--- title: "Make-Up Air for a Strong Range Hood: The 400 CFM Line and What It Protects" description: "Why a tight house fights a big blower, the code line above 400 CFM, backdrafting gas flues, where the intake goes and who sizes and tests it." url: "https://rangehooderepair.center/guides/make-up-air-for-a-strong-hood/" date_modified: "2026-09-26" --- Installation and setup # Make-Up Air for a Strong Range Hood: The 400 CFM Line and What It Protects Make-up air for a range hood is outdoor air let in to replace what the hood exhausts. Building code, as Broan's application guide summarizes it, requires an automatically controlled supply for hoods over 400 CFM. Without it a tight house is depressurized, the hood underperforms, and natural-draft gas appliances can backdraft. An HVAC contractor sizes and tests the system. By [RangeHoodeRepair.center technical team](https://rangehooderepair.center/authors/technical-team/) Published September 26, 2026 ## At a glance. Every cubic foot a hood sends outdoors has to be replaced from somewhere. In a drafty older house the gaps around windows do it without anyone noticing; in a tight new one, a powerful blower can pull against the whole building, and sometimes against the flue of the water heater. Read the whole guide ### The code line Over 400 CFM Broan's guide: code requires make-up air that starts and runs with hoods above 400 CFM. ### Why it matters most Natural-draft gas appliances Depressurization can pull flue gases back into the house instead of up the chimney. ### Intake location Outdoors, 10 feet from vents Broan's guide keeps it 10 feet from flues, stacks and exhaust outlets and 1 foot above grade. ### Who sets it up An HVAC contractor, then a test Broan recommends a qualified HVAC professional check the result after installation. In this guide 1. What make-up air is, in plain terms 2. The 400 CFM line in the building code 3. What goes wrong without it 4. How make-up air is brought in 5. Living with a make-up air system 6. Step by step 7. Questions ## What make-up air is, in plain terms A house is a closed box with small leaks. When the hood exhausts air, the pressure inside drops slightly and outdoor air pushes in through whatever openings exist to take its place. Broan's application guide defines make-up air as the outdoor and transfer air that replaces the exhausted air, and describes its main purpose as letting exhaust appliances work as designed. At 250 or 300 CFM the leaks in most homes keep up. Broan's guide notes that hoods over 300 CFM are often considered large, and gives the example of a kitchen hood around 1,000 CFM depressurizing the house whenever it runs, because it removes air faster than the leaks can replace it. ### Why newer houses feel it first Air sealing, tight windows and spray foam are good for energy bills and bad for a big blower. The tighter the shell, the less air slips in on its own and the more of the replacement air has to come through a deliberate opening. The same large hood that works well in a 1960s ranch may struggle in a new build. Source: [Broan — Automatic Make-Up Air Damper Application Guide](https://broan-nutone.com/getmedia/a31efbb7-8b9c-4931-ab22-2c905fe7c220/Broan-make-up-air-damper-application-guide.pdf) — Definition of make-up air; hoods over 300 cfm often considered large; a hood near 1000 cfm can depressurize the home; code requires automatically controlled make-up air for hoods over 400 cfm; intake outdoors only, 10 feet from vents and 1 foot above grade, screen openings 1/4 to 1/2 inch; climate-zone limits and humidity controls; maintenance of screen, filters and intake; qualified HVAC professional testing ## The 400 CFM line in the building code Planning make-up air for a range hood starts with its rating. Broan's guide summarizes the building code requirement: range hoods rated over 400 CFM need some of their make-up air supplied through a system that is automatically controlled to start and operate at the same time as the exhaust. Local codes can be stricter, and the local building department has the final word. The code also accepts that part of the replacement air arrives through ordinary leakage. Sizing tools such as Broan's make-up air specifier estimate how much the envelope supplies and how much the damper must add, and produce a sheet an installer can hand to the inspector. ### A second limit in the island manual Vent-A-Hood's island instructions cite ASHRAE's residential ventilation standard 62.2, which limits total exhaust fans to 15 CFM per 100 square feet of occupiable space unless a back-drafting test is done or make-up air is provided, and advise asking a local HVAC engineer to evaluate it. Source: [Broan — Automatic Make-Up Air Damper Application Guide](https://broan-nutone.com/getmedia/a31efbb7-8b9c-4931-ab22-2c905fe7c220/Broan-make-up-air-damper-application-guide.pdf) — Definition of make-up air; hoods over 300 cfm often considered large; a hood near 1000 cfm can depressurize the home; code requires automatically controlled make-up air for hoods over 400 cfm; intake outdoors only, 10 feet from vents and 1 foot above grade, screen openings 1/4 to 1/2 inch; climate-zone limits and humidity controls; maintenance of screen, filters and intake; qualified HVAC professional testing Close on a small louvered air inlet in a kitchen wall near a large range hood. ## What goes wrong without it The first sign is usually the hood itself. A blower working against a depressurized house moves far less than its rating, gets louder, and lets smoke roll out past the canopy. Doors to the outside may be hard to open while it runs, and the fireplace or a bathroom fan duct may whistle as air is pulled in backward. The serious risk is combustion. Broan's hood manuals warn that sufficient air is needed for the flue of fuel-burning equipment to exhaust properly and prevent backdrafting. A natural-draft water heater or furnace relies on warm gas rising up its flue; a strong hood can reverse that flow and draw the gases into the house. - The hood sounds strong and captures poorly. - Exterior doors pull shut or resist opening while it runs. - A whistle at windows, the fireplace or other exhaust grilles. - Soot or a smell of exhaust near a gas water heater or furnace. - A carbon monoxide alarm sounding while the hood is on high. ### If the carbon monoxide alarm sounds Treat it as an emergency, not as a ventilation question. NFPA's advice is to move at once to fresh air outdoors or by an open window or door, make sure everyone in the home is accounted for, and call for help from there, staying until emergency personnel arrive. Have the gas appliances and the make-up air arrangement checked before the hood is used on high again. Source: [Broan — BXT1 Series Range Hood Installation Instructions](https://broan-nutone.com/getmedia/e05acdba-b6be-4e9e-8182-e44db1a97192/Installation-Guide-BXT1.pdf?ext=.pdf) — Sufficient air is needed for proper combustion and exhausting of gases through the flue of fuel-burning equipment to prevent backdrafting ## How make-up air is brought in The usual arrangement is an insulated duct from an outdoor intake to a motorized damper that opens when the hood starts. Broan's damper, for example, can be wired directly to compatible hoods or triggered by a pressure switch in the exhaust duct. The fresh air is delivered either into the return side of the heating and cooling system or through a register near the kitchen. Broan's guide sets the rules for the intake: it must draw directly from outdoors, never from a garage, attic or crawl space; sit at least 10 feet from flues, plumbing stacks and exhaust outlets and at least a foot above the ground; and carry a screen with openings between a quarter and a half inch. - An outdoor intake with a screen, placed away from flues and exhaust outlets. - An insulated duct from the intake into the house. - A motorized damper that opens when the hood runs and closes when it stops. - A signal from the hood, or a pressure switch in its duct, that tells the damper when to open. - A delivery point: the return side of the central system or a room register. ### Cold and humid outdoor air Air brought in on a January evening is cold, and air brought in during a Gulf Coast summer is wet. Broan's guide gives limits on how much outdoor air a furnace's return can take by climate zone, and suggests controls that run the air conditioning when the damper opens in humid climates. Those decisions belong to a heating and cooling contractor who knows the local climate. Source: [Broan — Automatic Make-Up Air Damper Application Guide](https://broan-nutone.com/getmedia/a31efbb7-8b9c-4931-ab22-2c905fe7c220/Broan-make-up-air-damper-application-guide.pdf) — Definition of make-up air; hoods over 300 cfm often considered large; a hood near 1000 cfm can depressurize the home; code requires automatically controlled make-up air for hoods over 400 cfm; intake outdoors only, 10 feet from vents and 1 foot above grade, screen openings 1/4 to 1/2 inch; climate-zone limits and humidity controls; maintenance of screen, filters and intake; qualified HVAC professional testing ## Living with a make-up air system Once installed, it asks for little. Broan's maintenance list is short: keep the outdoor screen free of debris, keep snow, leaves and plants away from the intake, clean or replace any filter in the fresh-air path, and have the damper checked during routine heating and cooling service. If the hood starts to feel weaker than it did, look at the intake before blaming the blower. A screen matted with cottonwood fluff or leaves quietly turns a balanced system back into a depressurized one. Source: [Broan — Automatic Make-Up Air Damper Application Guide](https://broan-nutone.com/getmedia/a31efbb7-8b9c-4931-ab22-2c905fe7c220/Broan-make-up-air-damper-application-guide.pdf) — Definition of make-up air; hoods over 300 cfm often considered large; a hood near 1000 cfm can depressurize the home; code requires automatically controlled make-up air for hoods over 400 cfm; intake outdoors only, 10 feet from vents and 1 foot above grade, screen openings 1/4 to 1/2 inch; climate-zone limits and humidity controls; maintenance of screen, filters and intake; qualified HVAC professional testing ## Where to stop Make-up air touches the house's heating system and every gas appliance with a flue, so it is designed and tested by an HVAC contractor, not improvised. Stop using the hood on high and book a check if a flue appliance shows soot or exhaust smell, or a door strains while it runs. Adjusting the motorized damper, its wiring or the pressure switch at the hood is not owner work. Written by [RangeHoodeRepair.center technical team](https://rangehooderepair.center/authors/technical-team/). The crew that fixes kitchen range hoods here, writing from its own service visits and each maker's installation papers. Sources: [Broan — Automatic Make-Up Air Damper Application Guide](https://broan-nutone.com/getmedia/a31efbb7-8b9c-4931-ab22-2c905fe7c220/Broan-make-up-air-damper-application-guide.pdf) · [Broan — BXT1 Series Range Hood Installation Instructions](https://broan-nutone.com/getmedia/e05acdba-b6be-4e9e-8182-e44db1a97192/Installation-Guide-BXT1.pdf?ext=.pdf) · [Vent-A-Hood — Island Range Hood Installation Instructions](https://www.ventahood.com/installpdfs/island%20range%20hood%20installation%20instructions.pdf) · [NFPA — Carbon monoxide](https://www.nfpa.org/education-and-research/home-fire-safety/carbon-monoxide) ## Step by step. Four steps, in the order worth doing them. 01 ### Read the hood's rating Find the certified CFM on the label or specification sheet and note whether it exceeds 400. 02 ### List the fuel-burning appliances Note any gas water heater, furnace, boiler or fireplace inside the house, and how each is vented. 03 ### Ask the building department Confirm the local make-up air rule before buying or installing a larger hood. 04 ### Have it sized and tested Let an HVAC contractor size the damper and duct, then test that flues still draft with the hood on high. For the heart of your home. ## Questions owners ask. ### Does every range hood need make-up air? No. Broan's guide describes the code requirement as applying to hoods over 400 CFM, and many local codes follow that line. Smaller hoods in ordinary homes usually get enough replacement air through normal leakage. ### Can I just open a window while the hood runs? It does let air in, and it helps in the moment. It is not an automatic system that starts with the hood, which is what the code asks for above 400 CFM, and it depends on someone remembering every time. ### My house has no gas appliances. Do I still need it? The backdraft risk is lower, but a tight house still starves a big hood of air, so it captures poorly and runs loud. Whether code requires it depends on the rating and the local rules, not on the fuel. ### Where does the fresh air actually come out? Either through the return duct of the central heating and cooling system, which mixes and conditions it, or through a dedicated register near the kitchen. The HVAC contractor chooses based on the house and the climate. ## The problems behind it. Each has a page of its own: what it usually means and the causes in the order worth checking. [All 17 problems](https://rangehooderepair.center/problems/) Problem ### Range Hood Runs but Barely Pulls Smoke 7 causes, cheapest first Problem ### Range Hood Too Loud: Roar, Whine or Grinding 6 causes, cheapest first Problem ### Cold Draft or a Flapping Damper Through the Range Hood 6 causes, cheapest first ## When it needs a technician. The repairs on the right are where this guide ends when the checks do not settle it. ### Range hood diagnostic visit A visit that runs the hood through every speed and light, tests the draw at the filters and follows the air outside until one part is named. ### Range hood damper and duct repair Getting the air out of the house: the backdraft damper, the duct and its joints, the wall or roof cap, and make-up air in a tight house. ### Range hood blower motor repair Testing and renewing the fan that moves a hood's air, in the canopy, the duct or on the roof: its capacitor, bearings, wheel or the motor itself. ## More in installation and setup. Sizing a hood to the burners below, running the duct and fitting the backdraft damper, recirculating when no duct can reach outside, and the make-up air a strong blower needs in a tight house. [The whole section](https://rangehooderepair.center/guides/installation-and-setup/) Size, duct, damper and make-up air ### Sizing a Range Hood: Width, Mounting Height and CFM for Wall and Island HVI's 100 and 150 CFM per foot, a canopy that covers the burners, the height windows makers print, and the duct a rating depends on. Size, duct, damper and make-up air ### Range Hood Duct Runs and the Backdraft Damper: Size, Elbows and the Cap Outside Smooth metal at full diameter, five feet for every elbow, where a duct may and may not end, and how a flap that sticks lets in cold air or chokes the fan. Guides ### All 18 guides Six sections, by what you are trying to find out. --- **[Read on the site: Make-Up Air for a Strong Range Hood: The 400 CFM Line and What It Protects](https://rangehooderepair.center/guides/make-up-air-for-a-strong-hood/)** **[Book a visit with RangeHoodeRepair.center](https://rangehooderepair.center/schedule/)**