SYSTEM OPERATION
15
(3)
Outdoor Opening(s) Size.
The outdoor opening(s) size
shall be calculated in accordance with the following:
(a) The ratio of the interior spaces shall be the available
volume of all communicating spaces divided by the
required volume.
(b) The outdoor size reduction factor shall be 1 minus
the ratio of interior spaces.
(c) The minimum size of outdoor opening(s) shall be the
full size of outdoor opening(s) calculated in accor-
dance with 9.3.3, multiplied by the reduction factor.
The minimum dimension of air openings shall not be
less than 3 in. (80 mm).
9.3.5 Engineered Installations.
Engineered combustion air
installations shall provide an adequate supply of combus-
tion, ventilation, and dilution air and shall be approved by the
authority having jurisdiction.
9.3.6 Mechanical Combustion Air Supply.
Where all com-
bustion air is provided by a mechanical air supply system,
the combustion air shall be supplied form outdoors at the
minimum rate of 0.35 ft
3
/min per 1000 Btu/hr (0.034 m
3
/min
per kW) for all appliances located within the space.
9.3.6.1
Where exhaust fans are installed, additional air shall
be provided to replace the exhausted air.
9.3.6.2
Each of the appliances served shall be interlocked
to the mechanical air supply system to prevent main burner
operation where the mechanical air supply system is not in
operation.
9.3.6.3
Where combustion air is provided by the building’s
mechanical ventilation system, the system shall provide
the specified combustion air rate in addition to the required
ventilation air.
9.3.7 Louvers, Grilles, and Screens.
9.3.7.1 Louvers and Grilles.
The required size of openings
for combustion, ventilation, and dilution air shall be based
on the net free area of each opening. Where the free area
through a design of louver or grille or screen is known, it shall
be used in calculating the size opening required to provide
the free area specified. Where the louver and grille design
and free area are not known, it shall be assumed that wood
louvers will have 25 percent free area, and metal louvers and
grilles will have 75 percent free area. Nonmotorized louvers
and grilles shall be fixed in the open position.
9.3.7.2 Minimum Scree Mesh Size.
Screens shall not be
smaller than 1/4 in. mesh.
9.3.7.3 Motorized Louvers.
Motorized louvers shall be inter-
locked with the appliance so they are proven in the full open
position prior to main burner ignition and during main burner
operation. Means shall be provided to prevent the main burn-
er form igniting should the louver fail to open during burner
startup and to shut down the main burner if the louvers close
during burner operation.
9.3.8 Combustion Air Ducts.
Combustion air ducts shall
comply with 9.3.8.1 through 9.3.8.8.
9.3.8.1
Ducts shall be constructed of galvanized steel or a
material having equivalent corrosion resistance, strength,
and rigidity.
Exception: Within dwellings units, unobstructed stud and joist
spaces shall not be prohibited from conveying combustion air,
provided that not more than one fireblock is removed.
9.3.8.2
Ducts shall terminate in an unobstructed space, al-
lowing free movement of combustion air to the appliances.
9.3.8.3
Ducts shall serve a single space.
9.3.8.4
Ducts shall not serve both upper and lower combus-
tion air openings where both such openings are used. The
separation between ducts servicing upper and lower com-
bustion air openings shall be maintained to the source of
combustion air.
9.3.8.5
Ducts shall not be screened where terminating in an
attic space.
9.3.8.6
Horizontal upper combustion air ducts shall not slope
downward toward the source of combustion air.
9.3.8.7
The remaining space surrounding a chimney liner,
gas vent, special gas vent, or plastic piping installed within a
masonry, metal, or factory built chimney shall not be used to
supply combustion air.
Exception: Direct vent appliances designed for installation in
a solid fuel-burning fireplace where installed in accordance
with the manufacture’s installation instructions.
9.3.8.8
Combustion air intake openings located on the ex
-
terior of the building shall have the lowest side of the com-
bustion air intake openings located at least 12 in. (300 mm)
vertically from the adjoining grade level.
Category I Venting (Vertical Venting)
(80% Furnaces Only)
WARNING
To prevent possible personal injury or death due to asphyx
-
iation, non-condensing gas fired warm air furnaces must be
Category I vented. Do not vent any of these furnaces using
Category III venting.
Category I Venting is venting at a non-positive pressure. A
furnace vented as Category I is considered a fan-assisted
appliance and does not have to be “gas tight”.
NOTE:
Sin-
gle-Stage and Two-Stage gas furnaces with induced draft
blowers draw products of combustion through a heat ex
-
changer allowing in some instances common venting with
natural draft appliances (i.e. water heaters).
All installations must be vented in accordance with National
Fuel Gas Code NFPA 54/ANSI Z223.1 - latest edition. In
Canada, the furnaces must be vented in accordance with
the National Standard of Canada, CAN/CGA B149.1 and
CAN/CGA B149.2 - latest editions and amendments.
