17
VENTING (continued)
VERTICALLY VENTED DUCT FURNACES (CATEGORY III)
Observe the following precautions when venting the unit:
1. Use fl ue pipe of the same size as the fl ue connections
on the gas duct furnace (See Table 1). All heaters
should be vented with UL Listed (UL 1738 in United
States, UL S636 in Canada) double wall or single wall
vent pipe, a factory built chimney, or a lined brick and
mor tar chimney that has been constructed in
accordance with the National Building Code.
2. Provide as long a vertical run of fl ue pipe at the duct
furnace as possible. A minimum of 5 feet (1.5m) of
vertical fl ue is required. The top of the vent pipe
should extend at least 2 feet (0.61m) above the
highest point on the roof. Install a weather cap over
the vent opening; cap should be a Breidert Type L or
Fields Starkap vent cap. Consideration should be
made for anticipated snow depth. See Figure 8A.
3. Slope horizontal runs upward from the duct furnace
at least 1/4 inch per foot (21mm/m) minimum.
Horizontal runs should not exceed 75% of the vertical
height of the vent pipe, or chimney, above the fl ue
pipe connections, up to a maximum length of 10 feet
(3m). Horizontal portions of the venting system shall
be supported at minimum intervals of 4 feet (1.2m)
in the United States, and at minimum intervals of
3 feet (1m) in Canada. See Figure 8A.
4. Use as few elbows as possible.
5. Avoid running vent pipe through unheated spaces.
6. When this cannot be avoided, insulate the pipe to
prevent condensation of moisture on the walls of the
pipe.
7. Do not damper the fl ue piping. Failure to open such
damper prior to operating the duct furnace will result
in the spillage of fl ue gas into the occupied space.
8. Avoid installing units in areas under negative pressure
due to large exhaust fans or air conditioning. When
required, a flue vent fan should be installed in
accordance with the instructions included with the
fan.
9. Vent connectors serving Category I and Category II
heaters shall not be connected into any portion of
mechanical draft systems operating under positive
pressure.
