Common Venting
The 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 N provides the volume of flue products at
full fire for the calculation of appropriate vent sizing
and extractor 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 XTherm 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 150° F.
* NOTE: Data for 100% firing rate.
Table N: Volume of Flue Products Data
Fig. 24: Typical Common Venting
CAUTION: A listed vent cap terminal suitable for
connection to the Cat IV vent materials, adequately
sized, must be used to evacuate the flue products
from the heaters.
Model
N
o.
Vent Size
(
in.)
Volume of
Flue Products
(CFM)
1005 6 260
1505 8 390
2005 8 520
26
Note: Units rotated 90° for clarity. Typical multi-unit installations have the heaters installed side-by-side. Extractor is required.
