035-17438-001 Rev. A (901)
4
Unitary Products Group
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:
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.
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 2 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.
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.
Total Input of All Appliances
= Square Inches Free Area
4000
Table 1:
UNIT CLEARANCES TO COMBUSTIBLES
APPLICATION
TOP FRONT REAR
LEFT
SIDE
RIGHT
SIDE
FLUE
FLOOR/
BOTTOM
CLOSET ALCOVE
ATTIC
LINE
CONTACT
UPFLOW/HORIZONTAL MODELS - UPFLOW APPLICATION
UPFLOW
1
6
0
0
3
1
6
COMBUSTIBLE
YES
YES
YES
NO
UPFLOW B-VENT
1
3
0
0
0
1
COMBUSTIBLE
YES
YES
YES
NO
HORIZONTAL APPLICATION
HORIZONTAL
3
2
6
0
1
0
6
COMBUSTIBLE
NO
YES
YES
YES
3
HORIZONTAL B-VENT
0
3
0
1
0
1
COMBUSTIBLE
NO
YES
YES
YES
3
1.
14 1/2” cabinet models only - all other units “0” clearance
2.
14 1/2” cabinet left airflow applications only - all other units and all right hand air flow applications “0” clearance.
3.
Line contact only permitted between lines formed by the intersection of the rear panel and side panel (top in horizontal position) of the
furnace jacket and building joists, studs or framing.
Table 2:
ESTIMATED FREE AREA
Wood or Metal
Louvers or Grilles
Wood 20-25%
1
Metal 60-70%
1
Screens
2
1/4 in. mesh or larger 100%
1.
Free area or louvers an grilles varies widely; installer should
follow louver or grille manufacturer’s instructions.
2.
Do not use less than 1/4 in. mesh
