This article was originally published in the November/December 1997 issue of Home Energy Magazine. Some formatting inconsistencies may be evident in older archive content.
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Home Energy Magazine Online November/December 1997
Home, Home on the Range Hood
Homeowners and builders may not be aware that these higher-powered fans, as well as clothes dryers and even central vacuum systems, can cause backdrafting problems with fireplaces, wood stoves, fossil-fueled furnaces, boilers, and water heaters unless make-up air is provided (see Backdrafting Causes and Cures, HE May/June '91, p. 30). For example, a 1994 survey by the Bonneville Power Administration of new homes in Oregon, Washington, and Idaho showed that 56% of homes that were not specially air sealed were at risk of backdrafting problems from exhaust fans. A rash of carbon monoxide (CO) alarm events in Chicago led to a study that found that 14% of homes tested had possible depressurization problems. No national study of backdrafting potential has been done.
The way to avoid depressurization problems with a kitchen exhaust fan is to size it correctly (bigger is not better); install it correctly to get the most out of the fan you install; and, if needed, provide dedicated makeup air to replace the exhaust air. Although recirculating range hoods--the lowest-cost option--don't cause depressurization, they remove only a little grease at best.
How Big a Fan Do You Need? Kitchen exhaust is best accomplished by an overhead hood, because hot air rises. A stoveback fan pulls from a wide slot several inches above the burner surface, and is far less effective than a range hood. Least effective of all are barbecue downdraft range fans, since they must overcome natural convection. They are recommended only when most of the cooking consists of grilling, frying, or cooking in low pans. Another option for kitchen ventilation is a wall-mounted fan. Although they are not intended to remove grease, these fans provide overall air exchange for the kitchen, removing smoke and odors when needed. Such fans are common in older houses and typically move about 300 CFM.
Why install a bigger fan than a house needs? It will be noisier and potentially risky. The Home Ventilating Institute, a fan manufacturers trade association, recommends range hood capacity of 40-50 CFM per linear foot of range, or about 120-150 CFM for the standard 30-inch range-- far less than the 600+ CFM that dealers sometimes recommend. To ensure that lower-capacity range hoods are effective, they must be positioned correctly over the stove. Many of the range hood installations shown in architectural magazines look great but may not be effective, since they are located too far above the stove to capture the fumes and heat effectively.Correct Installation Range hoods should be as wide as the stove they are venting and should be located no more than 30 inches above the stove top. Twenty-inch deep hoods will pick up from the front area of the stove much better than the 17-inch models.
Researchers have shown that low-profile designer hoods and range hoods with a shelf for a microwave oven are far less effective at capturing steam and grease, since they may cover less of the surface of the range. The performance of this design doesn't improve much even when hoods were mounted only 15 inches from the top of the range.
Hoods over freestanding kitchen islands are usually installed 27 inches above the range to avoid blocking room sight lines. They must be considerably more powerful to compensate for the extra distance from the stove top. Tests at the University of Minnesota show that island hoods capture pollutants well, since they typically cover the entire range surface.
Maintaining rated air flows requires short duct runs--most manufacturers recommend less than 30 ft total equivalent length. Use the correct size smooth galvanized duct, secure it with screws and hangers, and tape all duct joints with long-life foil tape, such as one with a UL-181 rating.
After the hood is installed, the installer should verify that it is not depressurizing any vented combustion appliances. Any blower door service can provide a worst-case depressurization test of the home using standard measurement techniques. The American Gas Association (AGA) and others have prescribed an uninstrumented, albeit less reliable, version of this backdraft test for years (see Appendix H of the National Fuel Gas Code). With all interior doors closed except those between the kitchen and combustion appliances, the installer holds incense or a smoking splint up to the draft diverter or dilution port of each combustion appliance. He or she then turns on all exhaust devices in the house, starting with the smallest. The test must be continued with the furnace or heat pump blower both on and off, since unbalanced air flows in forced-air ductwork can contribute to depressurization problems.Providing Makeup Air If the test reveals depressurization, the installer should open a window within the depressurized zone until the backdrafting stops. If it requires only a little bit of opening (perhaps 10 to 20 in2), a passive duct with an automatic damper will supply adequate dedicated makeup air. The Canadian R-2000 program Make-Up Air Guidelines gives excellent instructions on providing makeup air. According to this manual, a passive duct for a 200 CFM fan would require a duct 10 inches in diameter; a 300 CFM fan would need a 12-inch diameter duct. Downdraft range fans, which can exceed 600 CFM, aren't even listed in the guidelines.
If it takes more than 20 in2 of opening area to alleviate the backdrafting, you probably need a fan-forced makeup air system. The R-2000 Make-up Air Guidelines provide a detailed prescription for installing a separate fan wired to blow outside air into the space exhausted by large fans. This fan-forced makeup air can be introduced into any adjacent space not blocked by a closable door, or into the return side of forced-air ductwork. In colder climates, partially preheating the incoming air is recommended practice; it is required in Canada.
The National Building Code of Canada requires only that the makeup air reduce the air flow difference to 160 CFM and does not require that all major exhaust devices be interlocked to the makeup air fan. The range hood installer can install either a sail switch in the duct or a relay to control both the exhaust and a makeup air fan. The Uniform Mechanical Code, widely used in the United States, acknowledges the possible need for makeup air for devices over 350 CFM, but it is rarely applied in residential situations.
--Dave BrookDave Brook is an energy agent at the Oregon State University Extension Service in Portland, Oregon.
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