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
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.
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