Overview of vent hoods
[Information first posted in the Appliances Forum (transferred from the Kitchen Forum) by Rick (Rick_Auricchio) on 7 November 2002 - thanks, Rick!]:
Noise: A sone is a measure of sound level used by appliance manufacturers. One sone is roughly equivalent to the noise level of a refrigerator. Fewer is better. Two things make up the noise in a hood: the greater component is the airflow, which makes noise whenever it must change direction. This happens in the filters, around the blower blades, and in the ductwork. The minor component is motor hum. Thus an external blower will not appreciably cut the noise level: you still have all the airflow.
Ducting: The terms ducted and vented appear to mean the same thing, that the air is carried outdoors. A hood that recirculates air back to the room will not remove moisture, heat, or some odors, making it of little use when compared to a hood that vents to the outdoors.
You should always check the manufacturer's specs to ensure that you install ductwork of the proper size. (An oversized duct is generally not a problem, but an undersized one will severely limit airflow.) In many cases, you can install a transition to convert the round duct exit on the hood to a rectangular duct that will fit inside a wall between the studs.
Airflow is measured in cubic feet per minute (CFM), where larger numbers mean more air movement. The rule-of-thumb suggests 10CFM per 1000BTU of burner capacity. Thus, for a 60K-BTU cooktop (4 x 15K), you should have a 600CFM hood. Since you rarely, if ever, have all burners on full at once, you have excess capacity that can be used when you really need it. Most hoods have variable-speed controls, which allow you to choose the airflow---and noise level---appropriate to the task.
If you have an indoor char-grill, however, you should double the CFM rating, because grills generate a lot of smoke.
Hood size: If possible, the hood should extend three inches to each side of the range; a 36" range should get a 42" hood. This overhang allows for better capture of the smoke, which spreads as it rises. Typical mounting height for a hood is 30" above the cooktop; if you mount the hood higher, you should definitely use an oversized hood. Island hoods are generally specified with an overhang, because the airflow patterns around the island tend to blow the smoke around where it can miss the hood.
Filters: The most common filter is the metal mesh filter, which looks like a cross between a screen and a scouring pad. This filter effectively traps grease, but cleaning requires a little effort. Many filters can be cleaned in the dishwasher, but you often need to find room to fit them in with a normal load. If you forget to plan the dish load to accommodate the filters, it's easy to defer cleaning for too long a period.
Baffle filters are metal plates with slots that cause the air to change direction as it passes through the slots. These are quieter than mesh...