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035-17477-001 Rev. A (801)
Air Source from Inside the Building
Two permanent openings, one within 12 inches of the top of
the confined space and one within 12 inches of the bottom,
shall each have a free area of not less than one square inch
per 1,000 Btuh of total input rating of all appliances located in
the space. The openings shall communicate freely with inte-
rior areas having adequate infiltration from the outside.
NOTE: At least 100 square inches free area shall be used for
each opening.
Air Source from Outdoors
1.
Two permanent openings, one within 12 inches of the
top of the confined space and one within 12 inches of the
bottom, shall communicate directly, or by means of
ducts, with the outdoors or to such crawl or attic spaces
that freely communicate with the outdoors.
a.
Vertical Ducts - Each opening must have a free
area of not less than one square inch per 4,000 Btuh
of total input of all appliances located in the space.
EXAMPLE:
Total Input of All Appliances
4000
b.
Horizontal Ducts - Each opening must have a free
area of not less than one square inch per 2,000 Btuh
of total input of all appliances located in the space.
NOTE: Ducts must have the same cross-sectional area as
the free area in the opening to which they are connected. The
minimum dimension of rectangular ducts shall be three
inches.
2.
One permanent opening, commencing within 12 inches
of the top of the enclosure shall be permitted where the
equipment has clearances of at least 1 inch from the
sides and back and 6 inches from the front of the appli-
ance. The opening shall communicate through a vertical
or horizontal duct to the outdoors, or spaces (crawl or
attic) that freely communicate with the outdoors and shall
have a minimum free area of:
a.
1 sq. in. per 3000 Btu per hr of the total input rating
of all equipment located in the enclosure.
b.
Not less than the sum of the areas of all vent con-
nectors in the confined space.
Unitary Products Group
= Square Inches Free Area
3.
Louvers, Grilles and Screens
a.
In calculating free area, consideration must be given
to the blocking effects of louvers, grilles and
screens.
b.
If the free area of a specific louver or grille is not
known, refer to Table 6, to estimate free area.
NOTE: If mechanically operated louvers are used, a means
to prevent main burner ignition and operation must be pro-
vided should louvers close during startup or operation.
Table 6: ESTIMATED FREE AREA
Wood or Metal
Louvers or Grilles
†
Screens
*.
Do not use less than 1/4 in. mesh
†.
Free area or louvers an grilles varies widely; installer should
follow louver or grille manufacturer's instructions.
Special Combustion and Ventilation Considerations
Operation of a mechanical exhaust, such as an exhaust fan,
kitchen ventilation system, clothes dryer or fireplace may cre-
ate conditions requiring special attention to avoid unsatisfac-
tory operation of gas appliances.
Specially Engineered Installations
The above requirements shall be permitted to be waived
where special engineering, approved by the authority having
jurisdiction, provides an adequate supply of air for combus-
tion, ventilation and dilution of flue gases.
Combustion Air Quality
The recommended source of combustion air is to use the out-
door air supply. Excessive exposure to contaminated com-
bustion air will result in safety and performance related
problems. However, the use of indoor air in most applications
is acceptable, except as follows:
1.
If the furnace is installed in a confined space it is recom-
mended that the necessary combustion air come from
the outdoors by way of attic, crawl space, air duct or
direct opening.
2.
If indoor combustion air is used, there must be no expo-
sure to the installations or substances listed in 3 below.
*
Wood 20-25%
*
Metal 60-70%
1/4 in. mesh or larger 100%
19
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