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 — 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 — 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 — 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 — 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 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 — 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 — 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 — 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 — 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
