Navien NFB Series Installation & Operation Manual page 70

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.
H
End cap
Common Vent Length Table [Total Length (L) =
W + H]
Vent Length for NFB Boiler Units
Model
Required Load
(Total BTU/H)
NFB-175/200
399,800
599,700
799,600
999,500
1,199,400
1,393,300
1,599,200
Every 90° elbow used is equivalent to 8 linear feet (2.4
Note
m) of vent length.
70 Installing a Common Vent System
Exhaust
D
W
Intake
Siphon loop
Drain hose
Drain
Total Length (ft)
D=3" D=4" D=6"
2 60 106 200
3 40 71 160
4 30 53 120
5 96
6 80
7 68
8 60
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.
The illustration is intended for reference purposes
Note
only.
D=8"
150
142
121
106