Rheem GF901U Series Installation Instructions Manual page 21

BTUH Free Area
Input Each Opening
45,000 100 square inches
60,000 100 square inches
75,000 100 square inches
90,000 100 square inches
105,000 105 square inches
120,000 120 square inches
B. USING OUTDOOR AIR FOR
COMBUSTION
IMPORTANT: Do not take air from
an attic space that is equipped
with power ventilation.
The confined space must
communicate with the outdoors in
accordance with Methods 1 or 2.
The minimum dimension of air
openings shall not be less than 3
inches. Where ducts are used,
they shall be of the same cross-
sectional area as the free area of
the openings to which they
connect.
Method 1
Two permanent openings, one
located within 12 inches of the top
and one located within 12 inches
of the bottom of the enclosure,
shall be provided. The openings
shall communicate directly, or by
ducts, with the outdoors or spaces
(crawl or attic) that freely
communicate with the outdoors.
a. Where directly communicating
with the outdoors or where
communicating to the outdoors
FIGURE 12
AIR FROM ATTIC/CRAWL SPACE
OUTLET AIR
IN ATTIC
MUST BE
ABOVE
INSULATION
1 SQ. INCH PER
4000 BTUH
OUTLET AIR
FURNACE
GAS
WATER
HEATER
1 SQ. INCH PER
4000 BTUH INLET AIR
FIGURE 11
AIR FROM HEATED SPACE
through vertical ducts as shown in
Figure 12, each opening shall
have a minimum free area of 1
square inch for each 4,000 BTUH
of total appliance input rating in the
enclosure.
BTUH
Input Each Opening
45,000 11.25 square inches
60,000 15.00 square inches
75,000 18.75 square inches
90,000 22.50 square inches
105,000 26.25 square inches
120,000 30.00 square inches
GABLE
VENT
VENTILATED
ATTIC GABLE OR
SOFFIT VENTS
12" MAX.
FURNACE
GAS
WATER
HEATER
Free Area Round Pipe
Size
4"
5"
5"
6"
6"
6"
FIGURE 13
OUTSIDE AIR USING A HORIZONTAL DUCT
GAS
WATER
HEATER
NOTE:
EACH OPENING SHALL
HAVE A FREE AREA OF
NOT LESS THAN ONE
SQUARE INCH PER 1,000
BTU PER HOUR OF THE
TOTAL INPUT RATING OF
ALL EQUIPMENT IN THE
ENCLOSURE, BUT NOT
LESS THAN 100 SQUARE
INCHES.
b. Where communicating with
outdoors through horizontal ducts,
each opening shall have a
minimum free area of 1 square
inch for each 2,000 BTUH of total
input rating of all equipment in the
enclosure (See Figure 13).
Here are some typical sizes:
BTUH Free Area
Input Each Opening
45,000 22.50 square inches
60,000 30.00 square inches
75,000 37.50 square inches
90,000 45.00 square inches
105,000 52.50 square inches
120,000 60.00 square inches
OUTLET AIR 1 SQ. INCH
PER 2000 BTUH
FURNACE
INLET AIR 1 SQ. INCH
12"
PER 2000 BTUH
MAX.
Round Pipe
Size
6"
6"
7"
8"
8"
9"
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