SYSTEM OPERATION
9
9.3.2.1* Standard Method.
The minimum required volume
shall be 50 ft
3
per 1,000/Btu/hour (4.8m
3
/kW).
9.3.2.2* Known Air Infiltration Rate Method.
Where the air
infiltration rate of a structure is known, the minimum required
volume shall be determined as follows:
(1) For appliances other than fan-assisted, calculate using
the following equation:
Required Volume
other
>
21 ft
3
I
other
ACH
1000 Btu/hr
(2) For fan-assisted appliances, calculate using the following
equation:
Required Volume
fan
>
15 ft
3
I
fan
ACH
1000 Btu/hr
where:
I
other
= all appliances other than fan-assisted input in Btu
per hour
I
fan
= fan-assisted appliances input in Btu per hour
ACH
= air change per hour (percent of volume of space
exchanged per hour, expressed as a decimal)
(3) For purposes of this calculation, an infiltration rate great
-
er than 0.60
ACH
shall not be used in the equations in
9.3.2.2(1) and 9.3.2.2(2).
9.3.2.3 Indoor Opening Size and Location.
Openings used
to connect indoor spaces shall be sized and located in accor-
dance with the following:
(1)*
Combining spaces on the same story.
Each opening shall
have a minimum free area of 1 in.
2
/1000Btu/hr (2200
mm
2
/kW) of the total input rating of all appliances in the
space but not less than 100 in.
2
(0.60m
2
). One opening
shall commence within 12 in. (300 mm) of the top, and
one opening shall commence within 12 in. (300 mm) of
the bottom, of the enclosure
[see Figure A.9.3.2.3(1)].
The minimum dimension of air openings shall be not less
than 3 in. (80 mm).
Furnace
Water
Heater
Opening
Chimney or Gas Vent
Opening
NOTE: Each opening must have
a free area of not less than one
square inch per 1000 BTU of
the total input rating of all equip-
ment in the enclosure, but not
less than 100 square inches.
Figure A.9.2.3.3.(1) All Combustion Air from Adjacent
Indoor Spaces through Indoor Combustion Air Openings.
(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.