. Dedicated tempered makeup air units. For higher CFM hoods or cold climates, a standalone unit with its own fan, motorized damper, and heating coil keeps incoming air close to room temperature and avoids overloading the main system.

Pro Tip: Size the makeup air path before you finalize hood selection, since a slightly smaller listed CFM can sometimes avoid triggering a dedicated tempered unit altogether.

Builder-focused guidance from Penn State's PHRC walks through these same three paths and consistently recommends automatic interlocks and tempering once flows get large.

Controls, Interlocks, and Keeping Makeup Air Comfortable

Code doesn't just require makeup air to exist. It requires a means of closure and automatic operation synchronized with the exhaust system, which in practice means a motorized damper that opens the moment the hood turns on and closes when it shuts off.

The common wiring approaches include:

• A current-sensing relay on the hood's exhaust fan circuit that triggers the damper actuator and, where used, a supply fan.
• A direct hood-switch interlock, wired so the makeup air damper and the exhaust fan share a single control point.
• Tempering sized to keep incoming air within about 10°F of room temperature, which matters more as flow volume climbs or in colder climates where untempered intake air chills the kitchen fast.

Whatever path you choose, check it against the central HVAC system's actual capacity. A makeup air load that seems small on paper can still push a marginal air handler past its comfortable operating range, especially in homes with multiple exhaust appliances already competing for conditioned air.

Sizing, Placement, and Avoiding Backdraft Problems

The baseline sizing rule is straightforward: makeup air volume should approximately match the hood's listed exhaust CFM, with the 400 CFM mark as the point where code stops treating it as optional. Getting the volume right is only half the job, though. Placement matters just as much.

• Keep outdoor intakes roughly 10 feet away from contaminant sources like vehicle exhaust, trash areas, or vent discharges, and never let the intake sit in the exhaust discharge path.
• Run a depressurization check before finalizing the design, using a worst-case multi-appliance scenario or a reference method such as ASTM E1998, especially in tighter building envelopes where small pressure imbalances show up fast.
• Account for duct sealing and distribution losses, since an undersized or leaky makeup air path can look adequate on paper but underperform in the field.

What Makeup Air Systems Typically Cost

Budgets scale with CFM and tempering needs more than with brand or finish. A simple damper-and-duct installation for a modest exhaust load typically runs $300 to $800 in combined materials and labor, according to PHRC's builder guidance, which is often enough for residential hoods just above the 400 CFM line.

• Small fan-assisted units without tempering generally fall in the $1,000 to $3,000 range.
• Fully tempered makeup air units with heating coils and dedicated controls run from roughly $1,500 up to $4,000 or more, depending on capacity and climate.
• Beyond equipment cost, factor in the energy penalty of conditioning outdoor air, any added load on the central HVAC system, and the time a permit reviewer spends verifying the installation.

A Design Checklist and When to Bring in an Engineer

Before a kitchen remodel or new hood installation goes to permit, confirm four things: the hood's listed CFM from its spec sheet, whether any atmospheric combustion appliances share the space, which makeup air method fits the flow and climate, and how the damper or fan interlock will be documented on the drawings.

For thermal sizing on tempering coils, a thermal modeling toolkit for heat-gain calculations can help estimate the heating load before committing to equipment.

Projects worth routing through a licensed mechanical engineer typically involve multiple gas appliances sharing a tight envelope, hoods rated well above the 400 CFM threshold, or jurisdictions layering extra tempering and verification requirements on top of base code. Our mechanical engineering team at Bazini Engineering handles exactly this kind of coordination between hood selection, HVAC capacity, and permit documentation.

What to Do in the Next Few Days

Pull the hood's spec sheet and confirm the listed CFM against the 400 CFM threshold before anything else moves forward. Document the makeup air method on your drawings now rather than after a plan reviewer flags it, since rework at that stage costs more than designing it right the first time.

Get a straightforward quote for the damper, duct, and controls before committing to a hood model, since a slightly different CFM rating can change which makeup air method you need. And skip the instinct to crack a window as a workaround. It isn't a code-compliant substitute and won't satisfy a plan reviewer or pass inspection.

— Joseph

How We Help You Design and Permit Makeup Air Systems

We design makeup air solutions that satisfy IRC and IMC requirements while fitting your project's actual HVAC capacity, not a generic template. Our engagement flow starts with a site and hood review, moves into equipment specification and control design, and finishes with stamped plans and permit support through filing.

If your project needs a makeup air system designed around your building's actual layout and HVAC capacity, our services overview is the place to start a conversation with our team.

FAQ

Is make-up air required by code?

Yes, both the IRC and IMC require makeup air once a kitchen exhaust hood's listed flow capacity crosses a set threshold. For residential hoods, that threshold is 400 CFM of listed capacity, and the supply must operate automatically with the exhaust system.

At what CFM do you need make-up air?

Code requires makeup air once a hood's listed flow capacity exceeds 400 CFM. That figure refers to the hood's maximum rated capacity on its spec sheet, not the speed setting it typically runs at.

How do you know if you need makeup air?

Check your hood's listed CFM rating against the 400 CFM code threshold first. If your home also has atmospheric combustion appliances like a gas water heater or furnace sharing the same air, a depressurization risk check is worth running even below that line.

How much does makeup air cost?

Simple damper and duct installations for modest exhaust loads typically run $300 to $800 in combined materials and labor. Dedicated tempered units with heating coils and controls run higher, generally from $1,500 to $4,000 or more depending on capacity.

Sources

• IRC 2024 — Chapter 15: Exhaust systems — Section M1503.6
• Exhaust systems and makeup air: Achieving balance under the 2024 International Mechanical Code® — ICC Building Safety Journal
• Providing Adequate Makeup Air — PHRC / Penn State