You must provide air sufficient for all gas appliances within furnace space. Locate one combustion and ventilation air opening within 12 inches of top of furnace space. Locate another within 12 inches of bottom of furnace space.
Ducts that supply air from outdoors must have the same
they connect.
Figure 10. All Air Directly From Spaces That Freely Access The Outdoors.
When directly communicating with the outdoors, each opening must have a minimum free area of one square inch per 4,000 Btu per hour of total combined input rating of all gas appliances within furnace space. For example: A 108,000 Btu per hour furnace requires two openings of at least 108,000 ÷ 4,000 or 27 square inches. See Figure 10 and Table 3.
Table 3. Free Opening Area Based On One Square Inch Per 4000 Btu/hr.
TOTAL | REQUIRED | MINIMUM | MAXIMUM |
INPUT | FREE | ROUND PIPE | INPUT FOR |
(Btu/hr) | AREA PER | DIAMETER | GIVEN PIPE |
| OPENING | (inches) | DIAMETER |
| (square inches) |
| (Btu/hr) |
54,000 | 13.5 | 5 | 78,000 |
72,000 | 18 | 5 | 78,000 |
90,000 | 22.5 | 6 | 113,000 |
108,000 | 27 | 6 | 113,000 |
126,000 | 31.5 | 7 | 153,000 |
144,000 | 36 | 7 | 153,000 |
When communicating with the outdoors through vertical ducts, openings and ducts must have a minimum free area of one square inch per 4,000 Btu per hour of total combined input rating of all gas appliances within furnace space. See Figure 11 and Table 3.
Figure 11. All Air Through Vertical Ducts From Spaces That Freely Access The Outdoors.
Figure 12. All Air From The Outdoors Through Horizontal Ducts.
When communicating with the outdoors through horizontal ducts, openings and ducts must have a minimum free area of one square inch per 2,000 Btu per hour of total combined input rating of all gas appliances within furnace space. See Figure 12 and Table 4.
Table 4. Free Opening Area Based On One Square Inch Per 2000 Btu/hr.
TOTAL | REQUIRED | MINIMUM | MAXIMUM |
INPUT | FREE | ROUND | INPUT FOR |
(Btu/hr) | AREA PER | PIPE | GIVEN PIPE |
| OPENING | DIAMETER. | DIAMETER |
| (square inches) | (inches) | (Btu/hr) |
54,000 | 27 | 6 | 56,000 |
72,000 | 36 | 7 | 76,000 |
90,000 | 45 | 8 | 100,000 |
108,000 | 54 | 9 | 127,000 |
126,000 | 63 | 9 | 127,000 |
144,000 | 72 | 10 | 157,000 |
Example: Furnace is 90,000 Btu per hour input and is to be installed in a confined space that contains no other gas appliances. Rectangular combustion and ventilation air ducts will run horizontally from outdoors to furnace space.
Installer’s Information Manual | Page 13 |