Your Range Hood Is Judged on Capture, Not CFM

The number on the box is the wrong number

Walk into any appliance showroom and the range hood conversation collapses into a single figure: cubic feet per minute. Six hundred CFM sounds twice as serious as three hundred, and a wall of chrome promising a thousand feels like insurance. Yet the laboratories that actually measure what a hood does have spent years quietly demolishing that logic. A hood's job is not to move a large volume of air through the room; it is to capture the plume of grease, moisture and combustion gases rising off the pan before it escapes sideways into the kitchen. Capture, it turns out, depends far more on where the hood sits and how much of the cooktop it covers than on the horsepower of its fan.

What capture efficiency actually measures

The honest metric is capture efficiency: the fraction of pollutants generated at the burner that the hood pulls outside rather than letting drift into the room. It now has a formal test, ASTM E3087, and the results are humbling. Lawrence Berkeley National Laboratory found that residential hoods vary from under 15 percent to more than 98 percent capture — an enormous spread among products whose CFM ratings often look similar. Higher airflow helps, but only within the same geometry; a badly shaped hood at 600 CFM routinely loses to a well-shaped one at 300. The single most decisive variable in study after study is not the fan at all.

Why the back burners do the real work

That variable is burner position. A hot plume rises and spreads as it climbs, so a pot on a back burner sits almost entirely under the hood, while a pan on a front burner sits at the hood's leading edge, its plume curling outward past the filter. The numbers are stark. Standard hoods capture perhaps 30 to 40 percent of what is emitted on the front burners and up to 90 percent on the back. Over-the-range microwaves — the shallow boxes most North American kitchens actually have — manage 40 to 85 percent at the back and drop badly at the front. The practical instruction that falls out of this is almost comically simple: fry and sear on the back burners. No amount of extra CFM rescues a searing pan parked at the front lip of an undersized hood.

Coverage and overhang beat raw suction

Because the plume widens as it rises, the hood needs to be larger than the cooktop, not merely equal to it. Designers who understand capture specify a hood that overhangs the cooking surface, ideally by a few inches on each side, and mount it low enough that the rising column has not yet spread beyond the canopy. A deep, wide canopy set at the manufacturer's minimum height will out-capture a shallow, flush unit running a bigger motor. This is why professional-style cooking demands a professional-depth hood: the extra coverage, not the extra noise, is what earns its keep.

Ducted, or it is just an expensive fan

Then there is the question of where the captured air goes. A recirculating hood pulls the plume through a charcoal filter and blows it straight back into the room. It removes some grease and odour and essentially none of the fine particles, nitrogen dioxide and carbon monoxide that gas cooking produces. Only a hood ducted to the outdoors removes those pollutants from the house. Public-health guidance is blunt on this point: vent outside, or accept that the appliance is redecorating your air rather than cleaning it. A modest ducted hood beats a powerful recirculating one on every measure that matters to health, because a recirculating unit was never designed to remove the gases that do the harm.

The 400 CFM trap

There is a ceiling on the brute-force approach, and it is written into building codes. Once a hood exhausts more than 400 CFM, most codes require dedicated make-up air, because a fan that powerful can depressurise a reasonably airtight house. The danger is not abstract: negative pressure can reverse the draft on a water heater or furnace flue and pull carbon monoxide back into the living space. A 1,200 CFM hood without a matched make-up air system is not twelve hundred CFM of ventilation; it is a source of backdraft risk and, often, a fan starved of the air it was sold to move. The high number on the box quietly becomes a liability.

Designing the hood into the room

The design lesson is that a kitchen's air is won at the canopy, not the motor. Choose a hood sized generously over the cooktop, hung at its rated height, ducted to the outside on a short straight run, and rated for capture rather than sheer flow — then cook on the back burners when the pan is smoking. Do that with a quiet 300 to 400 CFM hood and the room stays cleaner than it would under a roaring thousand. The most effective ventilation is the kind you barely hear, sized to the plume it is meant to catch rather than to the number that sells it.