Page 10
Installer’s Information Manual
ALL COMBUSTION AND VENTILATION AIR FROM
OUTDOORS
The furnace space must be provided with two permanent
air openings communicating directly, or by ducts, with the
outdoors or spaces that freely access the outdoors. Well-
ventilated attics or crawl spaces usually satisfy this
requirement. These openings will give the furnace free
access to fresh air for combustion and ventilation.
You must provide air sufficient for all gas appliances within
furnace space. Locate one combustion and ventilation air
opening within 12 inches of top of furnace space. Locate
another within 12 inches of bottom of furnace space.
Ducts that supply air from outdoors must have the same
cross-sectional area as the free area of openings to which
they connect.
When directly communicating with the outdoors, each
opening must have a minimum free area of one square inch
per 4,000 Btu per hour of total combined input rating of all
gas appliances within furnace space. For example: A
100,000 Btu per hour furnace requires two openings of at
least 100,000 ÷ 4,000 or 25 square inches. See Figure 12
and Table 3.
Figure 12. All Air Directly From Spaces That Freely Access
The Outdoors.
Table 3. Free Opening Area Based On One Square Inch
Per 4000 Btu/hr.
Total input
Btu/hr
Required Free
Area Per
Opening
(square
inches)
Minimum
Round Pipe
(inches dia.)
Maximum
Btu/hr Input
For Given
Pipe Diameter
40,000 10
4
50,000
60,000 15
5
78,000
80,000 20
6 113,000
100,000 25
6
113,000
120,000 30
7
153,000
140,000 35
7
153,000
160,000 40
8
201,000
180,000 45
8
201,000
When communicating with the outdoors through vertical
ducts, such openings and ducts must have a minimum free
area of one square inch per 4,000 Btu per hour of total
combined input rating of all gas appliances within furnace
space. See Figure 13 and Table 3.
Figure 13. All Air Through Vertical Ducts From Spaces
That Freely Access The Outdoors.
When communicating with the outdoors through horizontal
ducts, such openings and ducts must have a minimum free
area of one square inch per 2,000 Btu per hour of total
combined input rating of all gas appliances within furnace
space. See Figure 14 and Table 4.
Figure 14. All Air From The Outdoors Through Horizontal
Ducts.
