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