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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.
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. In
all cases, the minimum dimension of any combustion air
opening is 3 inches.
Air From Inside
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. See Example and Figure 1.
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 120,000 Btuh, each opening
must have a free area of at least 120 square inches.
/
Vent or
NOTES:
Each opening must be
at least 100 sq. in. or E
sq. in. per 1,000 Btuh
of total input rating,
whichever is greater.
Openings must start at
no more than 12 inches
from the top and bottom
of the enclosure.
Furnace
/
See _F
Notes
_
_
12" Max.
40,000
60,000
80,000
100,000
120,000
140,000
160,000
100 sq.
100 sq.
100 sq.
100 sq.
120 sq.
140 sq.
160 sq.
In
In
In
In
In
In
In
12inches
12inches
12inches
12inches
13inches
14inches
15inches
Figure 1. Combustion
Air Drawn from Inside
Outdoor Air from a Crawl Space or Vented Attic
When the openings can freely exchange air 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 exchange directly, or by ducts, with
the outdoor spaces (crawl or attic) that freely exchange
with the outdoors. See Figure 2.
Vent or
Chimney
Louvers
(each end of attic)
NOTE:
Air openings
shall
each have a free area of
Furnace
not less than one square
inch per 4,000 Btuh of the
Alternate,
Inlet Air
total input rating of all
Air Inlet
/
equipment in the enclosure.
j
Ventilation Louvers For
Unheated
Crawl Space
Figure 2. Combustion
Air Drawn from a Crawl
Space or Vented Attic
Outdoor Air Using Vertical Ducts
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. Attics or crawl spaces must connect freely with the
outdoors if they are the source of air for combustion and
ventilation. See Figure 3.
Vent or
Ventilation
Louvers
at each end of attic
\
Air Duct must be
sq. in.
per 4,000 Btuh of
total input rating.
urnace
_att__ill
!gl f
_12"
Max
40,000
60,000
80,000
100,000
120,000
140,000
160,000
10 sq. In
15 sq. In
20 sq. In
25 sq. In
30 sq. In
35 sq. In
40 sq. In
4inches
5inches
5inches
6inches
6inches
7inches
8inches
Figure 3. Combustion
Air Drawn from Outside
Through
Vertical Ducts
Outdoor Air Using Horizontal Ducts
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. Ducts must have cross - sectional area at least as
large as the free area of their respective openings to the
furnace space. See Figure 4 (page 8).
7
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