State SBS65 65NE - Manual (Page 9)

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combined space shall be considered in making this determination. Each
opening shall have a minimum free area of one square inch per 1,000 Btu
per hour (22 cm
2 /kW) of the total input rating of all gas utilization equipment
in the confined space, but not less than 100 square inches (645 cm 2 ). One
opening shall commence within 12 inches (30 cm) of the top and one
commencing within 12 inches (30 cm) of the bottom of the enclosures.
FIGURE 5.
B. ALL AIR FROM OUTDOORS: (See Figures 6, 7 and 8)
The confined space shall be provided with two permanent openings,
one commencing within 12 inches (30 cm) of the top and one
commencing within 12 inches (30 cm) from the bottom of the enclosure.
The openings shall communicate directly, or by ducts, with the outdoors
or spaces (crawl or attic) that freely communicate with the outdoors.
• When directly communicating with the outdoors, each opening
shall have a minimum free area of 1 square inch per 4,000 Btu
per hour (5.5 cm
2 /kW) of total input rating of all equipment in the
enclosure, see Figure 6.
FIGURE 6.
• When communicating with the outdoors through vertical ducts,
each opening shall have a minimum free area of 1 square inch
per 4,000 Btu per hour (5.5 cm
2 /kW) of total input rating of all
equipment in the enclosure, see Figure 8.
• When communicating with the outdoors through horizontal ducts,
each opening shall have a minimum free area of 1 square inch
per 2,000 Btu per hour (11 cm
2 /kW)) of total input rating of all
equipment in the enclosure, see Figure 9.
• Alternatively a single permanent opening may be used when
communicating directly with the outdoors, or with spaces that freely
communicate with the outdoors. The opening shall have a minimum
free area of 1 square inch per 3,000 BTU per hour (8.3 cm
2 /kW)
of total input rating of all equipment in enclosure. See Figure 7.
FIGURE 7.
FIGURE 8.
• When ducts are used, they shall be of the same cross-sectional
area as the free area of the openings to which they connect.
The minimum short side dimension of rectangular air ducts shall
not be less than 3 inches (76.2 mm), see Figure 9.
FIGURE 9.
• Louvers and Grilles: In calculating free area, consideration shall be
given to the blocking effect of louvers, grilles or screens protecting
openings. Screens used shall not be smaller than 1/4 inch (6.4 mm)
mesh. If the free area through a design of louver or grille is known, it
should be used in calculating the size opening required to provide the
free area specified. If the design and free area is not known, it may be
assumed that wood louvers will be 20-25 percent free area and metal
louvers and grilles will have 60-75 percent free area. Louvers and
grilles shall be fixed in the open position or interlocked with the equipment
so that they are opened automatically during equipment operation.
• Special Conditions Created by Mechanical Exhausting or Fireplaces:
operation of exhaust fans, ventilation systems, clothes dryers or
fireplaces may create conditions requiring special attention to avoid
unsatisfactory operation of installed gas utilization equipment.