Retrotec Residential Pressure & Air Leakage - Manual (Page 67)

Page 67 of 75
27
1.49
26
1.53
Cannot Reach 25 Pa Factors for ducts
Rather than using the equation in the situation where the 25 Pa test pressure cannot be achieved, you can use
tables with the factors already calculated and the following procedure. Take a flow (CFM) reading at the
pressure achieved and multiply by the "Cannot reach 25 Pa factor” from Table 24 to get an estimate of the flow
that would result at 25 Pa.
For example, if you read 600 CFM at a building pressure of 19 Pa, the "Cannot reach 25 factor" from Table 24 is
1.18. To get the estimated flow at 25 Pa, make the following calculation:
600 CFM × 1.18 = 708 CFM
Thus the estimated result for CFM at 25 Pa will be 708 CFM.
The factors depend on the “n” value chosen, and Table 24 is for “n” of 0.60 which is typical for ducts.
Table 24: Factors for when a building pressure of 25 Pa cannot be reached (“n” value of 0.6).
Pressure
achieved
Cannot reach
25 factor
25
1.00
24
1.02
23
1.05
22
1.08
21
1.11
20
1.14
19
1.18
18
1.22
17
1.26
16
1.31
15
1.36
14
1.42
13
1.48
12
1.55
11
1.64
10
1.73
9
1.85
8
1.98
7
2.15
6
2.35
5
2.63