9
Duct Length
Ducting design is in terms of maximum length of
straight metal duct. To ensure proper air movement,
limit fittings, other than straight, to 50% of total duct
length. Maximum allowable calculated duct length is
60 equivalent feet.
• Use “Duct Length Piece Equivalents” section to
calculate duct length.
•Flexible duct is not recommended because it can
create a fire hazard. However, if it is used, one foot of
flexible duct is equal to two feet of smooth metal
duct.
1'
3'
10'
5'
5'
1' of Duct 1'
90° Elbow 5'
3' of Duct 3'
90° Elbow 5'
10' of Duct 10'
Total Feet 24'
Typical Duct System
Duct Length Piece Equivalents3¼ x 10 inch ducting pieces
3¼ x 10-inch to 6 inch
transition =
5 equivalent foot
3¼ x 10-inch to 6 inch
transition = 4½
equivalent feet
3¼ x 10-inch flat elbow =
12 equivalent feet
3¼ x 10-inch 90° elbow =
5 equivalent feet
3¼ x 10-inch wall cap
(Unrestricted) = 0
equivalent feet