183
Venting for Outdoor Installations — The venting system is
designed for direct discharge of flue gases to the outdoors. The
vent discharge opening should be located to provide an unob
-
structed discharge to the outside and should be located as far from
the combustion air inlet as possible but in the same pressure zone.
The vent duct should pitch down toward the outlet, to ensure that
any condensate that occurs in the vent duct drains away from the
combustion blower fan housing. The duct opening should be pro
-
tected by a
1
/
2
in. x
1
/
2
in. (12 mm x 12 mm) mesh screen. An op
-
tional rainhood may be used over the discharge opening to prevent
wind-driven rain from entering the vent duct, but should not inter
-
sect the flue gas discharge path. See Fig. 145.
Venting For Indoor Installations — All duct furnaces must be
connected to a venting system to convey flue gases outside of the
heated space. Vent systems must be sized and installed in accor
-
dance with ANSI Z223.1 (NFPA 54) or in Canada CAN/CGA -
B149.
There are two methods for venting indoor furnace installations:
vertical venting and horizontal venting.
Vertically Vented Duct Furnaces (Category I)
— Proper vent
-
ing of the heating units is the installer’s responsibility. Vent piping
is supplied by others. When operated with the venting system in
place, proper duct furnace operation must be verified, including
flue gas analysis of each connected furnace.
Use single wall or double wall (Type B) vent pipe of diameters
listed in the following table.
Maximize the height of the vertical run of vent pipe. A minimum
of 5 ft (1.5 m) of vertical pipe is required. The top of the vent pipe
must extend at least 2 ft (0.61 m) above the highest point on the
roof. (Use Listed Type B vent for external runs.) An approved
weatherproof vent cap must be installed to the vent termination.
Horizontal runs must not exceed 75% of the vertical height of the
vent pipe, up to a maximum of 10 ft (3 m). Horizontal runs should
be pitched upward
1
/
4
in. per ft (21 mm/m) and should be support
-
ed at 3 ft (1 m) maximum intervals.
Design vent pipe runs to minimize the use of elbows. Each 90-de
-
gree elbow is equivalent to 5 ft (1.5 m) of straight vent pipe run.
Vent pipe should not be run through unheated spaces. If such runs
cannot be avoided, insulate vent pipe to prevent condensation in
-
side vent pipe. Insulation should be a minimum of
1
/
2
in. (12.7
mm) thick, foil-faced material suitable for temperatures up to
500°F (260°C). Vent connectors serving Category 1 heaters must
not be connected into any portion of a mechanical draft system op
-
erating under positive pressure. See Fig. 146.
IMPORTANT: Outdoor units must be
individually
vented.
INPUT RATING
(BTUH)
INPUT RATING (W)
VENT PIPE
DIAMETER
75,000 - 149,999
21,980 - 43,958
5 in. (126 mm)
150,000 - 400,000
43,960 - 117,228
6 in. (152 mm)
401,000 - 600,000
117,229 - 175,842
7 in. (178 mm)
IMPORTANT: Dampers must
not
be used in vent piping runs.
Spillage of flue gases into the occupied space could result.
COMBUSTION AIR OPENING
FLUE GAS FLOW
UNOBSTRUCTED
AHU
CABINET
VENT HOOD
COMBUSTION
AIR INLET
LOUVER
VENT HOOD
AHU
CABINET
COMB.
BLWR.
Fig. 145 —
Outdoor Horizontal Venting
a39-4395
