73
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 Lengths for an NFC Boiler with up to 7 NPE Water
Heaters
The following table provides information on the required load
amount and the total length for one NFC boiler with up to 7 NPE
water heaters installed.
Required
Load
(Total BTU/H)
Model
Total Length (ft)
NFC-
175/200
NPE-
240
D=3” D=4” D=6” D=8”
399,800
1
1
60
106
200
599,700
2
40
71
160
799,600
3
30
53
120
999,500
4
96
150
1,199,400
5
80
142
1,393,300
6
68
121
1,599,200
7
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 1 combi-boiler (NFC
Model) and 7 Navien water heaters (NPE Model).
Note
●
The illustration is intended for reference purposes
only.
●
The maximum allowable vent length of the pipe
branch connecting the boiler to the common vent
trunk is 20 feet.
Summary of Contents for NFC-175
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