--- title: "Range Hood Duct Runs and the Backdraft Damper: Size, Elbows and the Cap Outside" description: "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." url: "https://rangehooderepair.center/guides/duct-runs-and-the-backdraft-damper/" date_modified: "2026-09-26" --- Installation and setup # Range Hood Duct Runs and the Backdraft Damper: Size, Elbows and the Cap Outside A range hood duct and backdraft damper decide how much of the blower's rating reaches outdoors. Makers ask for smooth metal duct at full size, as short and straight as possible, ending at a wall or roof cap with a damper, never in an attic, wall cavity or chimney. The damper should open freely with the fan on and close when it stops. By [RangeHoodeRepair.center technical team](https://rangehooderepair.center/authors/technical-team/) Published September 26, 2026 ## At a glance. Between the blower and the open air sits a length of metal pipe, a few bends and a flap that should swing open when the fan starts and fall shut when it stops. Most complaints of a hood that is loud, weak or drafty begin somewhere along that path rather than in the hood itself. Read the whole guide ### Duct material Smooth metal only Broan and Vent-A-Hood both require metal; Vent-A-Hood rules out flexible and corrugated duct. ### One elbow Counts as about 5 feet of duct Vent-A-Hood's figure for a 90-degree elbow. ### Where it ends A wall or roof cap outdoors Never an attic, a chimney, a crawl space, a garage or the space inside a wall. ### The damper Opens with the fan, closes without it Vent-A-Hood requires a gravity damper at every cap to prevent back drafts. In this guide 1. What the duct is asked to do 2. Size and material: full diameter, smooth metal 3. Where the duct has to end 4. The backdraft damper: a flap with one job 5. Step by step 6. Questions ## What the duct is asked to do A blower is rated at a given resistance. Every foot of pipe, every bend and every grille at the end adds resistance, and the fan answers by moving less air. That is why two identical hoods can perform very differently in two kitchens: one on a short straight run to an outside wall, the other on a long run with four turns up to the roof. Both Broan and Vent-A-Hood open their ducting instructions with the same sentence in different words: use the shortest possible run with the fewest elbows. Treat the range hood duct and backdraft damper as one system: a flap that will not lift spoils a perfect run just as surely as a crushed elbow does, and a clean, free flap cannot rescue a duct that is too small. ### Counting a run the way the makers do Vent-A-Hood treats a 90-degree elbow as equal to 5 feet of straight duct, and asks for the diameter to grow by one inch for every ten feet once a run passes 20 feet. Add the straight lengths, add five feet for each full elbow, and compare the total with the limit in your own hood's manual. Source: [Vent-A-Hood — Wall Mount Range Hood Installation Instructions](https://www.ventahood.com/installpdfs/wall%20mount%20range%20hood%20installation%20instructions.pdf) — Never reduce the duct size; no flexible or corrugated duct, only smooth galvanized metal; short, straight runs with gradual turns; a 90-degree elbow equals 5 feet of duct; over 20 feet add one inch of diameter per ten feet; combined ducts keep the total area; a louver or roof jack with a gravity damper on every run; no screen wire or spring-loaded doors; do not terminate into an attic or chimney; [Broan — BXT1 Series Range Hood Installation Instructions](https://broan-nutone.com/getmedia/e05acdba-b6be-4e9e-8182-e44db1a97192/Installation-Guide-BXT1.pdf?ext=.pdf) — Shortest run and fewest elbows; metal ductwork only so a grease fire is contained; tape joints with metal duct tape; never vent into an attic, walls, ceilings, crawl spaces or garages; a hood damper and a wall-cap damper can interfere and cause poor airflow or back drafts, in which case remove the hood damper flap ## Size and material: full diameter, smooth metal The duct size printed in the hood's manual is the smallest the rating allows, and Vent-A-Hood's instructions say never to reduce it. A 7-inch outlet necked down to 6 inches to reuse an old wall cap, or a rectangular duct squeezed through a joist bay, throttles the fan from the first day. Broan requires metal ductwork to reduce the risk of fire and explains why: if a grease fire is drawn in, metal contains it. Vent-A-Hood goes further and asks for smooth galvanized duct, ruling out flexible and corrugated pipe because the ridges drag on the airflow and collect grease. - Smooth metal duct at the full diameter the manual names. - Gradual bends, such as adjustable elbows or two 45-degree turns, rather than sharp corners. - Joints sealed with metal foil duct tape so they stay tight and airtight. - Where two ducts combine, a total cross-section at least equal to both. ### An old duct behind a new hood Replacing a hood with a stronger one is where duct problems appear. The old run may be a size smaller, may be plastic or foil, or may simply end under the roof deck. A bigger blower pushed into it gets louder and barely moves more air. Source: [Vent-A-Hood — Wall Mount Range Hood Installation Instructions](https://www.ventahood.com/installpdfs/wall%20mount%20range%20hood%20installation%20instructions.pdf) — Never reduce the duct size; no flexible or corrugated duct, only smooth galvanized metal; short, straight runs with gradual turns; a 90-degree elbow equals 5 feet of duct; over 20 feet add one inch of diameter per ten feet; combined ducts keep the total area; a louver or roof jack with a gravity damper on every run; no screen wire or spring-loaded doors; do not terminate into an attic or chimney; [Broan — BXT1 Series Range Hood Installation Instructions](https://broan-nutone.com/getmedia/e05acdba-b6be-4e9e-8182-e44db1a97192/Installation-Guide-BXT1.pdf?ext=.pdf) — Shortest run and fewest elbows; metal ductwork only so a grease fire is contained; tape joints with metal duct tape; never vent into an attic, walls, ceilings, crawl spaces or garages; a hood damper and a wall-cap damper can interfere and cause poor airflow or back drafts, in which case remove the hood damper flap Close on the hinged flap of a backdraft damper inside a round metal duct collar on top of a range hood. ## Where the duct has to end Every manufacturer says the same: outdoors. Broan's instructions rule out spaces inside walls or ceilings, attics, crawl spaces and garages, and warn that venting into an attic lets grease build up into a fire hazard. Vent-A-Hood adds chimneys to the list. At the outside end, Vent-A-Hood asks for a wall louver or roof jack on each run with nothing restricting the airflow, and specifically no screen wire or spring-loaded doors, which trap grease and fight the fan. A cap on a side wall should sit where the discharge will not be pulled straight back in: not beside an openable window, under a deck or tight against a soffit vent. On a roof, the jack belongs where the duct can rise without a long horizontal stretch through the attic first, because every foot and every bend in that stretch counts against the blower. Source: [Vent-A-Hood — Wall Mount Range Hood Installation Instructions](https://www.ventahood.com/installpdfs/wall%20mount%20range%20hood%20installation%20instructions.pdf) — Never reduce the duct size; no flexible or corrugated duct, only smooth galvanized metal; short, straight runs with gradual turns; a 90-degree elbow equals 5 feet of duct; over 20 feet add one inch of diameter per ten feet; combined ducts keep the total area; a louver or roof jack with a gravity damper on every run; no screen wire or spring-loaded doors; do not terminate into an attic or chimney; [Broan — BXT1 Series Range Hood Installation Instructions](https://broan-nutone.com/getmedia/e05acdba-b6be-4e9e-8182-e44db1a97192/Installation-Guide-BXT1.pdf?ext=.pdf) — Shortest run and fewest elbows; metal ductwork only so a grease fire is contained; tape joints with metal duct tape; never vent into an attic, walls, ceilings, crawl spaces or garages; a hood damper and a wall-cap damper can interfere and cause poor airflow or back drafts, in which case remove the hood damper flap ## The backdraft damper: a flap with one job A backdraft damper is a light metal flap, sometimes two, in the hood's outlet collar, in the duct, or in the cap outside. When the blower starts, air pressure swings it open; when the blower stops, gravity drops it closed so outdoor air, insects and cold stay outside. Vent-A-Hood requires one at every wall louver or roof jack. Broan's instructions warn about one common mistake: a damper at the hood outlet and a second in a wall cap mounted directly behind it can collide, cutting airflow or causing back drafts. Where that happens, one flap is taken out rather than both left fighting. ### What a damper that is not working feels like - A cold draft falling from the hood on windy or winter days with the fan off. - A flap that taps or bangs in gusts, often loudest at night. - Weak airflow on every speed because the flap only opens partway. - A whistling note at high speed from a flap rubbing its collar. - Insects or leaves appearing on the filters from outside. ### Why flaps stick Grease is the usual cause. A thin coat hardens on the hinge and the flap stays partly open or refuses to lift. A bent flap from an earlier installation, paint on an outdoor cap, or a nest in the louver do the same. Broan's installation steps ask the installer to confirm the damper opens and closes freely before the hood is closed up. Source: [Vent-A-Hood — Wall Mount Range Hood Installation Instructions](https://www.ventahood.com/installpdfs/wall%20mount%20range%20hood%20installation%20instructions.pdf) — Never reduce the duct size; no flexible or corrugated duct, only smooth galvanized metal; short, straight runs with gradual turns; a 90-degree elbow equals 5 feet of duct; over 20 feet add one inch of diameter per ten feet; combined ducts keep the total area; a louver or roof jack with a gravity damper on every run; no screen wire or spring-loaded doors; do not terminate into an attic or chimney; [Broan — BXT1 Series Range Hood Installation Instructions](https://broan-nutone.com/getmedia/e05acdba-b6be-4e9e-8182-e44db1a97192/Installation-Guide-BXT1.pdf?ext=.pdf) — Shortest run and fewest elbows; metal ductwork only so a grease fire is contained; tape joints with metal duct tape; never vent into an attic, walls, ceilings, crawl spaces or garages; a hood damper and a wall-cap damper can interfere and cause poor airflow or back drafts, in which case remove the hood damper flap ## Where to stop Freeing a flap inside the hood means taking out the blower housing or reaching into the chimney cover beside the motor wiring, and that is book-a-visit work, with power off at the panel. So is anything on a roof or a ladder at the outside cap. An owner can look, listen and describe; a duct that ends in an attic or a joint that has come apart is a repair for the next visit, not a weekend. 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: [Vent-A-Hood — Wall Mount Range Hood Installation Instructions](https://www.ventahood.com/installpdfs/wall%20mount%20range%20hood%20installation%20instructions.pdf) · [Broan — BXT1 Series Range Hood Installation Instructions](https://broan-nutone.com/getmedia/e05acdba-b6be-4e9e-8182-e44db1a97192/Installation-Guide-BXT1.pdf?ext=.pdf) · [Broan — BCSD Series Range Hood Installation, Use and Care Manual](https://images.thdstatic.com/catalog/pdfImages/d0/d0c7024e-429d-40bf-b7bb-b697c67105be.pdf) ## Step by step. Four steps, in the order worth doing them. 01 ### Watch the outside cap With the hood on high, check from the ground that the cap's flap opens and air is felt or seen leaving. 02 ### Listen with the fan off On a windy day, note whether anything taps, bangs or lets cold air in at the hood. 03 ### Trace the route From the attic or basement, where the run is visible, look for plastic, foil or crushed duct and for joints that have parted. 04 ### Book the rest Leave the hood's outlet collar, the blower and any work on the roof to a technician. For the heart of your home. ## Questions owners ask. ### Can I use flexible foil duct to make an awkward turn? Vent-A-Hood says not to use flexible or corrugated duct at all, and Broan requires metal duct. Smooth metal with an adjustable elbow makes the same turn with far less resistance and without the ridges that hold grease. ### Why does cold air come down from my hood in winter? Most often a backdraft damper that no longer falls shut, held open by hardened grease, a bent flap or debris in the outside cap. A cap without a damper at all will do the same. ### Is it acceptable to vent the hood into the attic for now? No. Manufacturers forbid it, and Broan's manual explains that grease builds up in the attic into a fire hazard. The duct has to reach a cap outside the house. ### Why is my new, stronger hood no better than the old one? The duct behind it sets the limit. An undersized, long or foil run with several elbows can hold a larger blower to little more than the old one moved. ## 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 ### Cold Draft or a Flapping Damper Through the Range Hood 6 causes, cheapest first 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 ## When it needs a technician. The repairs on the right are where this guide ends when the checks do not settle it. ### 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 noise and vibration repair Finding what makes a hood roar, rattle or shake, from a grease-heavy wheel or loose screws to a flapping damper, and quieting it at the source. ### 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 ### Make-Up Air for a Strong Range Hood: The 400 CFM Line and What It Protects 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. Guides ### All 18 guides Six sections, by what you are trying to find out. --- **[Read on the site: Range Hood Duct Runs and the Backdraft Damper: Size, Elbows and the Cap Outside](https://rangehooderepair.center/guides/duct-runs-and-the-backdraft-damper/)** **[Book a visit with RangeHoodeRepair.center](https://rangehooderepair.center/schedule/)**