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364861-UIM-H-0712
Johnson Controls Unitary Products 27
Ambient Combustion Air Supply
This type installation will draw the air required for combustion from
within the space surrounding the appliance and from areas or rooms
adjacent to the space surrounding the appliance. This may be from
within the space in a non-confined location or it may be brought into the
furnace area from outdoors through permanent openings or ducts. It is
not piped directly into the furnace. A single, properly sized pipe from the
furnace vent connector to the outdoors must be provided. It is recom-
mended that the supplied intake coupling & 18” of pipe be attached to
the furnace to prevent accidental blockage of the combustion air intake.
An unconfined space is not less than 50 cu.ft (1.42 m
3
) per 1,000 Btu/
hr (0.293 kW/h) input rating for all of the appliances installed in that
area.
Rooms communicating directly with the space containing the appli-
ances are considered part of the unconfined space, if doors are fur-
nished with openings or louvers.
A confined space is an area with less than 50 cu.ft (1.42 m
3
) per 1,000
Btu/hr (0.293 kW/h) input rating for all of the appliances installed in that
area. The following must be considered to obtain proper air for combus-
tion and ventilation in confined spaces.
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 Table11, to estimate free area.
* Do not use less than 1/4” (6.4 mm) mesh
+ Free area of louvers and grille varies widely; the installer should follow
louver 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 1/4” (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.
FIGURE 38: Combustion Airflow Path Through The Furnace Casing
This type of installation requires that the supply air to the appliance(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 create 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 appliance to soot and gen-
erate dangerous levels of CARBON MONOXIDE, which can lead to
serious injury, property damage and / or death.
Vent pipe cements
into socket just
above top panel
COMBUSTION AIR
Or vent pipe may be
clamped into outlet
of drain coupling
Table 11: Estimated Free Area
Wood or Metal
Louvers or Grilles
Wood 20-25%*
Metal 60-70% *
Screens+
1/4” (6.4 mm)
mesh or larger 100%
When a Category I furnace is removed or replaced, the original vent-
ing 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
BTUH Input Rating
Minimum Free Area
Required for Each Opening
40,000
40 in
2
(258 cm
2
)
60,000
60 in
2
(387 cm
2
)
80,000
80 in
2
(516 cm
2
)
100,000
100 in
2
(645 cm
2
)
120,000
120 in
2
(742 cm
2
)
130,000
130 in
2
(838 cm
2
)
Table 13: 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)
40,000
20 in
2
(129 cm
2
) 10 in
2
(64 cm
2
)
4” (10 cm)
60,000
30 in
2
(193 cm
2
) 15 in
2
(97 cm
2
)
5” (13 cm)
80,000
40 in
2
(258 cm
2
) 20 in
2
(129 cm
2
)
5” (13 cm)
100,000
50 in
2
(322 cm
2
) 25 in
2
(161 cm
2
)
6” (15 cm)
120,000
60 in
2
(387 cm
2
) 30 in
2
(193 cm
2
)
7” (18 cm)
130,000
65 in
2
(419 cm
2
) 33 in
2
(213 cm
2
)
7” (18 cm)
EXAMPLE: Determining Free Area.
Appliance 1 Appliance 2 Total Input
100,000 + 30,000 = (130,000 4,000) = 32.5 Sq. In. Vertical
Appliance 1 Appliance 2 Total Input
100,000 + 30,000 = (130,000 2,000) = 65 Sq. In. Horizontal