Retrotec Residential Pressure & Air Leakage - Manual (Page 66)

Page 66 of 75
©Retrotec Inc. 2014
CFM
50
= CFM
P
∗ (
50
P
)
n
Typically for ducts, the n value is 0.60, and for houses the n value is 0.65. For the flow at 25 Pa, replace “50”
with “25.”
Rather than using the equation in the situation where the 50 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 50 Pa factor” from Table 23 to get an estimate of the flow
that would result at 50 Pa.
For example, if you read 1000 CFM at a building pressure of 30 Pa, the "Cannot reach 50 factor" from Table 23 is
1.39. To get the estimated flow at 50 Pa, make the following calculation:
1000 CFM × 1.39 = 1390 CFM
Thus the estimated result for CFM at 50 Pa will be 1390 CFM.
The factors depend on the “n” value chosen, and Table 23 is for “n” of 0.65 which is typical for houses.
Table 23: Factors for when a building pressure of 50 Pa cannot be reached (“n” value of 0.65)
Pressure
achieved
Cannot reach
50 factor
Pressure
achieved
Cannot reach
50 factor
50
1.00 25 1.57
49
1.01 24 1.61
48
1.03 23 1.66
47
1.04 22 1.71
46
1.06 21 1.76
45
1.07 20 1.81
44
1.09 19 1.88
43
1.10 18 1.94
42
1.12 17 2.02
41
1.14 16 2.10
40
1.16 15 2.19
39
1.18 14 2.29
38
1.20 13 2.40
37
1.22 12 2.53
36
1.24 11 2.68
35
1.26 10 2.85
34
1.28 9 3.05
33
1.31 8 3.29
32
1.34 7 3.59
31
1.36 6 3.97
30
1.39 5 4.47
29
1.42
28
1.46