Of all the appliances in a kitchen, the ventilation hood is the one people judge by the wrong evidence. A hood that is loud feels like a hood that is working, and the two have almost nothing to do with each other. What matters is whether the air actually leaves the house, and that is decided almost entirely by what happens after the air enters the hood.
We get called to hoods in Mandarin more often than the appliance itself deserves, and a large share of those visits end with the same finding: the hood is fine and the duct is not.
The shortcut that causes it
The story is nearly always the same. A kitchen built in the 1970s or 1980s had a modest builder hood over a thirty-inch range, and behind that hood was a duct sized for it. Twenty or thirty years later somebody put in a serious range and a serious hood, and connected the new hood to the duct that was already there, because it was already there.
That old duct may be narrower than the new hood expects. It may take a long route through the attic to reach a roof cap. It may make four or five turns to get there. It may end at a cap whose insect screen has been quietly closing up with lint and grease since the first administration that used it.
A blower rated to move a great deal of air can only move that air if there is somewhere for it to go. Push it down a restricted path and it will still spin at full speed, still make the noise you expect, and shift a fraction of what it should. The owner concludes the hood is failing. The hood is doing exactly what it was built to do.
Elbows are expensive
The single most surprising thing about duct design, for most people, is how much a turn costs. Air has momentum. Making it change direction sharply takes energy out of it, and every ninety-degree elbow in a run is equivalent to adding a substantial length of straight duct.
A hood ducted straight up through a short run to a roof cap will outperform an identical hood ducted across an attic with four elbows by a margin nobody would guess from looking at them. Two shorter runs with gentle bends will beat one short run with tight ones.
This matters in newer parts of the area as much as older ones. Deeper houses put kitchens in the middle of the plan, with no outside wall nearby, and the duct has further to travel and more corners to turn to get anywhere.
The version with no duct at all
Then there is the arrangement that cannot work in principle. The hood was installed venting into the cabinet above it, or through a grille back into the room, with a charcoal filter doing the work.
Recirculating filtration removes some odour. It removes no heat, no moisture and no combustion products, and it stops removing odour entirely once the filter is loaded, which happens faster than anyone replaces them. Over a serious cooking appliance it is not a compromise so much as a decoration.
We say so plainly when we find it, and we understand that it is unwelcome news, because the fix is construction rather than a part.
The damper nobody thinks about
Between hood and duct there is normally a backdraft damper, a light flap that the blower pushes open and gravity closes. It is a simple thing in an unpleasant environment, and in a humid climate the hinge stiffens and the flap becomes tacky with condensed grease.
Stuck shut, it stops the hood dead while everything still sounds perfect. Stuck open, it lets hot damp outside air fall back down into the kitchen whenever the blower is off, which people describe as a draught or a smell rather than as a hood problem. It is a five-minute check and it explains more complaints than its size suggests.
What you can do yourself
Clean the baffle filters far more often than feels necessary. They work by forcing greasy air to change direction so the grease hits metal and runs down, and a loaded baffle stops doing that and starts passing everything through to the blower wheel and the duct beyond.
Then go outside and look at where the duct comes out. If you can see the cap from the ground, look at the screen. If it is grey and matted, that is your airflow, and no amount of work on the appliance will get it back.