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SECTION C: Installation
24
Termination
The vent terminal should be vertical and should termi-
nate outside the building at least 2 ft above the highest
point of the roof within 10 ft. The vent cap should have
a minimum clearance of 4 ft horizontally from and in
no case above or below (unless a 4 ft horizontal dis-
tance is maintained) electric meters, gas meters,
regulators and relief equipment. The distance of the
vent terminal from adjacent public walkways, adjacent
buildings, open windows and building openings must
be consistent with the NFGC (U.S.) or B149.1 (Can-
ada). Gas vents supported only by flashing and
extending above the roof more than 5 ft should be se-
curely braced to withstand snow and wind loads.
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.
Fig. 20: Vertical Venting
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 L provides boiler discharge vent pressures
at vent pressure switch and volumes of flue products at
full fire for the calculation of appropriate vent sizing
for common venting.
WARNING: Vent connectors serving any other appli-
ances 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 LP gas and flue tem-
perature of 300° F.
Model
Vent Size
(in.)
Vent
Pressure
(in. WC)
Volume o
f
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 L: Vent Pressure and Volume of
Flue Products Data