Where hood extraction actually goes

When a hood stops clearing smoke, the instinct is to blame the blower. In our experience the blower is somewhere near last on the list. Airflow in a ventilation system is lost cumulatively, and it is lost in a fairly reliable order.

One: the baffle filters

Stainless baffle filters work by making air change direction sharply so grease separates out and drains. They are designed to be washed, and they are designed to be washed often. A baffle whose channels are half full of hardened grease still looks acceptable from below, because you are seeing the polished face, not the channel.

Take one down and look at it edge-on in good light. If the slots are narrowed, that is your first loss and it is free to fix. Wash them in hot water with a degreasing detergent, or run them in the dishwasher if the manufacturer’s instructions for your hood allow it, and dry them fully before refitting — on the coast, a filter refitted wet holds a damp film against the frame all night.

Two: the duct run

The duct is the part nobody can see and everybody assumes is fine. Losses live in flexible sections that have been crushed by insulation or a stored box in the attic, in transitions that were reduced in diameter at installation, in long lateral runs with more elbows than the design intended, and in joints that have separated so the hood is partly extracting into the attic rather than outside.

If the hood was fine for years and slowly got worse, a duct that has been disturbed is a strong candidate — particularly in a house that has had attic work done.

Three: the backdraft damper and the wall or roof cap

There is a damper somewhere in the run, and often a second set of flaps at the termination. Their whole job is to open under airflow and close when the fan stops. Grease stiffens the hinges, and salt air seizes them. A damper that opens halfway costs a large fraction of the system’s capacity.

A rattle in the wind is a useful diagnostic: it usually means the termination flaps are loose or the damper is not seating. Both are worth dealing with, since a damper that will not close lets humid outside air into the duct all day, and on the coast that air is carrying salt.

Four: make-up air

A powerful hood in a well-sealed modern house has to draw replacement air from somewhere. If the house is tight, and windows are shut, and the air conditioning is running, the fan spins at full speed and moves very little air. This presents exactly as a weak blower and is not one.

The quick check: run the hood on high with everything closed, then crack a window in the same room. If extraction visibly improves, the system is starved and the answer is a make-up air path, not a motor.

Five: the blower

Only now is the motor worth suspecting, and it usually announces itself by sound rather than by weakness. Louder without a change in pitch is imbalance, often grease loading on one side of the wheel. A whine or a growl is a bearing. Running at high but buzzing at low is the speed control rather than the motor.

Remote and inline blowers deserve a specific mention here. They are mounted in attics, on exterior walls or on roofs, which moves the noise out of the kitchen and moves the machine out of sight. In a coastal house that puts an aluminium and steel assembly in humid, salt-carrying air with no protection. We have opened remote blower housings that were structurally poor while the kitchen end of the system looked new.

A note on running time

Switch the hood on before you start cooking and leave it running for ten minutes after you stop. Grease and moisture that are captured while airborne never land on the housing, the filters or the cabinet fronts. The habit of switching the hood off with the burner is responsible for a large share of the grease loading described above, and grease loading is where the airflow went.