Figure 4 | When horizontal ducts are used to supply air from the |
CONFINED SPACE / OUTDOOR AIR | outdoors, they must be of the same cross sectional area |
| as the free area of the openings to which they connect. |
FROM ATTIC | The minimum dimension of rectangular air ducts must not |
| be less than 3 inches. Each opening must have a free |
| area of not less than one square inch per 2,000 Btu per |
| hour of total input of all appliances within the "Confined |
| Space". These openings must be located 12 in. from the |
| top and bottom of the furnace area. Neither opening can |
| be blocked at any time (See Figure 6). |
| Figure 6 |
| CONFINED SPACE / OUTDOOR |
| AIR THROUGH HORIZONTAL DUCTS |
Figure 5
CONFINED SPACE / OUTDOOR AIR
FROM ATTIC & CRAWL SPACE
For an attic installation it is important to keep insulation 12" or more away from any furnace openings. Some types of insulating materials may be combustible.
Furnaces installed with combustion air drawn from a heated space which includes exhaust fans, fireplaces, or other devices that may produce a negative pressure should be considered confined space installations.
20571201 | Issue 0527 | Page 10 of 28 |