Navien NHB Series Installation & Operation Manual page 76

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
Every 90° elbow used is equivalent to 8 linear feet
Note
(2.4 m) of vent length.
The pipe branch that connects the individual
appliance to the common vent trunk must be
within 20 equivalent feet.
76 Installing a Common Vent System
D
W
Exhaust
Intake
Common Vent Length Table [Total Length (L) =
W + H]
Vent Length for NFB Boiler Units
Required
Model
Load
NFB-301C
(Total BTU/h)
602,000 2
903,000 3
1,204,000 4
Required
Model
Load
NFB-399C
(Total BTU/h)
798,000 2
1,197,000 3
1,596,000 4
Total Length (ft)
D=4" D=6" D=8"
71 160
42 96 150
35 80 142
Total Length (ft)
D=4" D=6" D=8"
53 120
35 80 142
26 60 106