If your range hood is rated above 400 cubic feet per minute, both the International Residential Code and the International Mechanical Code treat it as powerful enough to depressurize your home, and the makeup air requirements for range hoods above that threshold kick in automatically. The rating that matters is the manufacturer’s maximum capacity, not the speed you usually run. A 600 CFM hood triggers the rule even if you never push past the second setting.
The 400 CFM Trigger
IRC Section M1503.6 and IMC Section 505.2 share the same cutoff: any kitchen exhaust system capable of moving more than 400 CFM requires replacement air.1UpCodes. GSA Residential Code 2024 Chapter 15 Exhaust Systems The two codes diverge on how much.
Under the IRC, makeup air must be “approximately equal to the exhaust air rate.” A 900 CFM hood needs roughly 900 CFM of replacement air coming back in. Under the IMC, makeup air only has to cover the volume “in excess of 400 cubic feet per minute,” so that same 900 CFM hood needs about 500 CFM of makeup air.2NC Department of Insurance Office of the State Fire Marshal. 0505.2 Kitchen Exhaust Makeup Air Exceptions Which code applies depends on your jurisdiction and whether the structure is a one- or two-family dwelling (IRC) or falls under the broader mechanical code (IMC).
When the IRC Rule Doesn’t Apply
M1503.6 only applies “where one or more gas, liquid or solid fuel-burning appliance that is neither direct-vent nor uses a mechanical draft venting system is located within a dwelling unit’s air barrier.”1UpCodes. GSA Residential Code 2024 Chapter 15 Exhaust Systems Translate that into plain language: if every combustion appliance in your home is sealed-combustion or power-vented, the IRC doesn’t require makeup air no matter how large your hood is.
All-electric homes are in the same position. No combustion appliance means no backdrafting risk, which is the hazard the section targets. The exemption disappears if your home has even one atmospherically vented gas water heater, a natural-draft furnace, or a wood-burning fireplace, since those appliances rely on hot exhaust buoyantly rising through a flue and will reverse direction when a strong hood drops the indoor pressure.
Local jurisdictions frequently amend the model codes, and some require makeup air for every hood above 400 CFM regardless of what else is in the house. Confirm with your local building department before relying on the exemption.
Passive or Powered
The IRC allows makeup air to be delivered “mechanically or passively.”1UpCodes. GSA Residential Code 2024 Chapter 15 Exhaust Systems A passive system is a duct from outside with a damper that opens when the hood pulls air; the negative pressure does the work. A powered system adds a supply fan.
Passive setups handle moderate hoods well, but as capacity rises, a passive duct has to grow large enough to deliver real volume on suction alone. Some local codes set explicit thresholds. Minnesota, for example, requires powered makeup air once the required passive duct would exceed 11 inches in diameter. Beyond code, contractors generally recommend powered systems above roughly 600 to 800 CFM, because a very large passive duct brings its own trouble: cold winter drafts, noise, and reduced flow through long runs or elbows.
Dampers and the Interlock
Every makeup air system needs a damper that opens automatically when the hood runs. The IRC allows either a gravity damper or an electrically operated one.3UpCodes. IRC M1503.6.2 Makeup Air Dampers Motorized dampers are not mandatory, but they end up in most installations for a practical reason: a passive gravity damper has to deliver its full rated airflow at a pressure differential of 0.01 inches of water column or less, and many inexpensive gravity dampers can’t meet that bar.
The interlock between hood and damper has to work every time. The common solution is a current-sensing relay wrapped around the power wire feeding the hood’s fan motor; when current flows, the damper opens. Variable-speed hoods sometimes use a pressure switch or a relay tied to the hood’s control board instead. Dampers that don’t open on their own are the most common inspection failure on these jobs, usually because the interlock wiring was skipped or the sensor was clamped on the wrong conductor.
Where the Intake Goes
Placement matters as much as sizing. IMC Section 401.4 requires air intakes to sit at least 10 feet horizontally from any hazardous or noxious contaminant source, including other exhaust vents, plumbing vents, and appliance flues. If that horizontal distance isn’t possible, the intake can be closer when it’s at least 3 feet below the contaminant source.4ICC. 2021 International Mechanical Code Chapter 4 Ventilation
The IRC applies the same logic from the exhaust side: an exhaust opening must sit at least 10 feet from any mechanical air intake, unless the exhaust is 3 feet or more above the intake.5ICC. 2021 International Residential Code Chapter 15 Exhaust Systems Treat the hood’s exhaust termination and the makeup air intake as a pair during design. Putting both on the same wall near each other is a frequent and avoidable mistake.
The IRC also requires makeup air to discharge into the same room as the exhaust, or into a connected room or duct with permanent openings sized to carry the full supply.1UpCodes. GSA Residential Code 2024 Chapter 15 Exhaust Systems Running the intake to a remote hallway and hoping the air finds the kitchen does not satisfy the code. Height above grade is a practical rather than code concern in most places; two feet is a common minimum, more in heavy-snow regions.
Why All of This Exists: Backdrafting
A 900 CFM hood in a tight house with no replacement air drops the indoor pressure. An atmospherically vented gas water heater normally depends on warm exhaust rising through its flue, and when the pressure flips, outdoor air pushes down the flue instead, carrying carbon monoxide into the home. A properly sized makeup air system keeps the indoor-outdoor pressure differential small enough that the flue keeps drafting correctly while the hood runs at full speed.6Pennsylvania Housing Research Center. Kitchen Ventilation Systems Part 2 Providing Adequate Makeup Air Contractors confirm this with a manometer during commissioning; a reading above the acceptable threshold means the system is undersized, the ductwork is too restrictive, or the damper isn’t opening fully.
Tempering the Incoming Air
The IRC doesn’t require you to heat or cool makeup air; tempering is a comfort and HVAC-protection measure, not a code mandate at the residential level.7Pennsylvania Housing Research Center. Residential Makeup Air Systems and Requirements That’s a budgeting point, not a design excuse. Dumping 600 CFM of zero-degree outdoor air into a kitchen creates problems no exemption solves: the HVAC system has to absorb the load, occupants are uncomfortable, and pipes near the intake can be at risk.
Electric duct heaters are the common fix. They install inline in the makeup air duct and modulate output to hold a set delivery temperature. Residential units range from roughly 1,100 watts on a 6-inch duct to 12,000 watts or more on a 10-inch duct.8Stelpro. Make-Up Air Unit MUAA A 12,000-watt heater at full output draws 50 amps on a 240-volt circuit, and retrofits often need a panel upgrade to accommodate it. The alternative, routing makeup air through the existing HVAC return plenum, works at lower volumes, but outdoor air generally should not exceed about 20% of the air handler’s design airflow in cold climates.
Permits and Inspection
Installing a makeup air system is permitted mechanical work in most jurisdictions. Inspectors verify three things: the hood’s rated CFM matches the makeup air system’s capacity, the damper opens automatically when the hood runs, and the intake meets clearance requirements from contaminant sources. A high-output hood installed with no makeup air system at all gets red-tagged, and the retrofit at that point is more expensive than doing it right the first time.
If a kitchen remodel is in your plans and you want a high-CFM hood, raise the makeup air question with your contractor before any ductwork is roughed in. Routing the supply duct, running electrical to a duct heater, and placing the intake vent are all much easier with open walls than after drywall and cabinets are in.