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The following table shows minimum free areas and round pipe sizes when drawing combustion air vertically
from the attic or crawl space for the furnace
only. If other fuel burning appliances are
present, their combustion air and ventilation
air requirements must be added to those of
the furnace.
TABLE 2 - VERTICAL AIR SUPPLY (CASE 3)
Input
Free Area
K/Btu/hr
Ea. Opening
40
15 in.² (97cm²)
60
80
20 in.² (129cm²)
100
25 in.² (161cm²)
120
30 in.² (194cm²)
IMPORTANT:
IF THE ATTIC HAS AN EXHAUST FAN (POWER VENT), IT MAY CREATE A NEGATIVE
PRESSURE SUFFICIENTLY LARGE ENOUGH TO PREVENT THE ATTIC FROM BEING
AN EFFECTIVE SOURCE OF COMBUSTION AND VENTILATION AIR. POWERED ATTIC
FANS DO NOT CUSTOMARILY RUN DURING THE HEATING SEASON; HOWEVER,
SOME ARE CONTROLLED BY A HUMIDISTAT AS WELL AS A THERMOSTAT, WHICH MAY
ALLOW SOME OPERATION DURING THE HEATING SEASON. THE CHOICES ARE (A)
USE THE DIRECT VENT OPTION; (B) OBTAIN OUTDOOR AIR FROM ELSEWHERE; OR
(C) INTERLOCK THE ATTIC EXHAUST FAN WITH THE FURNACE SUCH THAT THE TWO
CANNOT OPERATE SIMULTANEOUSLY.
8.1.4 CASE 4: FURNACE LOCATED IN A CONFINED SPACE, OUTDOOR AIR DUCTED
HORIZONTALLY
Similar to Case 3, outdoor air for combustion and ventilation may be drawn through horizontal ducting. The
free area for each opening is calculated on the basis of a minimum of 1 square inch (645mm²) per 2000 Btu/hr
input.
The following table shows minimum free areas and round pipe sizes when drawing combustion air horizontally
from the outdoors for the furnace only. If other fuel burning appliances are present, their combustion air and
ventilation air requirements must be added to those of the furnace.
FIGURE 6 - OUTDOOR AIR FOR COMBUSTION,
HORIZONTAL (CASE 4)
Combustion Air
Duct Terminates
12" (305mm)
above floor.
FIGURE 5 - OUTSIDE AIR FOR COMBUSTION, ATTIC OR
Round Pipe Size
5 in. (127mm)
6 in. (152mm)
7 in. (178mm)
H12.16.7A
CRAWL SPACE (CASE 3)
Alternate Combustion Air
from Attic. (12" / 305 mm
above floor)
H13.3B
TABLE 3 - HORIZONTAL AIR
SUPPLY (CASE 4)
Input
Free Area
K/Btu/hr
Ea. Opening
40
(194cm
60
80
(258cm
100
(323cm
120
(387cm
19
IOM
Round Pipe
Size
30 in
7 in
2
2
)
(178mm)
40 in
2
2
)
8 in
(203mm)
50 in
2
)
2
60 in
9 in
2
2
)
(229mm)
H12.9.7A
W415-2308 / B / 11.04.19
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