8
MEASURING AIR VOLUME CONT.
We recommend the following number of access holes depending on duct
size;
• 18” or less – Two access holes 6” apart
• 18”~30” – Five access holes
• 30”~36” – Six access holes
• Greater than 36” – Seven access holes
ACCESS HOLE SPACING
Use the Log-Tchebycheff Rule for rectangular ducts to determine spacing
for access holes.
Number of access
holes per side
Position relative to inner wall
(multiplier)
5
0.074, 0.288, 0.500, 0.712, 0.926
6
0.061, 0.235, 0.437, 0.563, 0.765, 0.939
7
0.019, 0.153, 0.217, 0.361, 0.639, 0.783,
0.847, 0.923, 0.981
Example
– for a 20” inside dimension select 5 access holes.
•
Access 1, 20 x 0.074 = 1.48” from wall (use 1 ½“)
•
Access 2, 20 x 0.288 = 5.76” from wall (use 5 ¾”)
•
Access 3, 20 x 0.500 = 10” from wall (use 10“)
•
Access 4, 20 x 0.712 = 14.24” from wall (use 14 ¼“)
•
Access 5, 20 x 0.926 = 18.52” from wall (use 18 ½“)
SAMPLE CALCULATED LOCATIONS
Access
Holes
5
Access
Holes
6
Access
Holes
7
Width
24
Width
30
Width
36
0.074
1.8
0.061
1.8
0.053
1.9
0.288
6.9
0.235
7.1
0.203
7.3
0.500
12.0
0.437
13.1
0.366
13.2
0.712
17.1
0.563
16.9
0.500
18.0
0.926
22.2
0.765
23.0
0.634
22.8
0.939
28.2
0.797
28.7
0.947
34.1
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