11
Installation Specifications
Calculating the Maximum Duct Run Length
• For optimal performance, consult a qualified HVAC
specialist when designing the duct system.
• Do not use duct work that is smaller in cross-sectional
area than the required duct sizes in the tables below.
• Keep the duct run as short as possible for best perfor-
mance and never exceed the maximums stated in the
table below right.
• The maximum duct length for the downdraft vent sys-
tem depends on the blower model used and the num-
ber of elbows and transitions. The
Equivalent Number
of Feet
for each elbow and transition (see table below)
must be subtracted from the maximum straight length
to compensate for wind resistance. To determine the
maximum allowable length of the duct work, subtract all
of the equivalent lengths of the elbows and transitions
listed below from the
Blower Maximum Duct Straight
Length
(see table right).
For example, for a downdraft vent system using 3 ¼ x 10
rectangular duct, two (2) 3 ¼ x 10 90° elbows, a 3 ¼ x 10
rectangular to 10” round transition, and a REMP16 remote
blower:
• From the
Blower Maximum Duct Straight Length
table below, the maximum length without transitions
and elbows is 60 feet.
• The equivalent length of each 90
°
elbow is 15 feet.
• The equivalent length of 45
°
elbow is 2 feet.
• The equivalent length of the transition is 4 feet.
• The total equivalent length of the above components is:
15 feet + 15 feet + 4 feet + 2 feet = 36 feet.
• The maximum amount of straight duct that can be used
with a REMP16 and the above components is: 60 feet -
34 feet = 24 feet.
Equivalent Number of Feet (Nominal) -
Duct Elbows and transitions
45° elbow
8 inch
3 feet
3 ¼ x 10
45° elbow
7 feet
45° elbow
10 inch
2 feet
3 ¼ x 10
90° elbow
15 feet
90° elbow
8 inch
7 feet
3 ¼ x 10
90° flat elbow
20 feet
90° elbow
10 inch
5 feet
3 ¼ x 10
to 8” round
transition
4 feet
90° 3 ¼ x 10
to 8” round
transition
25 feet
3 ¼ x 10
to 10” round
transition
4 feet
Roof cap
*
Wall cap**
*
* The equivalent lengths of roof and wall caps vary with
model and configuration. For equivalent length, contact the
manufacturer or a qualified HVAC specialist.
** Not applicable for REMP series blowers.
Duct Work Design Tips
• Wherever possible, reduce the number of transitions
and turns to as few sharp angles as possible. Two
staggered 45° angles are better than one 90°. Keep
turns as far away from the hood exhaust as possible,
with as much space between each bend as possible.
• For best performance, use round duct instead of rect-
angular when possible, especially when elbows are
required.
• If multiple elbows are used, try to keep a minimum of
24” of straight duct between them. Avoid “S” or “back to
back” configurations of adjacent elbows.
• Do not use flexible metal duct.
Blower
Used
Blower Maximum Duct Straight Length
8 Inch Duct
10 Inch Duct
3 ¼ x 10
Duct
CABP3
40 feet
(12.2 m)
30 feet
(9.2 m)
30 feet
(9.2 m)
ILHSF8
50 feet
(15.2 m)
40 feet
(12.2 m)
40 feet
(12.2 m)
ILHSF10
60 feet
(18.3 m)
70 feet
(21.3 m)
60 feet
(18.3 m)
REMP3
50 feet
(15.2 m)
40 feet
(12.2 m)
40 feet
(12.2 m)
REMP16
60 feet
(18.3 m)
70 feet
(21.3 m)
60 feet
(18.3 m)
