8
Total Input
Rating (Btu/hr)
40,000
60,000
80,000
100,000
120,000
140,000
160,000
Minimum
Free Area
(Each Opening)
10 sq. in.
15 sq. in.
20 sq. in.
25 sq. in.
30 sq. in.
35 sq. in.
40 sq. in.
Round Duct
Diameter
4"
5"
5"
6"
6"
7"
8"
Inlet 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.
Outlet Air Duct must
be at least 1 sq. in.
per 4,000 Btuh of
total input rating.
Ventilation Louvers at
each end of attic
Attic
Insulation
12" Max
a. All Combustion Air from Ventilated Attic.
Furnace
Water Heater
Vent or
Chimney
Figure 5. Equipment in a Confined Space with all
Combustion Air Drawn from the Outside
through Vertical Ducts
Air From Inside (See Figure 4)
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
combined input rate of all appliances is 126,000
Btuh, each opening must have a free area of at
least 126 square inches.
combustion air from that space. For example, a
space including a water heater rated at 45,000
Btuh input and a furnace rated at 72,000 Btuh
requires a volume of 5,850 cubic feet [50 x (45 +
72) = 5,850] to be considered unconfined. If the
space has an 8 foot ceiling, the floor area of the
space must be 731 square feet (5,850 / 8 = 731).
In general, a furnace installed in an unconfined
space will not require outside air for combustion.
However, in “tight” buildings (with weather stripping
and caulk to reduce infiltration), it may be
necessary to provide outside air to ensure
adequate combustion and venting, even though
the furnace is located in an unconfined space.
Installation In A Confined Space
A confined space is an area with volume less
than 50 cubic feet per 1,000 Btuh of the
combined input rates of all appliances drawing
combustion air from that space. Furnace
closets, small equipment rooms and garages
are confined spaces. Furnaces installed in a
confined space which supply heated air to
areas outside the space must draw return air
from outside the space and must have the
return air ducts tightly sealed to the furnace. A
confined space must have two openings
into the space for combustion air. One
opening must be within 12 inches of the
ceiling, and the other must be within 12
inches of the floor. The required sizing of
these openings is determined by whether inside
or outside air is used to support combustion,
the method by which the air is brought to the
space, and by the total input rate of all appliances
in the space.
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 4. Equipment in a Confined Space with all
Combustion Air Drawn from the Inside
Figure 6. Equipment in a Confined Space with all
Combustion Air Drawn from the Outside through
Exterior Wall
Each opening
to outside
must be at least
1 sq. in. per 4000
Btuh of total input
rating.
12" Max
12" Max
Total Input
Rating (Btuh)
40,000
60,000
80,000
100,000
120,000
140,000
160,000
Minimum
Free Area
(Each Opening)
10 sq. in.
15 sq. in.
20 sq. in.
25 sq. in.
30 sq. in.
35 sq. in.
40 sq. in.
Round Duct
Diameter
4"
5"
5"
6"
6"
7"
8"
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Furnace
Water Heater
Vent or
Chimney
