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Figure 4
CONFINED SPACE / OUTDOOR AIR
FROM ATTIC
Figure 5
CONFINED SPACE / OUTDOOR AIR
FROM ATTIC & CRAWL SPACE
For an attic installation it is important to keep
insulation 12" or more away from any furnace
openings. Some types of insulating materials may
be combustible.
When horizontal ducts are used to supply air from the
outdoors, they must be of the same cross sectional area
as the free area of the openings to which they connect.
The minimum dimension of rectangular air ducts must not
be less than 3 inches. Each opening must have a free
area of not less than one square inch per 2,000 Btu per
hour of total input of all appliances within the "Confined
Space". These openings must be located 12 in. from the
top and bottom of the furnace area. Neither opening can
be blocked at any time (See Figure 6).
Figure 6
CONFINED SPACE / OUTDOOR
AIR THROUGH HORIZONTAL DUCTS
Furnaces installed with combustion air drawn from
a heated space which includes exhaust fans,
fireplaces, or other devices that may produce a
negative pressure should be considered confined
space installations.
20571201 Issue 0527 Page 10 of 28