A Homeowner's Guide to Kitchen Vent Hoods

Beautiful young housekeeper cleaning the kitchen.
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Range hoods remove the grease, smoke, heat and moisture produced by cooking.

Kitchen vent hoods, or range hoods, have always been valuable appliances, but with the advent of energy-efficient, tightly constructed houses they have become even more important in kitchen design. They are the only reliable way to deal with the byproducts of cooking: heat, smoke, moisture, gases and airborne grease. There are many styles of range hoods, but they all fall into one of two categories: ducted and non-ducted (recirculating).

Ducted hoods use a fan to pull cooking byproducts from the kitchen to the outdoors through a wall or roof-mounted vent. Hoods that are vented into the attic or basement can lead to structural damage. This configuration is sometimes seen, although it is now forbidden by most building codes. The fan itself can be located in the range hood, which is the most common type; at the exterior vent; or within the ductwork. Remote fans tend to be quieter than those mounted in the hood.

Non-ducted hoods capture the products and then circulate them through charcoal filters to remove odors and grease and then recirculate the air back into the kitchen. They are not able to remove moisture or heat.

Range Hood Designs

Range hoods come in several designs. Except for downdraft units, where styles may be limited, you can find range hoods that match any kitchen style, from traditional to ultramodern.

  • Under-cabinet range hoods. Under-cabinet range hoods take up the least amount of space, and they are usually mounted to small cabinets above the range or cooktop. The cabinet usually hides the ducts that service the range hood.
  • Wall-mounted range hoods. To ventilate the cooking area while minimizing the visual interference, a wall-mounted hood is the way to go. The range hood is part of a chimney that hides the ducts.
  • Island range hoods. Like wall-mounted units, island range hoods feature a chimney that is suspended from the ceiling.
  • Downdraft range hoods. Unlike the other types, downdraft hoods remain out of sight until they are needed. When operating, the fan unit rises up a few inches above the cooktop behind the burner area to provide ventilation.

Sizing a Range Hood

When someone is cooking, the contaminants rise up and are captured by the hood with help from the hood fan. To do this job effectively, the range hood should be as large as the cooktop. Some experts say that one that extends a few inches to each side of the cooktop does a better job of capturing the contaminants.

Range hood fans are measured by the amount of air they can move in cubic feet per minute (CFM). According to the Home Ventilating Institute, the fan should be able to move 100 CFM for every linear foot of cooktop when the range is against a wall; 150 CFM for island cooktops. So, for example, a 36-inch range that is against a wall needs a 300 CFM fan.

Your requirements may vary from that. For example, systems that have unusually long duct runs may require a more powerful fan. Restaurant-style ranges with high BTU outputs from the burners may also require more powerful fans. One calculation method for ranges over 60,000 total BTUs is to add up the output from all of the burners and then drop the last two zeros—a 65,000 BTU range requires a 650 CFM fan if you are using this calculation method.

Installing cooker hood.
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When installing a range hood, follow the manufacturer's recommendations for the correct height above the cooktop.

No matter what size fan the hood contains, the hood should be installed within the specified minimum/maximum distances from the cooktop. And some maintenance is required. Ducted hoods have aluminum filters that trap grease. They need to be washed periodically. The filters on non-ducted hoods should be changed based on the manufacturer's recommendations.

Code Requirements

Some building codes require a range hood as part of new kitchen construction or major remodeling. The International Residential Code (IRC) requires that a makeup air system be installed with any range hood above 400 CFM. Here's the reasoning: When a fan exhausts air to the outside, new air must replace that air that is vented. In the past, leaky, drafty houses easily supplied the air from outside. But because today's tightly constructed houses limit outside air, a powerful fan could cause backdrafting, where the fan actually pulls in exhaust air from flues and chimneys, such as the flue for a gas water heater or fireplace.

To prevent this kind of dangerous backdrafting, a makeup air system can be installed to supply fresh outside air when the range hood fan is turned on. There are different systems available. Some are wall vents that are connected to the controls of the range hood, and many contain heating elements to temper the outside air before it enters the house.

The IRC is a model code that local jurisdictions use to develop their codes. A makeup air system is not required everywhere, but anyone installing a powerful range hood should seriously consider supplementing it with a makeup air system.