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a. If result is greater than or equal to 50 ft³ per 1,000
Btuh, space is considered an unconfined space.
b. If result is less than 50 ft³ per 1,000 Btuh, space is
considered a confined space.
4. Determine building type. A building of unusually
tight construction has the following characteristics:
a. Walls and ceiling exposed to outside atmosphere
have a continuous water vapor retarder with a
rating of 1 perm or less with openings gasketed
and sealed, and
b. Weather-stripping has been added on openable
windows and doors, and
c. Caulking or sealants applied in joints around
window and door frames, between sole plates
and floors, between wall-ceiling joints, between
wall panels, at plumbing and electrical
penetrations, and at other openings.
5. For boiler located in an unconfined space in a
building of other than unusually tight construction,
adequate combustion and ventilation air is normally
provided by fresh air infiltration through cracks
around windows and doors.
6. For boiler located in an unconfined space in a
building of unusually tight construction or in a
confined space, provide outdoor air through
permanent opening(s) which communicate directly
or by duct with the outdoors or spaces (crawl or
attic) freely communicating with the outdoors.
Minimum dimension of air opening(s) is 3" (7.6cm).
a. Two permanent openings: Locate one opening
within 12 " (30.5cm) of top of space. Locate
remaining opening within 12" (30.5cm) of bottom
of space. Size each opening per following:
i.
Direct communication with outdoors.
Minimum free area of each opening must be
1 square inch per 4,000 Btu per hour input
of all equipment in space.
ii. Vertical ducts. Minimum free area of each
opening must be 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.
iii. Horizontal ducts. Minimum free area of
each opening must be 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.
b. One permanent opening shall be permitted where
the boiler has clearances of at least 1" (2.5cm)
from the sides and rear and 6" (15.2cm) from the
front. Locate the opening within 12 " (30.5cm) of
top of space. Size opening per following:
i.
Minimum free area of 1 square inch per 3,000
Btu per hour input of all equipment in space.
ii. Free area shall not be less than the sum of
the areas of all vent connectors in the
confined space.
Alternate method for boiler located within confined
space. Use indoor air if two permanent openings
communicate directly with additional space(s) of
sufficient volume such that combined volume of all
spaces meet criteria for unconfined 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
fixed 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.
8. For Specially Engineered Installations. The above
requirements shall be permitted to be waived where
special engineering, consistent with good
engineering practice and approved by the authority
having jurisdiction, provides an adequate supply of
air for combustion, ventilation, and dilution of flue
gases.
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