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2. When communicating with the outdoors through vertical
ducts, each opening shall have a minimum free area of
1 sq. in. (6.5 cm
2
) per 4,000 Btu/h of total input rating of
all equipment in the enclosure.
Equipment Located in Confined Spaces: All Air from
Outdoors Through Ventilated Attic. See 5.3.3-b.
NOTE: The inlet and outlet air openings must each have a free
area of not less than 1 sq. in. (6.5 cm
input rating of all equipment in the enclosure.
A
F
A. Chimney or gas vent
B. Ventilation louvers
(each end of attic)
C. Outlet air
3. When communicating with the outdoors through
horizontal ducts, each opening shall have a minimum free
area of 1 sq. in. (6.5 cm
rating of all equipment in the enclosure.
Equipment Located in Confined Spaces: All Air from
Outdoors. See 5.3.3-b.
NOTES:
The air duct openings must each have a free area of not less
2
than 1 sq. in. (6.5 cm
) per 2,000 Btu of the total input rating
of all equipment in the enclosure.
If the appliance room is located against an outside wall and
the air openings communicate directly with the outdoors,
each opening shall have a free area of not less than 1 sq. in.
2
(6.5 cm
) per 4,000 Btu/h of the total input rating of all
appliances in the enclosure.
A
E
A. Chimney or gas vent
B. Outlet air duct
C. Inlet air duct
10
2
) per 4,000 Btu of the total
B
C
E
D
D. Inlet air duct (ends 12"
[30.5 cm] above floor)
E. Water heater
F. Furnace
2
) per 2,000 Btu/h of total input
B
D
C
D. Water heater
E. Furnace
4. When ducts are used, they shall be of the same cross-
sectional area as the free area of the openings to which
they connect. The minimum dimension of rectangular air
ducts shall not be less than 3" (7.6 cm).
Equipment Located in Confined Spaces: All Air from
Outdoors—Single Air Opening. See 5.3.3-b.
NOTE: The single opening must each have a free area of not less
than 1 sq. in. (6.5 cm
2
) per 3,000 Btu of the total input rating of all
equipment in the enclosure, but not less than the sum of the
areas of all vent connectors in the confined space.
A
E
A. Chimney or gas vent
B. Opening
C. Alternate opening location
5. When directly communicating with the outdoors, the
single opening shall have a minimum free area of 1 sq. in.
(6.5 cm
2
) per 3,000 Btu/h of total input rating of all
equipment in the enclosure.
5.3.4 Specially Engineered Installations:
The requirements of 5.3.3 shall not necessarily govern when
special engineering, approved by the authority having
jurisdiction, provides an adequate supply of air for combustion,
ventilation, and dilution of flue gases.
5.3.5 Louvers and Grilles:
In calculating free area in 5.3.3, consideration shall be given to
the blocking effect of louvers, grilles or screens protecting
openings. Screens used shall not be smaller than ¹⁄₄" (6.4 mm)
mesh. If the area through a design of louver or grille is known, it
should be used in calculating the size of opening required to
provide the free area specified. If the design and free area is not
known, it may be assumed that wood louvers will have 20 to
25 percent free area and metal louvers and grilles will have 60 to
75 percent free area. Louvers and grilles shall be fixed in the open
position or interlocked with the equipment so that they are
opened automatically during equipment operation.
5.3.6 Special Conditions Created by Mechanical
Exhausting or Fireplaces:
Operation of exhaust fans, ventilation systems, clothes dryers or
fireplaces may create conditions requiring special attention to
avoid unsatisfactory operation of installed gas utilization
equipment. Air from Inside Building. See 5.3.3-a.
B
D
C
D. Water heater
E. Furnace
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