W415-0973 / B / 08.28.13
17
IOM
8.1.3 CASE 3: FURNACE LOCATED IN A CONFINED SPACE, OUTDOOR AIR FROM
ATTIC OR CRAWL SPACE
H13.3
FIGURE 5 - OUTSIDE AIR FOR COMBUSTION, ATTIC OR
CRAWL SPACE (CASE 3)
Alternate Combustion Air
from Attic. (12” / 305 mm
above oor)
In this circumstance, the free area of each of the two combustion and ventilation air openings is based on a
minimum of 1 square inch (645mm²) per 4000 Btu/hr. In this confi guration, one opening can originate from the
fl oor drawing combustion and ventilation air from the ventilated crawl space.
The other opening may communicate freely with the ventilated attic. If using the attic air, ensure that the
opening is ducted from the ceiling high enough to be above the insulation. The attic must be adequately
vented with soffi t vents or gable vents (Figure 5).
As an alternative to creating an opening in the fl oor to draw air from a crawl space, a duct may be dropped from
the attic terminating 12” (305mm) above the fl oor.
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.
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.
TABLE 2 - VERTICAL AIR SUPPLY (CASE 3)
Input Free Area
Round Pipe Size
K/Btu/hr Ea. Opening
45
15 in.² (97cm²) 5 in. (127mm)
60
80 20 in.² (129cm²)
6 in. (152mm)
100 25 in.² (16cm²)
120 30 in.² (194cm²) 7 in. (178mm)
H12.16.3
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