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 filters, and cleaning them is easier. Grease typically drains into a trough or cup, which must be emptied and cleaned.
The Vent-A-Hood brand avoids all filters by way of its centrifugal-blower design. Rather than a fan blade, the VAH uses a squirrel-cage blower that slings grease onto the walls of the blower housing. The grease drains into a pan below the blowers, where it is removed by dismantling and cleaning the housing and pan. Disassembly of these parts is easy without tools, and they can be placed into the dishwasher for cleaning.
Without filters, VAH claims significantly increased performance and reduced noise levels. The manufacturer typically claims about a 50% more-effective CFM performance when compared to other hoods. (Thus, a 600CFM VAH would perform like filtered 900CFM hoods.) The noise level is reduced because the air doesn't pass through mesh filters or baffles.
Make-up air: When a hood draws air from the room, it creates a lowered air pressure in that room (and the house). In a tightly-built house, the hood performance will suffer as it struggles to draw air out. In some cases, this suction can cause air to be drawn into the house through a furnace flue or fireplace chimney. This can cause carbon monoxide to be drawn back into the house---a hazard.
The solution is to provide make-up air, which can be as simple as opening a kitchen window. In temperate climates, this solution works well. In cold winters or hot summers, however, admitting unconditioned air is generally not desirable. Ona alternative solution is to install a special intake duct near the range: outside air is drawn in near the range, where it's immediately exhausted by the hood. This minimizes the effect on the room environment. (In winter, cold air is probably a good thing near the hot range!)
A more complicated solution involves a fresh-air duct connected to the furnace air-return plenum. The fresh air passes through the HVAC system, where it gets heated or cooled before ending up in the house. Some systems even have an electrically-controlled damper on the fresh-air duct that opens when the hood is turned on.
Older, "loose," homes simply draw in outside air through every door and window frame. Make-up air will still prevent the intake of dust and drafts from everywhere in the house.
Hints: Turn on the hood before you realize it's needed; by then, you've let smoke escape into the room, where the hood cannot capture and remove it.
Clean filters periodically. A dirty filter limits airflow. As the filter clogs, you must turn up the blower speed, increasing noise.
[added to Appliances FAQ: 21 May 2003]