SYSTEM OPERATION
12
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 outdoors 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 calcu-
lated 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, mul
-
tiplied 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 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 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 combus-
tion, 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 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 openings
where both such openings are used. The separation between ducts servicing
upper and lower combustion air openings shall be maintained to the source
of combustion air.
