Planning Fans and Ducts

4-40 Banta Place Fair Lawn, NJ 07410 1-877-RCHOODS
 Transition of the duct to the outside should be designed according to the particular construction
situation. Some examples include the following. A wall transition should be covered with a “dryer
vent.” A sloped roof transition requires a cover like a duct with a sloped hat. A flat roof transition
needs a goose neck shape that blows the fumes directly onto the roof.
 No matter what transition the fumes have to pass through before they are released into the atmosphere,
they MUST always pass a back draft damper. This prevents the cold winter air from entering the
kitchen through the ventilation ducts when the hood fan is not running. Units with lighter smaller
louvers work better than those with a large closure.