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Figure 3: Clearance to Combustible Materials
5. For boiler located in an
unconned space in a
building of other than unusually tight construction,
adequate combustion and ventilation air is normally
provided by fresh air inltration through cracks
around windows and doors.
6. For boiler located within
unconned space in
building of unusually tight construction or within
conned space, provide outdoor air through two
permanent openings which communicate directly or
by duct with the outdoors or spaces (crawl or attic)
freely communicating with the outdoors. Locate one
opening within 12 inches of top of space. Locate
remaining opening within 12 inches of bottom of
space. Minimum dimension of air opening is 3
inches. Size each opening per following:
a. Direct communication with outdoors. Minimum
free area of 1 square inch per 4,000 Btu per hour
input of all equipment in space.
b. Vertical ducts. Minimum free area of 1 square
inch per 4,000 Btu per hour input of all
equipment in space. Duct cross-sectional area
shall be same as opening free area.
c. Horizontal ducts. Minimum free area of 1 square
inch per 2,000 Btu per hour input of all
equipment in space. Duct cross-sectional area
shall be same as opening free area.
Alternate method for boiler located within conned
space. Use indoor air if two permanent openings
communicate directly with additional space(s) of
sufcient volume such that combined volume of all
spaces meet criteria for unconned space. Size each
opening for minimum free area of 1 square inch per
1,000 Btu per hour input of all equipment in spaces,
but not less than 100 square inches.
7. Ventilation Duct Louvers and Grilles. Equip outside
openings with louvers to prevent entrance of rain
and snow, and screens to prevent entrance of insects
and rodents. Louvers and grilles must be xed in
open position or interlocked with equipment to open
automatically before burner operation. Screens must
not be smaller than ¼ inch mesh.
Consider the blocking effect of louvers, grilles and
screens when calculating the opening size to provide
the required free area. If free area of louver or grille
is not known, assume wood louvers have 20-25
percent free area and metal louvers and grilles have
60-75 percent free area.
H. Do not install boiler where gasoline or other ammable
vapors or liquids, or sources of hydrocarbons (i.e.
bleaches, cleaners, chemicals, sprays, paint removers,
fabric softeners, etc.) are used or stored.