Navien NFC-175 Installation & Operation Manual page 73

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
End cap
Siphon loop
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.
Model
Required
Load
NFC-
(Total BTU/H)
175/200
399,800
599,700
799,600
999,500 1
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.
Exhaust
D
Intake
Drain hose
Drain
Total Length (ft)
NPE-
D=3" D=4" D=6" D=8"
240
1 60 106 200
2 40 71 160
3 30 53 120
4 96
5 80
6 68
7 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 1 combi-boiler (NFC
Model) and 7 Navien water heaters (NPE Model).
The illustration is intended for reference purposes
Note
●
only.
The maximum allowable vent length of the pipe
●
branch connecting the boiler to the common vent
trunk is 20 feet.
150
142
121
106
Installing a Common Vent System 73