If the range hood is 70% size of cooktop, shouldn't the smoke from sides of cooktop still get sucked towards middle hence getting sucked inside?
So why people recommend range hood bigger than cooktop?
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The Code requires an overhead exhaust hood to be “at least as wide as the unit and shall extend over the entire unit”. (See ICC M1504) There are exceptions (i.e.: downdraft, etc.) but must be labeled for such use.
In addition, it goes on to say the exhaust duct shall extend to the exterior with a backdraft damper and cannot be plastic.
Interesting side note: Section ICC M1504 says there shall be a minimum clear distance from the cooktop to combustible material or cabinets of 24”, but Section ICC M1901 says the distance shall be a minimum of 30”. Go figure.