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Table 2–Minimum Free Area of Combustion Air Opening*
| AIR FROM INDOOR | OUTDOOR AIR THROUGH | OUTDOOR AIR THROUGH | OUTDOOR AIR THROUGH | ||||
FURNACE | UNCONFINED SPACE | VERTICAL DUCTS | HORIZONTAL DUCTS | SINGLE DUCT | ||||
INPUT | Free Area | Free Area of | Round Pipe | Free Area of | Round Pipe | Free Area of | Round Pipe | |
(BTUH) | of Opening | Opening and Duct | Opening and Duct | Opening and Duct | ||||
(in. Dia) | (in. Dia) | (In. Dia) | ||||||
| (Sq In.) | (Sq. In.) | (sq In.) | (Sq In.) | ||||
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44,000 | 100 | 11 | 4 | 22 | 6 | 14.7 | 5 | |
66,000 | 100 | 16.5 | 5 | 33 | 7 | 22.0 | 6 | |
88,000 | 100 | 22.0 | 6 | 44.0 | 8 | 29.3 | 7 | |
110,000 | 110 | 27.5 | 6 | 55 | 9 | 36.7 | 7 | |
132,000 | 132 | 33 | 7 | 66 | 10 | 44.0 | 8 | |
154,000 | 154 | 38.5 | 7 | 77 | 10 | 51.3 | 8 | |
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*Free area shall be equal to or greater than the sum of the areas of all vent connectors in the confined space. Opening area must be increased if other gas appliances in the space require combustion air.
Step 1—Unconfined Space
An unconfined space has a volume of at least 50 cu ft for each 1000 Btuh total input for all appliances (furnaces, clothes dryers, water heaters, etc.) in the space.
For example:
FURNACE | MINIMUM WITH |
INPUT (BTUH) | |
44,000 | 294 |
66,000 | 441 |
88,000 | 587 |
110,000 | 734 |
132,000 | 881 |
154,000 | 1028 |
If the unconfined space is constructed unusually tight, air for combustion and ventilation must come from either the outdoors or spaces freely communicating with the outdoors. Combustion and ventilation openings must be equivalent to those used for a confined space (defined below). Return air must not be taken from the room unless an equal or greater amount of air is supplied to the room.
Step 2—Confined Space
A confined space has a volume less than 50 cu ft per 1000 Btuh of total input ratings of all appliances installed in that space. A confined space must have provisions for supplying air for com- bustion, ventilation, and dilution of flue gases using one of the following methods in Table 2 and Fig. 7 and 8.
NOTE: When determining the free area of an opening, the blocking effect of louvers, grilles, and screens must be considered. If the free area of the louver or grille design is unknown, assume wood louvers have a 20 percent free area and metal louvers or grilles have a 60 percent free area. Screens must not be smaller than
The opening size depends upon whether air comes from outside of the structure or an unconfined space inside the structure.
1.Air from inside the structure requires 2 openings (for struc- tures not of unusually tight construction):
a.Each opening must have a minimum free area of not less than 1 sq in per 1000 Btuh of total input rating for all gas utilization equipment in the confined space, but not less than 100 sq in. The minimum dimension of air openings should be no smaller than 3 in. (See Table 2 and Fig. 7 ).
b.If building construction is unusually tight, a permanent opening directly communicating with the outdoors shall be provided. (See next section).
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| AIR | VENT THROUGH ROOF | ||
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| 12″ |
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| IN. | ||
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| 1 SQ | ||||||
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| PER 1000 | ||||||
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| CONFINED SPACE |
| BTUH* IN DOOR | |||||||
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| OR WALL | ||||||||
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INTERIOR |
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| SPACE | |||||||
HEATED |
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SPACE |
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| 6″ MIN | ||||||
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| 1 SQ IN. | ||||||
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| PER 1000 | ||||||
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| 12″ MAX | ||
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AIR DUCTS
*Minimum opening size is 100 sq in. with minimum dimensions of 3 in.
†Minimum of 3 in. when
A02038
Fig. 7—Confined Space: Air for Combustion and
Ventilation from an Unconfined Indoor Space
c.If the furnace is installed on a raised platform to provide a
2.Air from outside the structure requires 1 of the following:
a.If combustion air is taken from outdoors through 2 vertical ducts, the openings and ducts must have at least 1 sq in. of free area per 4000 Btuh of total input for all equipment within the confined space. (See Fig. 8 and Table 2.)
b.If combustion air is taken from outdoors through 2 hori- zontal ducts, the openings and ducts must have at least 1 sq in. of free area per 2000 Btuh of total input for all equipment within the confined space. (See Fig. 8 and Table 2.)
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