SYSTEM OPERATION
14
(2)
Combining spaces in different stories.
The volumes of
spaces in different stories shall be considered as com
-
municating spaces where such spaces are connected
by one or more openings in doors or floors having a total
minimum free area of 2 in.
2
/1000 Btu/hr (4400 mm
2
/kW)
of total input rating of all appliances.
9.3.3 Outdoor Combustion Air.
Outdoor combustion air shall
be provided through opening(s) to the outdoors in accordance
with the methods in 9.3.3.1 or 9.3.3.2. The minimum dimen-
sion of air openings shall not be less than 3 in. (80 mm).
9.3.3.1 Two Permanent Openings Method.
Two permanent
openings, one commencing within 12 in. (300 mm) of the top
and one commencing within 12 in. (300 mm) of the bottom, of
the enclosure shall be provided. The openings shall commu-
nicate directly, or by ducts, with the outdoors or spaces that
freely communicate with the outdoors, as follows:
(1)*Where directly communicating with the outdoors or
where communicating to the outdoors through vertical
ducts, each opening shall have a minimum free area of
1 in.
2
/4000 Btu/hr (550 min
2
/kW) of total input rating of all
appliances in the enclosure.
[See Figure A.9.3.3.1(1)(a)
and Figure A.9.3.3.1(1)(b).]
Furnace
Water
Heater
Outlet Air
Chimney or Gas Vent
NOTE: The inlet and outlet air
openings must each have a free
area of not less than one square
inch per 4000 BTU of
the
total input rating of all equipment
in the enclosure.
Inlet Air
Ventilation louvers for
unheated crawl space
Alternate
air inlet
Ventilation louvers
(each end of attic)
Figure A.9.3.3.1(1)(a) All Combustion Air From Outdoors - Inlet Air
from Ventilated Crawl Space and Outlet Air to Ventilated Attic.
Furnace
Water
Heater
Outlet Air
Chimney or Gas Vent
NOTE: The inlet and outlet air
openings must each have a free
area of not less than one square
inch per 4000 BTU of
the
total input rating of all equipment
in the enclosure.
Inlet air duct
[ends 1 ft (300 mm)
above floor]
Ventilation louvers
(each end of attic)
Figure A.9.3.3.1(1)(b) All Combustion Air
From Outdoors through Ventilated Attic.
(2)*Where communicating with the outdoors through horizon-
tal ducts, each opening shall have a minimum free area
of 1 in.
2
/2000 Btu/hr (1100 min
2
/kW) of total input rating of
all appliances in the enclosure.
[See Figure A.9.3.3.1(2).]
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 equipm
ent 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 perma-
nent 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.
