INSTALLATION CONSIDERATIONS
14
Furnace
Water
Heater
Chimney or Gas Vent
NOTE: The air duct openings
must have a free area of not
less than one square inch per
2000 BTU of the total input
rating of all equipment in the
enclosure*.
Outlet air duct
Inlet air duct
Figure A.9.3.3.1(2) All Combustion Air From Outdoors
through Horizontal Ducts.
9.3.3.2* One Permanent Opening Method.
One permanent openings,
commencing within 12 in. (300 mm) of the top of the enclosure, shall
be provided. The appliance shall have clearances of at least 1 in. (25
mm) from the sides and back and 6 in. (150 mm) from the front of the
appliance. The opening shall directly communicate with the outdoors
or shall communicate through a vertical or horizontal duct to the out-
doors or spaces that freely communicate with the outdoors
(see Figure
A.9.3.3.2)
and shall have a minimum free area of the following:
(1) 1 in.
2
/3000 Btu/hr (700 mm
2
per kW) of the total input rating of all
appliances located in the enclosure, and
(2) Not less than the sum of the areas of all vent connectors in the space.
Furnace
Water
Heater
Opening
Chimney or Gas Vent
NOTE: The single opening must have
a free area of not less than one
square inch per 3000 BTU of
the total input rating of all equip-
ment in the enclosure, but not less than
the sum of the areas of all vent
connectors in the confined space.
Alternate
Opening
Location
Figure A.9.3.3.2 All Combustion Air
From Outdoors through Single Combustion Air Opening.
9.3.4 Combination Indoor and Outdoor Combustion Air.
The use
of a combination of indoor and outdoor combustion air shall be in
accordance with (1) through (3)
(see example calculation in Annex J]:
(1)
Indoor Openings:
Where used, openings connecting the interior
spaces shall comply with 9.3.2.3.
(2)
Outdoor Opening(s) Location.
Outdoor opening(s) shall be located
in accordance with 9.3.3.
(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 accordance 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 installa-
tions shall provide an adequate supply of combustion, ventilation, and
dilution air and shall be approved by the authority having jurisdiction.
9.3.6 Mechanical Combustion Air Supply.
Where all combustion 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 me-
chanical 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 com-
bustion, 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 open-
ing 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 interlocked
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 burner 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, allowing 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 combustion air open-
