IRC 2024 Kitchen Makeup Air: When High-CFM Range Hoods Need Replacement Air

What IRC 2024 § M1503.4 requires

Under IRC 2024 Section M1503.4, makeup air is required when a kitchen exhaust hood is rated at more than 400 CFM. The makeup air system must supply replacement air approximately equal to the exhaust volume so that the kitchen and house remain at neutral pressure during hood operation. Without makeup air, a high-CFM hood creates negative pressure (depressurization) inside the house that causes combustion appliances to backdraft, makes exterior doors difficult to open, draws unfiltered air through gaps in the building envelope, and pulls contaminated air from attached garages or crawl spaces into the living space.

Under IRC 2024, makeup air may be provided passively (through transfer grilles) or actively (through a powered supply fan), and it must be conditioned or delivered in a way that avoids discomfort or thermal penalty.

Section M1503.4 establishes the threshold: when the exhaust rate of a kitchen hood exceeds 400 CFM, the installation must include a source of makeup air. The makeup air must be “approximately equal” to the exhaust air rate — in practice, this means the makeup air system must be capable of delivering at or near the exhaust volume when the hood is at maximum speed. The makeup air must not create an indoor pressure condition that is excessively positive, which would cause the opposite problem of forcing conditioned air out through the building envelope.

The code does not mandate a specific makeup air system type, leaving system design to the designer or contractor. However, M1503.4 specifies that makeup air systems must deliver air in a manner that does not adversely affect the operation of combustion appliances. This means the makeup air cannot be simply a large uncontrolled gap in the building envelope — it must be a deliberate, controlled system that delivers the required airflow without creating cross-currents that disrupt combustion equipment.

Passive makeup air systems use transfer grilles, transfer ducts, or permanently open low-resistance pathways from an adjacent conditioned space to the kitchen. These systems work when the pressure difference created by the running exhaust hood is sufficient to draw the required air volume through the passive pathway. Passive systems are simple and have no moving parts, but they require that the adjacent conditioned space be large enough to act as an air reservoir, and they may not perform adequately for very high CFM hoods (600 CFM and above) in tight construction.

Active makeup air systems use a powered supply fan that is interlocked with the exhaust hood. When the hood turns on, the supply fan turns on and delivers an approximately equal volume of replacement air. Active systems can be integrated with the home’s HVAC system (using the air handler to supply additional air to the kitchen when the hood is running) or can be a dedicated fan-and-duct system. The supply air must be delivered in a way that does not short-circuit the exhaust hood — supply air discharged directly toward the hood’s intake will be immediately exhausted without ventilating the kitchen, which is ineffective.

Conditioning of makeup air is not explicitly required by IRC 2024 M1503.4, but energy codes in many states require that makeup air for hoods exceeding 400 CFM be tempered (heated in winter, cooled in summer) to avoid excessive heating or cooling loads. Introducing 400 to 1,000 CFM of unconditioned outdoor air directly into a kitchen during cold weather would impose a massive heating load on the HVAC system and create serious occupant discomfort. Energy code compliance for makeup air systems is a separate analysis from the mechanical code requirement but equally important in design.

Why This Rule Exists

A range hood exhausting more than 400 CFM from a tight modern house can depressurize the building to the point where combustion appliances cannot draw the combustion air they need through their natural draft pathways. This condition, called backdrafting, causes combustion byproducts — including carbon monoxide — to be drawn back into the living space through the vent pathway rather than expelled to the exterior. Backdrafting of a gas water heater, furnace, or fireplace is a serious carbon monoxide poisoning hazard. The 400 CFM threshold is the point at which research and field experience indicates that depressurization in typical residential construction becomes severe enough to reliably trigger backdrafting in naturally-drafted combustion equipment.

Beyond combustion safety, house depressurization from a high-CFM hood has additional consequences. When the house is at significant negative pressure relative to outside, air infiltrates through every gap in the building envelope — through electrical outlets, at window frames, through wall penetrations — at a rate determined by the pressure difference and the total leakage area. This uncontrolled infiltration bypasses the insulation and air barrier system, importing outdoor humidity, pollutants, and temperature extremes directly into wall cavities. In attached-garage situations, negative house pressure actively draws garage air — which may contain vehicle exhaust, gasoline vapors, and other contaminants — through the garage-to-house connection. The makeup air requirement is thus both a combustion safety rule and an indoor air quality rule.

Physical effects of severe house depressurization are also significant. A house at -50 Pascals of negative pressure relative to outside will have exterior doors that require significant force to open, a fireplace that cannot draft (or that flows backward), and HVAC equipment that performs below its rated capacity because the air handler cannot achieve its designed airflow against a significant negative pressure. Makeup air eliminates these operational problems by keeping the house at or near neutral pressure.

What the Inspector Checks at Rough and Final

At rough-in, the inspector verifies that a makeup air pathway has been included in the design when the specified hood is rated over 400 CFM. If the hood is built-in to the kitchen design (typically on a custom island hood or professional-grade wall-mount hood), the makeup air duct or transfer grille location must be planned at this stage. They confirm that the active makeup air fan, if specified, has been allocated an appropriate circuit and duct route.

At final inspection, the inspector verifies that the makeup air system is installed and functional. For passive systems, they check that the transfer grille or duct pathway is open and properly sized for the required CFM. For active systems, they verify that the supply fan is interlocked with the exhaust hood so that both operate together. They may perform a simple combustion appliance zone test — operating the hood at full speed and verifying that the water heater and furnace do not backdraft — though not all jurisdictions require this performance test at inspection. They verify that the makeup air inlet is not located near a vehicle exhaust source, combustion vent, or other pollutant source.

What Contractors Need to Know

Design the makeup air system concurrently with the hood specification, not as an afterthought. The makeup air system type — passive or active — affects framing, duct routing, and electrical rough-in. Waiting until after the drywall is up to address a makeup air requirement for a 600 CFM island hood is costly. When the hood specification is confirmed, determine the required makeup air volume (equal to hood CFM at max speed), select the system type, and rough in the ductwork, transfer grilles, and electrical supply simultaneously with the hood duct.

When designing passive makeup air, calculate the required grille or duct size for the target CFM. A transfer grille that is too small will not flow enough air to prevent depressurization even if it is positioned correctly. ACCA Manual J or ASHRAE 62.2 references provide guidance on grille free area sizing for given airflow requirements. For 400 CFM of passive makeup air through a transfer grille, a grille with at least 2 to 3 square feet of free area is typically required at low pressure drops.

Active makeup air systems should always be interlocked with the exhaust hood rather than controlled independently. A homeowner who forgets to turn on the makeup air fan while running the exhaust hood achieves no benefit from the system. Most active makeup air systems use a current-sensing relay on the exhaust hood circuit to automatically start the supply fan whenever the hood is energized. Some smart home systems can achieve the same interlock digitally, but verify that the interlock is reliable and not dependent on a software update or Wi-Fi connection.

What Homeowners Get Wrong

The most pervasive misunderstanding is that any range hood that fits the opening and looks appropriate is suitable for the kitchen. A 1,200 CFM professional-grade hood in a tight, well-insulated modern house without a makeup air system is not just a code violation — it is a potential carbon monoxide poisoning hazard every time it is run at full speed. Homeowners who purchase high-output hoods for aesthetic reasons without understanding the makeup air requirement create dangerous conditions. If your hood is rated over 400 CFM, verify whether a makeup air system was installed.

Homeowners sometimes address house depressurization by cracking a window when the high-CFM hood is running. While this works somewhat in principle, it is not a code-compliant makeup air solution and does not provide controlled, measured makeup air. More importantly, opening a window in winter introduces very cold air that imposes a large heating load. A properly designed makeup air system delivers tempered or conditioned air to the kitchen in a controlled manner that does not create a discomfort or energy penalty.

A common misconception is that recirculating (ductless) hoods over 400 CFM do not need makeup air because they do not exhaust to the exterior. This is correct: a recirculating hood, regardless of its rated airflow, does not depressurize the house because it returns all air to the kitchen. However, recirculating hoods are not acceptable for new construction per the energy code requirements in most jurisdictions. If your recirculating hood is in an existing kitchen and is grandfathered, the makeup air requirement does not apply to it, but neither is it removing moisture or combustion byproducts from the kitchen air.

State and Local Amendments

California’s Title 24 Energy Code requires that makeup air for kitchen exhaust hoods exceeding 400 CFM be tempered to within 10 degrees Fahrenheit of the indoor temperature before being delivered to the kitchen. This effectively prohibits unconditioned outdoor air makeup systems in California and requires either heat recovery ventilation (HRV), an HVAC system integration, or an electric or hydronic tempering coil on the makeup air supply. The tempering requirement significantly increases the system complexity and cost compared to a simple passive grille approach.

Some Pacific Northwest jurisdictions require active makeup air systems for all hoods over 400 CFM, regardless of whether passive makeup air could theoretically provide adequate airflow, because of the documented backdrafting risk with passive-only systems in tight construction with multiple combustion appliances. Verify local requirements before specifying a passive system for a high-output hood in a jurisdiction with stringent indoor air quality standards.

When to Hire a Professional

Makeup air system design and installation requires a licensed mechanical engineer or HVAC contractor when the system involves ductwork, powered fans, or integration with the existing HVAC system. A simple passive transfer grille between adjacent rooms may be within the scope of a kitchen remodeling contractor, but the sizing calculation should be reviewed by a mechanical engineer or experienced HVAC contractor to confirm that the grille free area is adequate for the hood’s CFM. Combustion safety testing of the completed system — verifying that combustion appliances do not backdraft when the hood is running at full speed — should be performed by a Building Performance Institute (BPI) certified contractor or equivalent, particularly in tight modern construction.

Common Violations Found at Inspection

• Kitchen hood rated over 400 CFM installed with no makeup air system of any kind — the most common violation with high-output residential range hoods.
• Passive makeup air grille present but undersized for the hood’s CFM rating, providing inadequate airflow to prevent depressurization.
• Active makeup air fan installed but not interlocked with the exhaust hood, requiring the homeowner to manually start the supply fan.
• Makeup air intake located near a combustion vent outlet or a vehicle exhaust source, importing contaminated air into the kitchen supply.
• Makeup air supply duct discharging directly into the exhaust hood intake, short-circuiting the makeup air so it is immediately exhausted without ventilating the kitchen.
• Passive makeup air using an open gap in the building envelope rather than a controlled, screened, and sized opening — creates pest, moisture, and uncontrolled infiltration problems.
• No makeup air designed into a kitchen where a high-CFM island hood is specified, identified at framing inspection after it is too late to easily rough in the required system.
• Makeup air system installed but delivering unconditioned exterior air in a jurisdiction that requires tempered makeup air.