Catalog CFM vs face velocity at the pans
A higher CFM number suggests more air movement through the hood’s opening, but “more air somewhere” is not the same as “clean air at the cooktop.” Capture—the hood’s ability to pull the rising plume into the inlet—depends on geometry and turbulence as much as on bulk flow. If you have not read it yet, capture and plume geometry is the companion piece to this article.
Face velocity is only meaningful relative to where the contaminated air actually is. A hood that is too high, too shallow, or misaligned with front burners can leave the highest concentrations outside the high-velocity zone—no matter what the brochure says. For layout specifics, see hood width, depth, and capture area.
Where the duct steals performance
Flexible duct, tight-radius elbows, reducers, long horizontal runs, and undersized diameter all raise static pressure. The fan responds by moving less air along its operating curve; you may hear more noise while seeing less improvement at the cooktop. That is why installers often insist on short, smooth paths with rigid metal where code allows—especially in ducted exhaust systems that are supposed to move contaminants outdoors rather than through filters indoors.
If your hood feels weak, the duct run—not the motor alone—is often the first place to investigate. In recirculating setups, clogged grease layers and saturated charcoal packs raise resistance too; our guide on charcoal filter life indicators explains why “replace when the light says so” can be optimistic.
Rule-of-thumb formulas—and their blind spots
You may encounter heuristics such as a certain CFM per linear foot of hood width, or multipliers for gas versus electric cooking. These shortcuts help start a conversation; they are not substitutes for ceiling height, island geometry, burner placement, or local code. In tightly sealed homes, large exhaust rates also raise questions about makeup air and depressurization, which can interact with fireplaces, fuel-burning equipment, and whole-house drafts.
Treat rules of thumb as triage, not verdicts. The useful question for a professional is not only “Is this hood big enough?” but “Will this hood, with this duct path and this house leakage, behave safely and comfortably at the speeds we will actually use?”
Competing air movers in the same home
Kitchen hoods do not operate in a vacuum. Bath fans, clothes dryers, central return paths, and windy facades can all shift pressure and flow patterns. Open windows near the kitchen can help or hurt: sometimes they relieve depressurization; sometimes they steer plumes away from the hood inlet. There is no universal prescription—only site-aware testing and observation.
Noise as a hidden derating factor
Manufacturers publish sones at given speeds, but human tolerance matters more than charts. If the highest speed is rarely used because it is unpleasant, your effective ventilation rate is whatever people actually select—often the middle speeds. That behavioral reality ties directly to grease loading and maintenance cadence discussed in filters, grease, noise, and fire sense.