Range Hoods Affect Indoor Air Quality
Home ventilation is supposed to improve indoor air quality. In another case of more is not necessarily better, high-powered kitchen range hoods could be hazardous to health. Many new fans remove air at a rate of more than 600 CFM (cubic ft. per minute)--and some exceed 1,000 CFM. In comparison typical range hoods are rated at 175 to 250 CFM.
"These high-capacity fans are easily powerful enough to pull exhaust gases out of a fireplace, wood stove, water heater or furnace," says Dave Brook, Extension Energy Agent with the Oregon State University Extension Service. The problem, called backdrafting, exposes occupants to fumes containing carbon monoxide (CO), oxides of nitrogen (NOx) and other pollutants. Prolonged exposure to low levels of odorless CO can cause symptoms similar to flu, and high levels can even cause death. (See April 1994).
"Backdrafting has the potential to be a serious problem in new homes," says Brook. In 1994, the Bonneville Power Administration conducted a survey of new homes without special air sealing in Oregon, Washington and Idaho. The survey showed that 56 percent of the homes could easily have backdrafting problems from typical exhaust fans.
The rising popularity of commercial-style ranges is partly responsible for the increasing air-flow capacity of range fans. More air flow is required to remove the heat from high-output ranges and to make up for the reduced "capture" effectiveness of the hoods that are more stylish, but slimmer than traditional hoods.
Since hot air rises, range exhaust is best accomplished by an overhead hood. "Stove-back" fans that pull from several inches above the burner surface and down-draft fans, including those on indoor grills, require significantly more air flow to be effective.
This schematic from the R-2000 Make-up Air Guidelines shows how air switches can trigger the make-up air fan.
Your best bet is to install a fan that's no bigger than you need. The Home Ventilating Institute (HVI), a fan manufacturers' trade association, recommends range hood capacity of 40 to 50 CFM per linear foot of range, or about 120 to 150 CFM for the standard 30-in. range. To work properly, the range hood should be at least as wide as the stove with an extra three to six inches for good measure. It should be located no more than 20 to 24 inches above the stove top. A 20-in.-deep hood will capture fumes better than the typical 17-in.-deep models. Wall-mounted hoods are generally more effective than free-standing or "island" hoods, because there are fewer air currents to blow fumes away from the hood.
Dealing with Depressurization
To protect against backdrafting, you must be sure to provide a reliable source of make-up air to replace the air that is being exhausted. Otherwise the fan will pull air from the most available source--often a chimney or flue in an adjacent area. This is critical for high-capacity fans, such as stove-back and down-draft models.
Suggesting that occupants "open a window" doesn't qualify. Even if they remember, it's unlikely that occupants would open the window wide enough, especially during extreme weather. According to standards established by the Canadian R-2000 program, a 200-CFM range hood would require a 24-in.-wide window to be raised 5 inches to create about 113-sq.-in. of ventilation area. The Uniform Mechanical Code contains a similar although rarely enforced provision. It requires exhaust fans greater than 350 CFM to have dedicated make-up air. The UMC is vague about how to provide this make-up air.
Canada's national building code requires a separate fan wired to blow outside air into the same space when any exhaust device, including fans and clothes dryers, exceed 160 CFM. This fan-forced make-up air can be delivered into the ductwork of a forced-air heating system or to an adjacent living space that is not blocked by a closable door. In colder climates, preheating the incoming air can eliminate cold drafts.
No range hood manufacturer currently provides an integrated make-up air solution, so the range hood installer has to find a way to activate the supply fan when the exhaust fan starts. A relay can be wired to the exhausting device's switch, but this can be difficult and could void the warranty. Another option is an "air switch" in the exhaust duct that senses when air is flowing. (See diagram.) The best guidance is published in R-2000 Make-up Air Guidelines, which can be purchased for $40 by calling the Canadian Home Builders Association at 613-230-3060.
After installation, it's important to verify that the exhaust fan is not depressurizing chimneys or flues. A pressure measuring device called a "manometer" is the best tool for this job. Contractors specializing in blower door testing and duct sealing will generally have one.
"It's possible to get a rough idea whether backdrafting is occurring by using a stick of incense or a smoking match," says Brook. To perform a simple test, close all interior doors except between the kitchen and combustion appliances. While the fan is running, watch to see if the smoke rises up the flue. Also perform the test while the furnace blower is operating, because unbalanced air flows in ductwork can also contribute to depressurization problems.
With proper sizing, selection and installation, you can make sure that the range hood promotes good indoor air quality instead of making it worse.
This article appeared in Energy Source Builder #53 October 1997,