Revised 3/27/20 Copyright © 2020 La Cornue Page 14
c.
Duct runs for 600 – 1200 CFM ventilator models should not exceed 35 linear feet with two 90-degree
elbows and two 45-degree elbows, a damper and a roof or wall cap. Longer runs or additional elbows
will result in decreased ventilation performance. Each 90-degree elbow is the equivalent of 6 linear
feet of duct pipe; each 45-degree elbow is equivalent to 3 linear feet of duct pipe.
d.
Always run ventilator ducts to the outdoors. DO NOT terminate a duct into an attic, basement,
garage, crawl space under a house, a chimney, other ducting or an enclosed room.
4.
Dampers
IMPORTANT:
DO NOT USE SCREWS TO ATTACH ANY TYPE OF DAMPER AS THE SCREWS MAY BLOCK THE
DAMPER BLADES.
IMPORTANT:
DO NOT USE MORE THAN ONE DAMPER IN THE VENTILATION SYSTEM.
NOTE
: Many styles of
roof caps and wall caps have built-in dampers. See
Roof Caps and Wall Caps
section for more detail.
Always use carefully crafted, tightly wrapped Aluminum Foil Metal Duct Tape on all connections and physically
view and test the damper blades to make certain they are opening and closing correctly. Make sure that the
damper blades do not touch the duct walls and that there is no debris blocking the free movement of the
damper mechanism. Common things to look for include screws protruding into the blade’s path, overspray of
paint, plaster and insulation. If using rectangular duct, be sure that all four sides of the duct are on the
outside of the damper’s start collar or frame. On ventilator installations designed to recirculate the exhaust
air back into the kitchen, be sure to remove the aluminum damper blade before attaching the duct to the
damper frame.
600 – 1200 CFM Ventilator Models
600-1200 CFM models have either an 8" or 10" round start collar that requires use of a damper with
the corresponding dimension.
Damper in Roof Cap or Wall Cap
In ventilation systems utilizing a roof cap or wall cap
with
a built-in damper, do not install an
additional in-line damper. Two dampers are unnecessary and will cause increased static pressure (air
resistance), that may result in rattling, vibration and air buffeting noises, as well as inadequate
ventilation.
In-line Damper
(for ventilation systems utilizing a roof cap or wall cap
without
a built-in damper)
In cold weather areas, installing an “in-line” damper may be the preferred type of installation. This is
because in-line dampers installed just above the perimeter of the heated space (the ceiling) will
reduce the amount of cold air traveling down the duct into the heated space and into the kitchen
through the ventilator. In-line dampers must be installed so that the exhaust air flow will open the
butterfly blades.
For
horizontal duct runs
, the in-line damper must be installed so that the hinge between the two
butterfly blades is
vertical
—the hinge pin must point up and down. Otherwise, because of gravity,
the damper’s blades will not close and the damper will not prevent backdrafts.
For
vertical duct runs
, the in-line damper’s hinge will be
horizontal
(sideways), which is correct for
vertical duct runs. Gravity will help close the damper blades after each use.
For
upward slanted duct runs
, the in-line damper’s hinge must point to the top and bottom sides of
the duct. In his position, gravity will help close the damper blades after each use. Otherwise, because
of gravity, the damper’s blades will not close and the damper will not prevent backdrafts.
