David McNulty uses hoods with an airflow of 600 cfm for standard cooktops but upgrades to a 1,200-cfm hood for commercial-style ranges. He cautions that hoods over 900 cfm exert a powerful pull that can cause problems with air quality. In Minnesota, if designers specify 1,200-cfm hoods, they must provide another opening to create positive pressure to prevent back draft.
Another cause of back draft is incorrect ductwork. McNulty says some installers use 6-inch ducts that can't handle 600-cfm-and-higher fans. Anything over 600 cfm, he says, requires an 8-inch duct. He gives his subcontractors detailed plans that include duct size and location.
Larry Bucher of Don Crowell Inc., Austin, Texas, installed an 1,100-cfm blower in a kitchen that was open to the living room. The fireplace flue didn't work at full capacity if the fire wasn't roaring, and if the homeowners turned on the blower, the vent would suck smoke into the room. "It was a three-speed fan. We disconnected the high speed so they would never mistakenly use it and the fireplace at the same time," says the project manager. He says powerful vents can also suck conditioned air out of the kitchen area, meaning the A/C unit can't keep the room cool.
Bucher will increase the cfms depending on several factors: if the homeowners have a grill, if the hood is installed above the standard height from the cooktop, or if he adds an architectural detail that requires a small capture area. Ann Arbor, Mich., also has strict local ventilation codes. Designer Mary DeTar says newer homes have fresh-air makeup on the furnaces. But for older homes that don't have fresh-air makeup capabilities, building inspectors nix the use of 1,300-cfm hoods. "Altering the furnace could add $1,000 to the price of the job," DeTar says.
Air Quality Do's and Don'ts
* DO shelter the cooking area to prevent drafts.
* DON'T position the cooking area in or near a source of strong air currents, like windows, patio doors, or garage entrances.
* DO increase the cfms of the hood to compensate for higher mounting heights or when there are greater cooking demands.
* DON'T assume that a higher-cfm hood will cure all. Most likely, you'll also need increased capture area or modified hood design.
* DO use ductwork that is at least as large as the exhaust outlet of the hood.
* DON'T ignore the importance of the ductwork system. The majority of hood performance problems are directly related to poor ducting systems.
* DO keep ductwork as short and straight as possible. Use cylindrical ductwork where possible, because curves have lower losses than angles.
* DON'T lay out ductwork so two 90-degree elbows occur within 2 feet of each other.
--List courtesy Broan-NuTone