Bradford White M-2-TW-75T6SX - Manual (Page 26)

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Installation (Combustion Air Supply) continued-
All Air From Inside the Building: The confined space must be provided with
two permanent openings communicating directly with an additional room(s) of
sufficient volume so that the combined volume of all spaces meets the criteria
for an unconfined space. The total input of all gas utilization equipment
installed in the combined space must be considered in making this
determination. Each opening must have a minimum free area of 1 square inch
per 1000 BTU (6.5 cm
2 /0.29 kw) per hour 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 must be within 12 inches (31 cm) of the top and one
within 12 inches (31 cm) of the bottom of the enclosure.
All Air From Outdoors: The confined space must be provided with two
permanent openings, one commencing within 12 inches (31 cm) of the top and
one commencing within 12 inches (31 cm) from the bottom of the enclosure.
The openings must communicate directly, or by ducts, with the outdoors or
spaces (crawl or attic) that freely communicate with the outdoors.
1. When directly communicating with the outdoors, each opening must have a
minimum free area of 1 square inch per 4000 BTU (6.5 cm 2 /1.2 kw) per
hour of total input rating of all equipment in the enclosure.
2. When communicating with the outdoors through vertical ducts, each
opening must have a minimum free area of 1 square inch per 4000 BTU
(6.5 cm
2 /1.2 kw) per hour of total input rating of all equipment in the
enclosure.
3. When communicating with the outdoors through horizontal ducts, each
opening must have a minimum free area of 1 square inch per 2000 BTU
(6.5 cm
2 /0.6 kw) per hour of total input rating of all equipment in the
enclosure.
4. When ducts are used, 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 be not less than 3 inches (7.5 cm).
Specially Engineered Installations
The requirements noted under CONFINED SPACES above must not
necessarily govern when special engineering, approved by the authority having
jurisdiction, provides an adequate supply of air for combustion, ventilation, and
dilution of flue gases.