14
See the venting section for venting guidelines
and specifications.
Air From Inside (See Figure 9)
If combustion air is taken from the heated
space, the two openings must
each have a free
area of at least one square inch per 1,000 Btuh
of total input of all appliances in the confined
space, but
not less than 100 square inches
of free area.
For example, if the combined input
rate of all appliances is less than or equal to
100,000 Btuh, each opening must have a free
area of at least 100 square inches. If the com-
bined input rate of all appliances is 120,000
Btuh, each opening must have a free area of at
least 120 square inches.
Outdoor Air Using Vertical Ducts
(See Figure 10)
If combustion air is taken from outdoors through
vertical ducts, the openings and ducts must
have a minimum free area of one square inch
per 4,000 Btuh of total appliance input. In instal-
lations drawing combustion air from a ventilated
attic, both air ducts must extend above the attic
insulation.
If the unit is installed in an area with an exhaust
fan, provide sufficient ventilation to prevent
negative pressures from occurring in the room.
The combustion air openings must not be re-
stricted in any manner.
! CAUTION:
Do not supply combustion air from an
attic space that is equipped with power
ventilation or any other device that may
produce a negative pressure.
Figure 11. Equipment in a Confined
Space with all Combustion Air Drawn
from the Outside through Exterior Wall
Each openin
g
to out
s
ide
mu
s
t be at lea
s
t
1
s
q. in. per 4000
Btuh of total input
ratin
g
.
12" Max
12" Max
Total Input
Ratin
g
(Btuh)
40,000
60,000
8
0,000
100,000
120,000
140,000
160,000
Minimum
Free Area
(Each Openin
g
)
10
s
q. in.
15
s
q. in.
20
s
q. in.
25
s
q. in.
3
0
s
q. in.
3
5
s
q. in.
40
s
q. in.
Round Duct
Diameter
4"
5"
5"
6"
6"
7"
8
"
-----
----
-----
----
Furnace
Water Heater
Vent or
Chimney
Total Input
Ratin
g
(Btu/hr)
40,000
60,000
8
0,000
100,000
120,000
140,000
160,000
Minimum
Free Area
(Each Openin
g
)
10
s
q. in.
15
s
q. in.
20
s
q. in.
25
s
q. in.
3
0
s
q. in.
3
5
s
q. in.
40
s
q. in.
Round Duct
Diameter
4"
5"
5"
6"
6"
7"
8
"
Inlet Air Duct mu
s
t
be at lea
s
t 1
s
q. in.
per 4,000 Btuh of
total input ratin
g
.
Inlet and Outlet
Duct
s
mu
s
t
extend above
attic in
s
ulation.
Outlet Air Duct mu
s
t
be at lea
s
t 1
s
q. in.
per 4,000 Btuh of
total input ratin
g
.
Ventilation Louver
s
at
each end of attic
Attic
In
s
ulation
12" Max
a. All Combu
s
tion Air from Ventilated Attic.
Furnace
Water Heater
Vent or
Chimney
Figure 10. Equipment in a Confined
Space with all Combustion Air Drawn
from the Outside through Vertical Ducts
Total
Input
Rating (Btu/hr)
40,000
60,000
80,000
100,000
120,000
140,000
160,000
Round Duct
Diameter
12"
12"
12"
12"
13"
14"
15"
Minimum
Free Area
(Each Opening)
100 sq. in.
100 sq. in.
100 sq. in.
100 sq. in.
120 sq. in.
140 sq. in.
160 sq. in.
Furnace
Openings to
adjacent space.
Each opening must
be at least 100 sq. in.
or 1 sq. in. per 1000
Btuh of total input
rating, whichever is
greater. See minimum
area per table.
12" Max.
12" Max.
Water Heater
Vent or
Chimney
Figure 9. Equipment in a Confined
Space with all Combustion
Air Drawn from the Inside
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