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364861-UIM-B-0708
When a Category I furnace is removed or replaced, the original
venting system may no longer be correctly sized to properly vent
the attached appliances.
An improperly sized vent system can cause CARBON MONOXIDE
to spill into the living space causing personal injury, and or death.
Table 12: Unconfined Space Minimum Area in Square Inch
Minimum Free Area in Square Feet
BTUH Input Rating
40,000
60,000
80,000
100,000
120,000
130,000
Gas
Vent
Gable
Vent
Ventilated
Attic
Optional
Top Above
Inlet (a)
Insulation
Outlet
Soffit
Air (a)
Vent
Gas
Water
Heater
Ventilated
Crawl Space
FIGURE 36: Outside and Ambient Combustion Air
Air Supply Openings and Ducts
1.
An opening may be used in lieu of a duct to provide to provide the
outside air supply to an appliance unless otherwise permitted by
the authority having jurisdiction. The opening shall be located
within 12" (30.5 cm) horizontally from, the burner level of the appli-
ance. Refer to "AIR SOURCE FROM OUTDOORS AND VENT
AND SUPPLY AIR SAFETY CHECK" in these instructions for
additional information and safety 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 supply inlet opening to which it connects.
4.
The duct shall terminate within 12 in (30.5 cm) above, and within
24 in (61 cm) horizontally from, the burner level of the appliance
having the largest input.
5.
A square or rectangular shaped duct shall only be used when the
required free area of the supply opening is 9 in
larger. When a square or rectangular duct is used, its small dimen-
sion shall not be less than 3 in (7.6 cm).
6.
An air inlet supply from outdoors shall be equipped with a means
to prevent the direct entry of rain and wind. Such means shall not
reduce the required free area of the air supply opening.
26
Required for Each Opening
2
40 (258 cm
)
2
60 (387 cm
)
2
80 (516 cm
)
2
100 (645 cm
)
2
120 (742 cm
)
2
130 (838 cm
)
Gable
Vent
Soffit
Vent
Inlet
Air (b)
2
(58.06 cm
Table 13: Free Area
BTUH Input
Horizontal Duct
Rating
(2,000 BTUH)
40,000
20 in
60,000
30 in
80,000
40 in
100,000
50 in
120,000
60 in
130,000
65 in
EXAMPLE: Determining Free Area.
Appliance
1 Appliance
+ 30,000 = (130,000 ÷ 4,000) = 32.5 Sq. In. Vertical
100,000
Appliance
1 Appliance
+ 30,000 = (130,000 ÷ 2,000) = 65 Sq. In. Horizontal
100,000
Gas
Vent
Ventilated
Attic
Top Above
Insulation
Gas
Water
Heater
Inlet
Air (a)
7.
An air supply inlet opening from the outdoors shall be located not
less than 12" (30.5 cm) above the outside grade level.
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,
Two permanent openings, shall communicate directly or by means
of ducts with the outdoors, crawl spaces or attic spaces.
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 directly with the outdoors and shall have a min-
imum free area of:
a.
1 square in per 3000 Btu per hour (6.45 cm3 per 0.879 kW) of
the total input rating of all equipment located in the enclosure.
b.
Not less than the sum of all vent connectors in the confined
2
) or
space.
3.
The duct shall be least the same cross-sectional area as the free
area of the air supply inlet opening to which it connects.
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 or grille is not known. Refer to Table 11, "Estimated Free
Area".
Minimum Free Area Required for Each Opening
Vertical Duct or
Opening to Outside
(4,000 BTUH)
2
2
2
(129 cm
)
10 in
(64 cm
2
2
2
(193 cm
)
15 in
(97 cm
2
2
2
(258 cm
)
20 in
(129 cm
2
2
2
(322 cm
)
25 in
(161 cm
2
2
2
(387 cm
)
30 in
(193 cm
2
2
2
(419 cm
)
33 in
(213 cm
2 Total Input
2 Total Input
Outlet
Air (b)
Gas
Water
Inlet
Heater
Air (b)
Johnson Controls Unitary Products
Round Duct
(4,000 BTUH)
2
4" (10 cm)
)
2
5" (13 cm)
)
2
5" (13 cm)
)
2
6" (15 cm)
)
2
7" (18 cm)
)
2
7" (18 cm)
)
Gas
Vent
Outlet
Air (a)
Inlet
Air (a)
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