Rheem OBF Series Installation Instructions Manual page 8

conditions, two openings of
approximately equal area (one located
near the top and one located near the
bottom of the enclosure) must be
provided each with a total free area of not
less than 1 square inch per 4,000 BTU's /
Hr. of total input rating of all appliances in
the enclosure (see figure 5).
FIGURE 5
ALL AIR FROM VENTILATED ATTIC
If horizontal ducts are used, each
opening shall have a free area of not
less than 1 sq. in. per 2,000 BTUH of
total input of all appliances in the
enclosure (see figure 6).
FIGURE 6
DIRECTLY CONNECTING HORIZONTAL
DUCTS TO OUTDOORS
Appliances installed in confined
spaces may be installed with ventilation
air from inside the building and
combustion air from outdoors. Consult
state and local codes and the NFPA
No. 31, Standard for Installation of Oil
Burning Equipment for specific details
(see figure 7 for an example).
Appliances installed in confined
spaces with all required air coming
from the outdoors may also get inlet
air from continuously ventilated crawl
spaces and outlet air to a ventilated
attic (see figure 8).
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FIGURE 7
INSIDE VENTILATION AIR, OUTSIDE
COMBUSTION AIR
"NFPA 31, Standard for Installation of
Oil Burning Equipment" defines
"confined space" and "unconfined
space" as follows.
Confined Space - Any space whose
volume is less than 50 cu. ft. per 1,000
BTUH of the aggregate input rating of
all fuel-burning appliances installed
therein.
Unconfined Space - Any space whose
volume is equal to or greater than 50
cu. ft. per 1,000 BTUH of the aggregate
input rating of all fuel-burning
appliances installed therein. Rooms
connecting directly with the space in
which the appliances are located by
means of openings that have no doors
FIGURE 8
OUTSIDE AIR FROM ATTIC AND CRAWL SPACE
or closures, unless fully louvered, shall
be considered part of the unconfined
space.
When determining if the furnace is
located in a confined or unconfined
space it is important to realize that
some buildings are so tight that normal
infiltration does not meet air
requirements for proper combustion or
venting and outside air must be
introduced.
Important: All applicable codes must
be followed when providing air to the
confined space.