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Duct Leakage Calculations

Design Criteria
Duct System Costs
Duct Construction
Leakage Calculations
A New Problem for Rectangular Duct System
Spiral and Flat Oval Duct
Duct Heat Gain
Noise Control

Duct system air leakage is based on the amount of total duct surface and leakage classes established by ASHRAE/SMACNA research (see Table below).

Round duct has the smallest perimeter per air volume, Flat oval and rectangular duct perimeters are greater in that order, depending on aspect ratio.
Duct
Length
Size
Inches
Round
Sq. Ft.
Oval
Sq. Ft.
Rect
Sq. Ft.
Average
pressure

90 Feet
60 Feet
66 Feet
33 Feet
80 Feet
150 Feet

32 or 60x16
26 or 38x16
26 or 38x16
18 or 20x14
16 or 16x14
14 or 12x14

754
409
449
146
336
550

1037
471
518
164
336
550

1140
540
594
187
400
650

2.5 in. w.g.
2.5 in. w.g.
1.5 in. w.g.
1.5 in. w.g.
2.5 in. w.g.
1.5 in. w.g.

Sq. ft. Totals
@ 2.5 in. w.g.

1499

1844

2080
 

Sq. ft. Totals
@1.5 in. w.g.

1155

1232

1431
 

Total Square Footage

2654

3076

3511
 
ROUND DUCT
Class 6 1499 sq.ft x 11 cfm/1OO sq.ft. = 165 cfm Leakage Class 12 1155 sq.ft. x 15 cfm/100 sq.ft. = 173 cfm Leakage
Total = 338 cfm
Leakage 338 cfm/2O,1O0 cfm = 1.68% Leakage

FLAT OVAL DUCT
Class 6 1844 sq.ft. x 11 cfm/1OO sq.ft. = 203 cfm Leakage Class 12 1232 sq.ft. x 15 cfm/1OO sq.ft. = 185 cfm Leakage
Total = 388 cfm
Leakage 388 cfm/20,100 cfm = 1.93% Leakage

RECTANGULAR DUCT
Class 12 2080 sq.ft. x 22 cfm/100 sq.ft. = 458 cfm Leakage Class 24 1431 sq.ft, x 30 cfm/lOO sq,ft. = 429 cfm Leakage
Total = 887 cfm
Leakage 887 cfm/20,100 cfm = 4.41% Leakage