105561-UIM-B-0605
16 Unitary Products Group
An unconfined space is not less than 50 cu.ft (1.42 m
3
) per 1,000 Btu/
hr (0.2928 kW) 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 openings are not
furnished with doors.
A confined space is an area with less than 50 cu.ft (1.42 m
3
) per 1,000
Btu/hr (0.2928 kW) 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 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 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 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% *
Screens+
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)
40,000 20 sq. in. (51cm) 10 sq. in. (25 cm) 4” (10 cm)
60,000 30 sq. in. (76 cm) 15 sq. in. (38 cm) 5” (13 cm)
80,000 40 sq. in. (102 cm) 20 sq. in. (51 cm) 5” (13 cm)
100,000 50 sq. in. (102 cm) 25 sq. in. (64 cm) 6” (15 cm)
120,000 60 sq. in. (152 cm) 30 sq. in. (76 cm) 7” (18 cm)
135,000 70 sq. in. (178 cm) 35 sq. in. (89 cm) 7” (18 cm)
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
40,000
250 (635 cm
2
)
60,000
375 (953 cm
2
)
80,000
500 (1270 cm
2
)
100,000
625 (1588 cm
2
)
120,000
750 (1905 cm
2
)
135,000
875 (2222 cm
2
)
EXAMPLE: Square feet is based on 8 foot ceilings.
28,000 BTUH X 50 Cubic Ft. = 1,400 = 175 Sq. Ft.
1,000
8’ Ceiling Height
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