When Does a Range Hood Need Makeup Air?

A ducted range hood sends indoor air outside with grease, heat, moisture, and odors. That outgoing air has to be replaced. If replacement air cannot enter readily enough, indoor pressure falls and the building begins pulling air through cracks, open flues, attached garages, or other unintended routes.

Makeup air is the controlled alternative: an intentionally sized outdoor-air route that opens as the hood runs. Depending on the installation, it may be a wall intake with a motorized damper, a tempered-air unit, or a system tied to the HVAC equipment. Its job is not simply to add fresh air; it is to keep the hood moving its rated airflow without creating excessive negative pressure.

Why the furnace supply is not automatically makeup air

A normal forced-air supply register delivers air only while the heating or cooling system is operating, and its airflow was typically balanced for comfort rather than for a 600- or 1,200-CFM kitchen exhaust load. Even when the blower is running, the supply may be distributed throughout the home instead of available near the kitchen. It also does not guarantee an equal volume of outdoor replacement air; much of that air may simply be recirculated indoor air.

A recirculating hood is different again. It passes air through grease filters and, where fitted, charcoal filters, then returns that air to the kitchen. It can reduce some odors and particles, but it creates no meaningful exhaust imbalance because no air is leaving the building.

When a dedicated path becomes sensible

A modest ducted hood in a leaky older house may obtain replacement air naturally without operational problems. A powerful hood in a newer, tightly sealed house is far less forgiving. A dedicated system is especially important when testing or local requirements identify one or more of these conditions:

• The hood is high-capacity, particularly above the local code threshold.
• The home is tightly built or recently air-sealed.
• Atmospheric-combustion fireplaces, water heaters, or furnaces could backdraft.
• Doors become difficult to open, the hood sounds strained, or smoke capture worsens at high speed.

The correct design follows the local code and hood rating, then is verified in operation—not assumed from the presence of HVAC registers.

In jurisdictions using the commonly adopted International Residential Code language, a kitchen exhaust hood rated over 400 CFM generally requires a makeup-air system. A 400-CFM hood is not automatically exempt from sound building-science practice, however, and a 401-CFM listing does not settle the design by itself. The locally adopted code edition, amendments, permit conditions, and inspector’s interpretation control.

The usual code concept is straightforward: the makeup-air system must supply approximately the same airflow as the exhaust system and must have an automatic damper that opens when the hood operates. It should not simply be a permanently open wall vent. In practice, the installer should verify the hood’s rated airflow, not assume its highest marketing number or an unmeasured field estimate.

Before selecting a hood, confirm these items with the local authority or mechanical contractor:

• Whether the 400-CFM provision is adopted or amended
• Whether airflow must be matched at the hood’s maximum speed
• How the damper is interlocked with hood operation
• Whether combustion appliances or a fireplace require additional depressurization testing

A tight home with a 350-CFM hood may still need planned replacement air; an older, leakier home may behave differently, but leakage is not a reliable design strategy.

Size the hood for the cooking, not the threshold

Selecting a 300-CFM hood solely because it falls under a local makeup-air trigger is usually false economy. A hood must capture the heat, moisture, grease, and combustion by-products produced at the cooktop—not merely avoid a permitting requirement.

A modest electric range used for simmering and occasional sautéing may be well served by 300 CFM, assuming adequate hood width, capture depth, and short, smooth ducting. That is not comparable to a 36-inch gas range with high-output burners, a grill or griddle, frequent wok cooking, or several burners operating across a wide cooktop. Those loads create fast-rising, high-velocity plumes that a small blower often lets spill into the room.

For example, a 600-CFM hood may be the appropriate capture tool for a high-output gas setup; choosing 300 CFM can mean smoke alarms, greasy cabinets, and poor indoor air despite nominal compliance. First calculate the hood CFM required for the cooking load, then design the makeup-air system around that result. A correctly sized hood with makeup air is preferable to an undersized hood that fails at cooking time.

A hood’s advertised CFM is usually measured under favorable test conditions. Installed airflow can be materially lower once air passes through a grease filter, a backdraft damper, long duct runs, elbows, roof or wall caps, and a restrictive transition. A nominal 600-CFM blower may therefore move far less air at the hood opening—and still be treated as a 600-CFM hood by the permitting authority.

Capture is the practical test. Smoke and heat must be pulled into the hood canopy before they roll past its front edge, especially with front burners, wok cooking, or high-output gas appliances. For ventilating a 48-inch gas range, canopy coverage, mounting height, and a short, smooth duct often matter as much as selecting a larger blower.

Treat the listed maximum as the compliance number

Variable-speed controls reduce noise and airflow during routine cooking, but they do not necessarily remove a makeup-air requirement. Many jurisdictions use the hood’s listed maximum CFM, because the unit can be operated at that setting. Others assess the installed system or require a documented reduced maximum; that decision belongs to the local code official, not a sales claim.

Before purchase, obtain the manufacturer’s certified airflow data and duct requirements. Then confirm whether the jurisdiction bases its threshold on rated capacity, field-measured airflow, or a permanently limited and approved setting. Do not undersize the hood merely to stay below a threshold: poor capture trades a permitting concern for moisture, grease, and combustion by-products indoors.

A motorized wall damper is the lowest-cost option: the hood opens it whenever the blower runs, admitting raw outdoor air. It can work in mild climates and occasional cooking, but winter drafts, summer humidity, and pressure swings are expected.

A hood-linked ducted supply brings that air to a planned register rather than a random wall opening. It offers better distribution, but the supply grille must be sized for low velocity and located away from the hood and cooktop—often high on an adjacent wall. Air blown across the burners can push smoke past the capture zone and make a powerful hood perform poorly.

HVAC-integrated tempered makeup air uses the heating or cooling system to moderate incoming air. It is more comfortable in four-season climates, but requires enough blower capacity, duct space, control coordination, and verification that the HVAC system can run safely with the hood.

A dedicated conditioned unit provides the most reliable temperature and humidity control for large hoods, cold climates, or high-performance homes. It also brings the most design, cost, and commissioning work.

Regardless of approach, controls should be interlocked: hood operation opens the damper and starts any required supply equipment. Commissioning should confirm delivered supply airflow, damper operation, pressure response, and smoke capture at each hood speed.

A powerful hood can turn a house into a competing-exhaust system. Bath fans, a dryer, a central vacuum, and an atmospheric water heater or furnace all draw on the same pressure reserve. A tightly sealed enclosure has little accidental replacement air, so the hood’s performance and the safety of combustion appliances must be checked together.

Fireplaces deserve particular caution. With the hood at its intended high speed, a fireplace may spill smoke into the room or become difficult to light; naturally drafted gas appliances can spill combustion gases at their draft hood. Large gas ranges add another reason to verify capture at the burners, rather than reducing fan speed merely to avoid pressure problems.

This is part of whole-home ventilation and indoor-air-quality planning, not an accessory decision. It also prevents apparent hood failures that are later blamed on range-hood maintenance problems, when the real cause is a stuck damper, failed interlock, or inadequate supply path.

Test at the conditions that matter

Commissioning should run the hood at every intended operating speed, especially maximum, while other likely exhaust devices operate. A qualified contractor should measure pressure, confirm makeup-air delivery, and observe whether the supply air disrupts the hood’s capture plume. Combustion safety testing should include draft and spillage checks on every naturally vented appliance; a fireplace should be evaluated with the doors or damper in its normal operating position.