IRC 2024 Makeup Air: Preventing Backdrafting from Large Exhaust Fans

What IRC 2024 § M1503.4 requires

IRC 2024 Section M1503.4 requires makeup air when a kitchen exhaust hood exceeds 400 CFM. Without replacement air entering the home to compensate for what the hood removes, the house depressurizes — creating negative pressure that can backdraft atmospheric combustion appliances such as gas water heaters, furnaces with draft hoods, and gas fireplaces. Makeup air can be supplied passively through a transfer grille or exterior passive inlet, or actively through a motorized damper synchronized with the hood or a dedicated supply fan with tempering.

Under IRC 2024, the specific approach must be designed to prevent depressurization of the combustion zone and to maintain building pressure within acceptable limits during hood operation.

Section M1503.4 states: when the flow capacity of a kitchen exhaust hood exceeds 400 CFM, makeup air shall be provided. The code does not specify the exact method of makeup air delivery but requires that the system be designed to prevent the spillage of combustion gases from combustion appliances and the loss of conditioned air from distribution systems. Supplemental makeup air systems serving hoods over 400 CFM must be identified on the construction documents and submitted for plan review in most jurisdictions.

The building science literature and engineering guidance accompanying Section M1503.4 identify three general categories of makeup air systems. Passive systems use pressure-equalization pathways: a passive exterior air inlet (a louvered or dampered wall vent that opens when pressure differential exceeds a threshold), a transfer grille from an adjacent conditioned space, or an undercut door of sufficient area between the kitchen and an adjacent room connected to an exterior air source. These approaches work when the hood is operating at moderate negative pressures and when the home is not extremely tight.

Active systems use mechanical means to introduce outdoor air in synchrony with the hood fan. A motorized damper on an outdoor air duct, wired to open whenever the hood fan switch is energized, is the most common approach. The damper opens, and outdoor air is drawn or pushed through the duct into the kitchen or adjacent space. Some installations add a small supply fan on the same duct to provide positive-pressure makeup air regardless of outdoor wind conditions. High-end cooking suites with hoods rated above 1,000 CFM often require a dedicated makeup air unit with a heating coil or heat exchanger (tempering unit) to condition the incoming outdoor air before it reaches the kitchen, preventing cold-air drafts during winter operation.

The 400 CFM threshold applies to the hood’s listed flow capacity, not necessarily to its set flow rate. A hood with a maximum rated capacity of 600 CFM triggers the Section M1503.4 requirement even if the homeowner typically uses it at 200 CFM, because the hood could be operated at full capacity.

Why This Rule Exists

In a tight modern home, a 600 CFM range hood operating at full speed removes 600 cubic feet of air per minute from the building. The laws of physics require that this air be replaced — it cannot simply disappear. In a leaky older home, replacement air infiltrates through hundreds of small cracks and gaps in the envelope, and the resulting negative pressure is relatively small. In a well-sealed modern home with a tested air leakage rate below 2 ACH50, there are very few pathways for air to enter. The 600 CFM hood creates a significant negative pressure in the building that must be relieved through whatever pathway is available.

If the nearest available pathway is the flue of an atmospheric gas water heater or a fireplace chimney, outdoor air enters through the flue in the direction opposite to its intended flow — a phenomenon called backdrafting or spillage. Backdrafting draws products of combustion, including carbon monoxide, nitrogen dioxide, and unburned fuel, back into the living space. Carbon monoxide poisoning incidents and fatalities have been directly attributed to high-CFM kitchen exhausts operating in tight homes without makeup air. This is not a theoretical risk — it is documented in insurance records and incident investigations.

Even in homes without atmospheric combustion appliances, sustained depressurization during hood operation degrades comfort (cold drafts through envelope gaps), reduces the thermal performance of the building, and can stress weather-sealing at doors and windows. At extreme negative pressures, sealed combustion appliances (which are otherwise safe from backdrafting) can experience reduced combustion efficiency and increased NOx production.

What the Inspector Checks at Rough and Final

At plan review and rough-in, the inspector looks for construction documents identifying the makeup air system when the specified hood exceeds 400 CFM. A hood specification sheet showing maximum flow capacity above 400 CFM without a corresponding makeup air design is a plan-review deficiency that can stop a project. The inspector verifies that the makeup air duct (if active) is roughed in at the correct location, that the motorized damper is installed and wired to the hood fan circuit, and that the duct penetrates the exterior with a listed cap.

At final inspection, the inspector operates the hood at full capacity and verifies that the makeup air system activates. For a motorized damper system, the damper must open when the hood fan is switched on and close when the fan is switched off. For passive inlets, the inspector may use a manometer or incense smoke to verify that the inlet is open and drawing air when the hood is running. The inspector also looks for combustion appliances in or adjacent to the kitchen and may verify that the appliances are sealed combustion (direct vent) type, reducing the backdraft risk even if the makeup air system is marginally sized.

In jurisdictions that have adopted performance-based verification, the inspector may require a pressure differential test with the hood at full capacity and all other exhaust systems off. The measured building-to-outdoor pressure differential must not exceed −5 Pascals or a similar locally adopted threshold that protects combustion appliances.

What Contractors Need to Know

The 400 CFM trigger is based on listed capacity, not operating mode. Do not specify a hood with a listed maximum above 400 CFM and then tell the inspector the owner will only use it at low speed. The code requirement is capacity-based. If the client insists on a 600 CFM hood, design the makeup air system and put it on the plans.

In practice, the most reliable active makeup air approach for residential use is a motorized damper on a 6- to 8-inch insulated outdoor air duct, wired in parallel with the hood fan so it opens whenever the hood is energized. This adds $300 to $600 in materials and about two hours of labor. The return air from the makeup air duct should be delivered into the kitchen or the return plenum of the HVAC system, not into a bedroom or bathroom, to keep the replacement air in the same pressure zone as the exhaust.

For hoods above 800 CFM, passive outdoor air entry during winter creates a comfort problem: 800 CFM of −10°F outdoor air entering through an untempered inlet creates a cold waterfall in the kitchen. A makeup air unit with a hot-water coil, electric resistance coil, or heat-pump heat exchanger tempers the incoming air to approximately 55°F before it reaches the kitchen. This adds $1,500 to $4,000 to the project depending on the tempering approach and the hood CFM rating.

Where the home uses atmospheric combustion appliances — conventional gas water heater with a draft hood, unlined masonry fireplace, or gas furnace with a conventional flue — the makeup air system must be designed to keep building pressure from falling enough to reverse draft these appliances under any operating condition, including hood at full capacity with all other exhaust fans running simultaneously. This multi-system analysis is best performed by a mechanical engineer or experienced HVAC designer familiar with building pressure balance.

What Homeowners Get Wrong

The most common homeowner misconception is that opening a kitchen window is an adequate substitute for a designed makeup air system. Opening a window does introduce replacement air, but the amount is uncontrolled, varies with wind direction and speed, and is only available when the homeowner remembers to open the window and when weather permits. A designed makeup air system provides a reliable, consistent replacement air path regardless of outdoor conditions and does not require any action by the occupant.

Another frequent misunderstanding is about sealed combustion appliances. Many homeowners believe that installing a direct-vent water heater or a power-vented furnace completely eliminates the backdraft risk from a high-CFM hood. Sealed combustion appliances do eliminate the primary backdraft pathway, but the negative pressure still affects other pathways: unlined masonry fireplaces, bath fans with stuck dampers, recessed lights in cathedral ceilings, and other leakage points. A comprehensive pressure-balance design is still appropriate for homes with very high CFM hoods even when all combustion appliances are sealed combustion.

Homeowners purchasing high-end kitchen suites with 1,200 CFM professional-grade hoods sometimes discover after installation that the makeup air system was not included in the scope of work. The hood installer and the general contractor each assumed the other was responsible. The result is a spectacular hood with no legal makeup air system — a code violation and a safety problem. Specify makeup air in the construction contract and verify it is included in the appliance installer’s and HVAC contractor’s scope of work before signing either contract.

State and Local Amendments

California Title 24 Part 6 has adopted makeup air requirements that are more stringent than the base IRC in several respects: in some California climate zones, makeup air for hoods above 400 CFM must be tempered to within a specific temperature range, and the makeup air system must be verified by a HERS rater as part of the residential energy code compliance documentation. California also limits untempered passive makeup air to climates where outdoor air temperature does not regularly fall below 35°F during the heating season — a restriction that effectively requires tempered makeup air in Northern California mountain zones.

Washington state energy code requires that any outdoor air opening exceeding 1 square inch in area, including makeup air inlets, be equipped with a motorized damper that closes when the associated exhaust fan is off. This prevents uncontrolled infiltration through makeup air inlets during non-operation periods, which would undermine the home’s thermal envelope. Check with the local building official for state-specific requirements on makeup air tempering, controls, and verification before finalizing the system design.

When to Hire a Professional

Any kitchen exhaust system exceeding 400 CFM should be designed by a mechanical engineer or experienced HVAC professional who can perform a building pressure balance analysis. This is especially important when the home has any atmospheric combustion appliances (gas water heater with draft hood, masonry fireplace, gas range with standing pilot), when the home is tested at below 3 ACH50, or when the installed hood capacity exceeds 800 CFM. A pressure balance analysis models the negative pressure at various exhaust fan combinations and verifies that the makeup air system keeps building pressure within safe limits under the worst-case operating scenario. Hiring a certified home performance contractor (BPI Building Analyst credential) or a residential mechanical engineer for this analysis is appropriate for any high-performance home with a large hood.

Common Violations Found at Inspection

• Range hood with listed capacity above 400 CFM installed with no makeup air system on the construction documents or in the field.
• Passive makeup air inlet sized inadequately — the free area of the passive inlet is too small to prevent significant negative pressure at the listed hood capacity.
• Motorized damper on the makeup air duct wired independently of the hood fan switch rather than in parallel, so the damper is not reliably open when the hood is running.
• Makeup air delivered into a bedroom or bathroom rather than the kitchen zone, creating pressure imbalances that reduce effectiveness and disrupt comfort in sleeping rooms.
• Atmospheric combustion appliances (gas water heater with draft hood) present in the same structure as a high-CFM hood with no makeup air — a combustion-safety code violation.
• Untempered passive makeup air inlet in a cold climate, creating cold drafts in the kitchen during winter hood operation and causing homeowners to disable the inlet.
• High-CFM hood (over 800 CFM) specified without a tempering unit, leaving the makeup air system non-functional in practice during cold weather because occupants cannot tolerate the cold air delivery.
• Makeup air system installed but not commissioned — motorized damper stuck in the closed position and not verified to open during hood operation before certificate of occupancy was issued.