Makeup Air Requirements: What Happens When Your Hood Exceeds 400 CFM

1. The depressurization mechanism above 400 CFM

When a kitchen range hood exhausts air from a tightly-sealed modern home at 400 CFM or more, the home's interior envelope becomes depressurized relative to the outdoor ambient pressure. The pressure differential is small (typically 1-5 pascals), but it is sufficient to reverse the natural draft in the atmospheric vent stacks throughout the home. The stacks that reverse draft are the natural gas furnace flue, the natural gas water heater flue, the fireplace chimney, the clothes dryer vent, and any other combustion or vent stack connected to the home envelope. When the draft reverses, combustion byproducts (carbon monoxide from the furnace, the water heater, and the fireplace) flow back into the home instead of up the chimney. The carbon monoxide exposure is the health and safety hazard that drives the makeup air code requirement; the depressurization also pulls outdoor air in through every uncontrolled gap in the home envelope (door frames, window frames, electrical penetrations, plumbing penetrations), which makes the home less energy-efficient and less thermally comfortable.

The 400 CFM threshold corresponds to a specific physics calculation. A typical tightly-sealed modern home has an envelope air leakage rate of 3-5 air changes per hour (ACH) under normal operating conditions, which means the home leaks 3,000-5,000 cubic feet of air per hour through the envelope gaps. At 400 CFM, a kitchen hood exhausts 24,000 cubic feet per hour, which is 4-8x the natural envelope leakage rate. At a hood rating of 600 CFM (which is the 30-inch standard residential gas cooktop requirement from the 30 vs 36 vs 48 cooktop sizing chart), the hood exhausts 36,000 cubic feet per hour, which is 7-12x the natural envelope leakage rate. At 900 CFM (the 48-inch prosumer gas cooktop requirement), the hood exhausts 54,000 cubic feet per hour, which is 11-18x the natural envelope leakage rate. The makeup air requirement at each of these CFM levels is to supply 80-100% of the hood exhaust rate as controlled intake air, so that the home stays neutral or slightly positive relative to outdoor pressure.

2. The IRC M1505 residential requirement

The International Residential Code (IRC) M1505 requires that "where the rated airflow of the kitchen exhaust hood is greater than 400 cfm, makeup air shall be provided." The makeup air shall be "discharged into the same room in which the kitchen exhaust hood is located, or into a room or hallway that opens to the kitchen, and shall be sized to provide not less than 400 cfm of makeup air." The makeup air may be supplied by a passive intake (a wall vent with a motorized damper that opens when the range hood is operating) or by an active makeup air unit (a small inline fan that introduces tempered outdoor air into the kitchen at a controlled rate). The passive intake is the typical residential solution (cost USD 200-800 installed); the active makeup air unit is the typical prosumer solution (cost USD 800-2,500 installed). The IRC M1505.4 specifies that the makeup air inlet must be at least 10 feet from the range hood inlet to avoid short-circuiting the hood exhaust back into the makeup air stream.

The enforcement of M1505 varies by jurisdiction. The 2018 IRC adopted M1505 as a national requirement; the 2021 and 2024 IRC confirmed the 400 CFM threshold and added the clarification that the makeup air must be controlled (motorized damper or active unit) rather than a passive wall opening. The State of California adopted M1505 with the additional requirement that the makeup air inlet must be at least 25 feet from any opening of the home (door, window, garage door) to avoid pulling combustion gases into the makeup air stream. Other jurisdictions (e.g., the Canadian National Building Code Section 9.32) require makeup air above 300 CFM rather than 400 CFM, with tighter control requirements. The JILU North American kitchen range hood size standards and codes guide provides the per-jurisdiction makeup air threshold and the per-jurisdiction control requirement, with the strictest case (California) being the typical prosumer requirement.

Pairing reference across JILU product lines. For residential buyers, the makeup air requirement maps to the JILU under-cabinet range hood specification: A13 and other under-cabinet residential models in the 280-450 CFM band fall below the 400 CFM threshold and typically do not require makeup air. For commercial buyers, the makeup air requirement maps to the JILU commercial kitchen exhaust hood line: commercial models in the 900-1,800 CFM band always require the makeup air upgrade per the IMC Section 505 and the active inline centrifugal makeup air unit pairing. The JILU main product page and the JILU hot products page provide the full per-model CFM and per-model makeup air pairing reference.

3. The IMC Section 505 commercial requirement

The International Mechanical Code (IMC) Section 505 covers commercial kitchen exhaust hoods and is triggered when the hood is rated above 1,200 CFM or when the hood serves a commercial cooking appliance (as opposed to a residential cooktop). The IMC Section 505 requires makeup air for any commercial kitchen exhaust system, regardless of the CFM rating, with the makeup air sized to 80-100% of the hood exhaust rate. The makeup air must be tempered (heated or cooled to within 10 degrees Fahrenheit of the kitchen ambient temperature) before being discharged into the kitchen; untempered outdoor air can cause thermal discomfort for kitchen workers and condensation on cold surfaces. The IMC also requires the makeup air to be filtered if it is being drawn from a polluted environment (urban street, garage, loading dock); the filter specification is typically MERV 8 to MERV 13 depending on the local air quality.

The commercial-grade makeup air differs from residential-grade in three ways. First, the CFM capacity is larger (typically 800-2,000 CFM vs. 400-1,000 CFM residential). Second, the unit is typically an inline centrifugal fan rather than a wall vent with motorized damper, because the inline configuration supports the larger CFM capacity and the active tempering. Third, the commercial makeup air unit is integrated with the building management system (BMS) and interlocks with the hood operation; the residential unit is typically standalone and interlocked with the hood via a relay or current sensor. The JILU commercial kitchen exhaust hood line ships with the BMS interlock wiring prepared for the inline centrifugal makeup air unit; the residential wall-mount line ships with the current-sensor relay prepared for the residential motorized damper. The AHRI commercial ventilation certification covers the inline makeup air unit performance, with the JILU commercial hood installation guide cross-referenced to the AHRI certified model selection.

4. What happens when makeup air is not installed (the failure case)

When a residential or commercial range hood exceeds 400 CFM without makeup air, the failure scenario unfolds over a typical cooking session as follows. In the first 5-10 minutes of hood operation, the kitchen becomes slightly depressurized; the depressurization is small (1-3 Pa) and is not directly observable. The hood operates normally and captures cooking smoke and grease. At 10-30 minutes into the cooking session, the home envelope begins to draw makeup air from uncontrolled sources: the basement door frame, the attic hatch, the electrical penetrations around the meter, the plumbing penetrations around the water heater. The air drawn in from these sources is dirty (carrying insulation particles, dust, outdoor pollutants), unfiltered (no filter in the envelope gaps), and unconditioned (cold in winter, hot in summer). The kitchen worker or residential cook experiences drafts from unexpected locations and a slight change in the kitchen air quality (musty smell from the basement, dust on the kitchen counter). At 30-90 minutes into the cooking session, the depressurization grows stronger (3-5 Pa) and begins to reverse the natural draft in the atmospheric vent stacks. The water heater flue shows spillage at the draft hood (visible soot or condensation at the water heater base); the furnace flue shows the same. The carbon monoxide level in the home rises from a baseline of 0-2 ppm to 5-15 ppm. At 1-4 hours into the cooking session, the carbon monoxide exposure may reach the 25 ppm level that triggers the UL 2034 carbon monoxide alarm (if installed) and at 50-70 ppm it triggers the OSHA 8-hour permissible exposure limit for commercial workers.

The CO exposure is the primary health hazard, but the depressurization also creates secondary issues: combustion inefficiency in the furnace and water heater (cold air dilution drops the combustion temperature and produces more CO and less CO2), condensation on cold surfaces (the cold makeup air condenses on the kitchen windows and exterior walls in winter), and increased HVAC runtime (the furnace or AC runs longer to compensate for the cold or hot air infiltration). The condensation can produce mold growth on the bathroom walls and the kitchen ceiling if the cooking session repeats daily over weeks. The CPSC residential fire and CO safety data and the EPA Indoor Air Quality guidance both reference the depressurization-induced CO and IAQ risk as the primary residential kitchen hood safety issue. The JILU field service data shows that roughly 8-12% of 600-1,200 CFM hood installations in tightly-sealed homes exhibit some degree of depressurization symptom in the first year of operation, and that the symptoms are typically resolved by a paired makeup air upgrade (USD 200-800 residential, USD 800-2,500 commercial).

5. Makeup air sizing and the per-CFM calculation

The makeup air sizing follows the rule of 80-100% of the rated hood CFM. The 80% lower bound comes from the ASHRAE 62.2 mixed-mode calculation: a kitchen that draws 80% of its exhaust volume from controlled makeup air and 20% from envelope infiltration stays within 1-2 Pa of neutral pressure. The 100% upper bound is the safer calculation for tightly-sealed homes (under 3 ACH envelope leakage) and for commercial installations; it corresponds to perfect pressure neutrality. The per-jurisdiction rule varies: the IRC M1505.4 specifies that the makeup air shall be at least 400 CFM for a hood above 400 CFM (a fixed floor rather than a percentage); the IMC Section 505 specifies that the makeup air shall be at least 80% of the hood exhaust CFM (a percentage rather than a fixed floor); the California Building Code specifies the makeup air shall be at least 100% of the hood exhaust CFM. The JILU code guide recommends the 100% rule as a safe baseline across all jurisdictions.

The pairing rule of thumb is: 400-700 CFM hoods pair with residential-grade passive or active makeup air; 700-1,200 CFM hoods pair with commercial-grade active makeup air; 1,200+ CFM hoods pair with commercial-grade inline centrifugal makeup air with BMS interlock. The cost of the makeup air upgrade is roughly 5-10% of the hood installation cost at the residential end and 15-25% at the commercial end. The Intertek lab certification covers the makeup air unit performance and the hood-and-makeup-air interlock functionality; the ENERGY STAR certification covers the CFM-per-watt efficiency for both the hood and the makeup air unit.

6. The exception case: kitchen layout with multiple ventilation sources

The exception to the per-CFM rule is the kitchen layout with multiple ventilation sources that collectively provide the makeup air. A kitchen that has a kitchen exhaust hood rated at 800 CFM paired with a bathroom exhaust fan rated at 80 CFM and a clothes dryer vent rated at 150 CFM and a fresh air introduction vent rated at 200 CFM may not need a dedicated makeup air upgrade, because the fresh air introduction vent provides 200 CFM of controlled intake and the bathroom and dryer vents provide additional uncontrolled exhaust that draws the makeup air from the fresh air vent. The IRC M1505.4 explicitly allows this exception: "where the building is provided with a fresh air introduction system that provides not less than the required makeup air, the dedicated makeup air damper is not required." The exception is most common in newer LEED-certified homes and in ENERGY STAR certified homes that are required to have a fresh air introduction system for indoor air quality.

The exception also applies to commercial kitchens that have a dedicated makeup air handler. A restaurant that has a 1,500 CFM kitchen hood and a 2,000 CFM rooftop makeup air unit does not need a separate wall-mounted makeup air damper; the rooftop unit supplies the makeup air to the kitchen via the HVAC ductwork. The exception requires that the rooftop makeup air unit be interlocked with the hood operation (typically via a building management system relay), so that the makeup air is supplied only when the hood is operating. The JILU commercial kitchen exhaust hood line ships with the interlock wiring ready to connect to a rooftop makeup air unit or a wall-mounted inline makeup air unit, with the field wiring configuration documented in the installation guide. The exception is documented with the local code official at the permit application stage; the field installation is verified at the final inspection stage by the local code official with reference to the JILU installation guide and the makeup air unit specifications.

About the author

Mr. Zheng | Technical Director
Mr. Zheng has spent more than 30 years working on kitchen ventilation, stainless steel fabrication, and performance-focused exhaust solutions for demanding cooking environments. His practical experience covers airflow design, durability planning, and the details that make outdoor BBQ hoods last in real-world conditions.
Experience: 30+ years in kitchen ventilation, stainless steel fabrication, airflow design, durability planning, and commercial exhaust solutions.
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Editorial basis: ASHRAE 62.2 referenced via ASHRAE Standards. HVI referenced via HVI. CPSC and EPA indoor air quality referenced via CPSC and EPA IAQ. ENERGY STAR referenced via ENERGY STAR. AHRI referenced via AHRI. Product specifications: jilukitchen.com.