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Installer’s Information Manual Page 13
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.
Figure 10. All Air Directly From Spaces That Freely Access
The Outdoors.
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
108,000 Btu per hour furnace requires two openings of at
least 108,000 ÷ 4,000 or 27 square inches. See Figure 10
and Table 3.
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
DIAMETER
(inches)
MAXIMUM
INPUT FOR
GIVEN PIPE
DIAMETER
(Btu/hr)
54,000 13.5 5 78,000
72,000 18 5 78,000
90,000 22.5 6 113,000
108,000 27 6 113,000
126,000 31.5 7 153,000
144,000 36 7 153,000
When communicating with the outdoors through vertical
ducts, 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 11 and Table 3.
Figure 11. All Air Through Vertical Ducts From Spaces
That Freely Access The Outdoors.
Figure 12. All Air From The Outdoors Through Horizontal
Ducts.
When communicating with the outdoors through horizontal
ducts, 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 12 and Table 4.
Table 4. Free Opening Area Based On One Square Inch
Per 2000 Btu/hr.
TOTAL
INPUT
(Btu/hr)
REQUIRED
FREE
AREA PER
OPENING
(square inches)
MINIMUM
ROUND
PIPE
DIAMETER.
(inches)
MAXIMUM
INPUT FOR
GIVEN PIPE
DIAMETER
(Btu/hr)
54,000 27 6 56,000
72,000 36 7 76,000
90,000 45 8 100,000
108,000 54 9 127,000
126,000 63 9 127,000
144,000 72 10 157,000
Example: Furnace is 90,000 Btu per hour input and is to be
installed in a confined space that contains no other gas
appliances. Rectangular combustion and ventilation air
ducts will run horizontally from outdoors to furnace space.