Why A Recirculating Range Hood Moves Grease, Not Air

Two range hoods can look identical and do fundamentally different jobs. One removes the air above a stove from the building, and the other cleans part of it and puts it back in the kitchen.

Cooking produces more than smell

A stove releases water vapor, aerosolized grease, fine particles from hot oil, and, on a gas burner, the products of combustion.

Only some of these are particles. Water vapor and gases pass through any filter designed to catch droplets, because there is nothing physical there to trap.

A hood's usefulness therefore depends on whether the air leaves the building or merely passes through a filter.

A ducted hood exports the whole problem

Ducted installations pull air into a canopy and push it through a duct to an exterior wall or roof, where it leaves along with everything suspended in it.

Heat and humidity go with it, which is why a ducted hood keeps a kitchen noticeably cooler during a long cook and prevents moisture settling on cabinets.

The cost is a hole in the building envelope, a duct run, and the conditioned air being exhausted, which the house has to replace from somewhere.

A recirculating hood is a filter with a fan

Air is drawn through a metal baffle or mesh that condenses grease, then through a charcoal element that adsorbs some odor compounds, and is returned to the room.

Particles and grease are genuinely captured, so the cabinets stay cleaner. Heat, steam and combustion gases return to the kitchen unchanged.

Charcoal has a finite capacity and stops adsorbing once it is loaded, which happens quietly, so a neglected element reduces the hood to a noisy fan.

Capture depends on geometry as much as airflow

Rising plumes spread as they travel, so a hood mounted high needs a much larger canopy and much more airflow to catch the same plume as a lower one.

A canopy narrower than the cooktop lets the front burners escape entirely, which is why the front of a stove is the hardest position for any hood.

Running the fan before cooking and for several minutes afterward matters for the same reason, since the plume continues after the burner is off.

Powerful hoods need makeup air

Air pushed outdoors has to be replaced, and in a tightly built house the replacement is drawn through whatever openings exist.

If the easiest opening is a chimney or a naturally vented appliance flue, the hood can pull combustion products back into the house, which is why building codes require dedicated makeup air above a threshold airflow.

That requirement is the practical reason many kitchens end up recirculating: it is not that filtering is better, it is that ducting a large hood properly is an alteration to the building.