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Calculating Duct Length Table
For maximum efficiency, use the shortest and straightest duct possible. The system will operate most efficiently when
the ductwork does not exceed 60 feet of equivalent duct. The downdraft will operate properly with ductwork up to 100
equivalent feet of duct.
Calculations are approximate and based on HVAC industry standards.
DUCT PIECES EQUIVALENT NUMBER EQUIVALENT
LENGTH X USED = LENGTH
1 Ft. (0.3m) Ft. or m
6"
(15.2cm)
Round
Straight **
1.5 Ft. (0.45m) Ft. or m
6"
(15.2cm)
Round Metal Flex
No Bends **
(_ " (15.2cm) 10 Ft. (3m) Ft. or m
90° Elbow
(_ 6" (15.2cm)
45° Elbow 5 Ft. (1.5m) Ft. or m
31/4" x 10" (8.2cm x 25Acm)
Straight ** 1 Ft. (0.3m) Ft. or m
[_ 10 Ft. (3m) Ft. or m
31/4" 10"
(8.2cm 25Acm)x x
90° Elbow
31/4" x 10" (8.2cm x 25.4cm)45° Elbow 5 Ft. (1.5m) Ft. or m
31/4'' x 10" (8.2cm x 25.4cm) 10 Ft. (3m) Ft. or m
90° Flat Elbow
31/4'' x 10" (8.2cm x 25.4cm) to 6"
(15.2cm) Round Transition 90 ° Elbow 30 Ft. (9m) Ft. or m
6" (15.2cm) Round to 31/4'' x 10"
(8.2cm x 25.4cm) Transition 30 Ft. (9m) Ft. or m
90° Elbow
31/4'' x 10" (8.2cm x 25.4cm) to 6"
(15.2cm) Round Transition 5 Ft. (1.5m) Ft. or m
6" (15.2cm) Round to 31/4'' x 10"
(8.2cm x 25.4cm) Transition 5 Ft. (1.5m) Ft. or m
(_ 6" (15.2cm) Round Wall Cap with
Damper 30 Ft. (9m) Ft. or m
[_ 31/4'' x 10" (8.2cm x 25.4cm) Wall
Cap with Damper 30 Ft. (9m) Ft. or m
> 6" (15.2cm) Round Roof Cap 30 Ft. (9m) Ft. or m
TOTAL DUCTWORK Ft. or m
** For Straight Round / Rectangular Duct, measure actual linear feet
used and then multiply by Equivalent Length shown.
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