70 Installing a Common Vent System
9.1.2 Determining the Length of a Common
Vent System
Follow the instructions listed below to determine the length of a
common vent.
1. Add the BTU/H input ratings for each unit in the cascading
system to determine the total BTU/H rating.
2. Determine the total length (L) of the common vent, which
consists of the horizontal width (W) and the vertical height (H):
Total length (L) = W + H.
Exhaust
D
W
H
Intake
Siphon loop
Drain hose
End cap
Drain
Common Vent Length Table [Total Length (L) =
W + H]
Vent Length for NFB Boiler Units
Required Load
(Total BTU/H)
Model
Total Length (ft)
NFB-175/200 D=3” D=4” D=6” D=8”
399,800
2
60
106
200
599,700
3
40
71
160
799,600
4
30
53
120
999,500
5
96
150
1,199,400
6
80
142
1,393,300
7
68
121
1,599,200
8
60
106
Note
Every 90° elbow used is equivalent to 8 linear feet (2.4
m) of vent length.
Example of a Typical Installation (Direct Vent Only)
The following illustration depicts an example of a common vent
system installed for a cascade system of 8 Navien NFB boiler units.
Note
The illustration is intended for reference purposes
only.
