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Outdoor Air Using Vertical Ducts
(See Figure 8)
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 installations 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.
Vent or
Chimney
Water Heater
Total Input
Rating (Btu/hr)
40,000
60,000
80,000
100,000
120,000
140,000
160,000
Figure 7. Equipment in a Confined Space with all
Combustion Air Drawn from the Inside
Vent or
Chimney
Attic
Insulation
Water Heater
a. All Combustion Air from Ventilated Attic.
Minimum
Total Input
Free Area
Rating (Btu/hr)
(Each Opening)
40,000
10 sq. in.
60,000
15 sq. in.
80,000
20 sq. in.
100,000
25 sq. in.
120,000
30 sq. in.
140,000
35 sq. in.
160,000
40 sq. in.
Figure 8. Equipment in a Confined Space with all
Combustion Air Drawn from the Outside
through Vertical Ducts
12
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.
Furnace
12" Max.
Minimum
Free Area
Round Duct
(Each Opening)
Diameter
100 sq. in.
12"
100 sq. in.
12"
100 sq. in.
12"
100 sq. in.
12"
120 sq. in.
13"
140 sq. in.
14"
160 sq. in.
15"
Ventilation Louvers at
each end of attic
Outlet Air Duct must
be at least 1 sq. in.
per 4,000 Btuh of
total input rating.
Inlet and Outlet
Ducts must
extend above
attic insulation.
Inlet Air Duct must
be at least 1 sq. in.
per 4,000 Btuh of
total input rating.
Furnace
12" Max
Round Duct
Diameter
4"
5"
5"
6"
6"
7"
8"
The combustion air openings must not be restricted 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.
Air Directly Through An Exterior Wall
(See Figure 9)
If combustion air is provided directly through an exterior
wall, the two openings must each have free area of at least
one square inch per 4000 Btuh of total appliance input.
Outdoor Air Using a Crawl Space and Ventilated
Attic (See Figure 10)
When directly communicating with the outdoors, each
opening shall have a minimum free area of 1 square inch per
4,000 Btuh of total appliance input. The openings shall
communicate directly, or by ducts, with the outdoor spaces
(crawl or attic) that freely communicate with the outdoors.
Outdoor Air Using Horizontal Ducts (See Figure 11)
If combustion air is taken from outdoors through horizontal
ducts, the openings and ducts must have a minimum free
area of one square inch per 2,000 Btuh of total appliance
input.
12" Max
- - -
- - -
- - -
- - -
- - -
- - -
12" Max
Total Input
Rating (Btuh)
40,000
60,000
80,000
100,000
120,000
140,000
160,000
Figure 9. Equipment in a Confined Space with all
Combustion Air Drawn from the Outside
through Exterior Wall
Vent or
Chimney
Furnace
Water Heater
Minimum
Free Area
Round Duct
(Each Opening)
Diameter
10 sq. in.
4"
15 sq. in.
5"
20 sq. in.
5"
25 sq. in.
6"
30 sq. in.
6"
35 sq. in.
7"
40 sq. in.
8"
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