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26
C
ommon Venting
T
he NFGC does not address sizing guidelines for the
common venting of multiple Category IV heaters. This
is covered in the NFGC under “Engineered Vent Sys-
tems”. Table M provides boiler discharge vent
p
ressures at vent pressure switch and volumes of flue
products at full fire for the calculation of appropriate
vent and extractor sizing for common venting.
WARNING: Vent connectors serving any other
appliances shall not be connected into any portion of
mechanical draft systems operating under a positive
pressure. If an MVB heater is installed to replace an
existing heater, the vent system MUST be verified to
be of the correct size and of Category IV AL29-4C
vent material construction. If it is NOT, it MUST be
replaced.
NOTE: For extractor sizing, typical CO
2
levels are
8.5% for natural gas and 9.5% for propane gas and
flue temperature of 300° F.
Model
Vent
Size (in.)
Vent
Pressure
(in. WC)
V
olume of
Flue Products
(CFM)
503 0.1 160
753 0.2 240
1003
6
0.3 320
1253 0.2 400
1503 0.3 480
1753 0.4 560
2003
8
0.5 640
* NOTE: Data for 100% firing rate.
Table M: Typical Vent Pressure and Volume
of Flue Products
Fig. 24: Typical Common Venting