292278-UIM-A-0108
Unitary Products Group
19
Combustion Air Source From Outdoors
The blocking effects of louvers, grilles and screens must be given con-
sideration in calculating free area. If the free area of a specific louver or
grille is not known, refer to Table 10, to estimate free area.
*
Do not use less than 1/4” (0.635 cm) mesh.
+
Free area or louvers and grille varies widely; the installer should follow lou-
ver or grille
manufacturer’s instructions.
Dampers, Louvers and Grilles (Canada Only)
1.
The free area of a supply air opening shall be calculated by sub-
tracting the blockage area of all fixed louvers grilles or screens
from the gross area of the opening.
2.
Apertures in a fixed louver, a grille, or screen shall have no dimen-
sion smaller than 0.25” (6.4 mm).
3.
A manually operated damper or manually adjustable louvers are
not permitted for use.
4.
A automatically operated damper or automatically adjustable lou-
vers shall be interlocked so that the main burner cannot operate
unless either the damper or the louver is in the fully open position.
This type of installation requires that the supply air to the appli-
ance(s) be of a sufficient amount to support all of the appliance(s)
in the area. 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 unsatisfactory
operation of gas appliances. A venting problem or a lack of supply
air will result in a hazardous condition, which can cause the appli-
ance to soot and generate dangerous levels of CARBON MONOX-
IDE, which can lead to serious injury, property damage and / or
death.
TABLE 10:
Estimated Free Area
Wood or Metal
Louvers or Grilles
Wood 20-25%*
Metal 60-70% *
1/4” (0.635 cm)
mesh or larger 100%
TABLE 11:
Free Area
BTUH
Input
Rating
Minimum Free Area Required for Each Opening
Horizontal Duct
(2,000 BTUH)
Vertical Duct or
Opening to Outside
(4,000 BTUH)
Round Duct
(4,000 BTUH)
60,000
30 sq. in. (194 cm
2
)
15 sq. in. (97 cm
2
)
5” (13 cm
2
)
80,000
40 sq. in. (258 cm
2
) 20 sq. in. (129 cm
2
)
5” (13 cm
2
)
100,000
50 sq. in. (323 cm
2
) 25 sq. in. (161 cm
2
)
6” (15 cm
2
)
120,000
60 sq. in. (387 cm
2
) 30 sq. in. (194 cm
2
)
7” (18 cm
2
)
EXAMPLE: Determining Free Area.
Appliance 1Appliance
2Total
Input
100,000
+ 30,000 = (130,000
÷
4,000) = 32.5 Sq. In. Vertical
Appliance 1Appliance
2Total
Input
100,000
+ 30,000 = (130,000
÷
2,000) = 65 Sq. In. Horizontal
TABLE 12:
Unconfined Space Minimum Area in Square Inch
BTUH Input Rating
Minimum Free Area Required for Each Opening
60,000
60 (387 cm
2
)
80,000
80 (516 cm
2
)
100,000
100 (645 cm
2
)
120,000
120 (774 cm
2
)
FIGURE 20:
Outside and Ambient Combustion Air
GABLE
VENT
GAS
VENT
SOFFIT
VENT
VENTILATED
ATTIC
TOP ABOVE
INSULATION
OPTIONAL
INLET (a)
OUTLET
AIR (a)
VENTILATED
CRAWL SPACE
GAS
WATER
HEATER
VENTILATED
ATTIC
TOP ABOVE
INSULATION
GAS
VENT
GABLE
VENT
SOFFIT
VENT
GAS
WATER
HEATER
INLET
AIR (a)
INLET
AIR (b)
GAS
VENT
OUTLET
AIR (a)
OUTLET
AIR (b)
INLET
AIR (a)
INLET
AIR (b)
GAS
WATER
HEATER
FURNACE
1. An opening ma
y
be used in lieu of a duct to provide to provide the outside air
suppl
y
to an appliance unless otherwise permitted b
y
the authorit
y
having
jurisdiction. The opening shall be located within 12” (30.5 cm) horizontall
y
from,
the burner level of the appliance. Refer to “AIR SOURCE FROM OUTDOORS
AND VENT AND SUPPLY AIR SAFETY CHECK” in these instructions for
additional information and safet
y
check procedure.
2. The duct shall be either metal, or a material meeting the class 1
requirements of CAN4-S110 Standard for Air Ducts.
3. The duct shall be least the same cross-sectional area as the free
area of the air suppl
y
inlet opening to which it connects.
4. The duct shall terminate within 12 in (30.5 cm) above, and
within 24 in (61 cm) horizontall
y
from, the burner level of
the appliance having the largest input.
5. A square or rectangular shaped duct shall onl
y
be used
when the required free area of the suppl
y
opening is
9 in (58.06 cm ) or larger. When a square or rectangular
duct is used, its small dimensionshall not be less than
3 in (7.6 cm).
2
2
6. An air inlet suppl
y
from outdoors shall be equipped with
a means to prevent the direct entr
y
of rain and wind.
Such means shall not reduce the required free area of
the air suppl
y
opening.
7.
An air suppl
y
inlet opening from the outdoors shall
be located not less than 12” (30.5 cm) above the
outside grade level.
FURNACE
FURNACE
AIR SUPPLY OPENINGS AND DUCTS
COMBUSTION AIR SOURCE FROM OUTDOORS
1. Two permanent openings, one within 12 in (30.5 mm) of the top and
one within 12 in (30.5 mm) of bottom of the confined space,
shall communicate directl
y
or b
y
means of ducts
with the outdoors, crawl spaces or attic spaces.
Two
permanent openings,
2. One permanent openings, commencing within 12 in (30.5 mm)of the
top of the enclosure shall be permitted where the equipment has
clearances of at least 1 in (2.54 cm) from the sides and back and
6 in (15.24 cm) from the front of the appliance. The opening shall
communicate directl
y
with the outdoors and shall have a minimum
free area of:
3. The duct shall be least the same cross-sectional area as the free
area of the air suppl
y
inlet opening to which it connects.
1 square in per 3000 Btu per hour (6.45
0.879 kW) of the
total input rating of all equipment located in the enclosure.
cm per
3
a.
Not less than the sum of all vent connectors in the confined space.
b.
4. The blocking effects of louvers, grilles and screens must be given
consideration in calculating free area. If the free area of a specific
louver aor grille is not known.
