Chemical vapor corrosion of the flue, blower assembly and vent system may occur if air for combustion contains certain chemical vapors. Spray can propellants, cleaning solvents, refrigerator and air conditioner refrigerants, swimming pool chemicals, calcium and sodium chloride, waxes, bleach and process chemicals are typical compounds which are potentially corrosive.
A. ALL AIR FROM INSIDE BUILDINGS: (See Figure 11 and 12)
The confined space shall 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 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 cm2/kW) of the total input rating of all gas utilization equipment in the confined space, but not less than 100 square inches (645 cm2). One opening shall commence within 12 inches (31 cm) of the top and one commencing within 12 inches (31 cm) of the bottom of the enclosures.
FIGURE12.
B. ALL AIR FROM OUTDOORS: (See Figures 13, 14 and 15)
The confined space shall 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 shall 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 shall have a minimum free area of 1 square inch per 4,000 Btu per hour (5.5 cm2/kW) of total input rating of all equipment in the enclosure, see Figure 13.
FIGURE13.
2.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 cm2/kW) of total input rating of all equipment in the enclosure, see Figure 14.
3.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 cm2/kW)) of total input rating of all equipment in the enclosure, see Figure 15.
FIGURE14.
4.When ducts are used, they shall be of the same
FIGURE15.
5.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
6.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.
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