Allied A93UH1D - Page 8

507270-03Page 8 of 60 Issue 1621
Air from Outside
If air from outside is brought in for combustion and
ventilation, the conned space shall be provided with two
permanent openings. One opening shall be within 12” (305
mm) of the top of the enclosure and one within 12” (305 mm)
of the bottom. These openings must communicate directly
or by ducts with the outdoors or spaces (crawl or attic) that
freely communicate with the outdoors or indirectly through
vertical ducts. Each opening shall have a minimum free area
of 1 square inch per 4,000 Btu (645 mm² per .59 kW) per
hour of total input rating of all equipment in the enclosure
(See Figures 6 and 7). It is also permissible to bring in air
for combustion from a ventilated attic (Figure 8) or ventilated
crawl space (Figure 9).
When communicating with the outdoors through horizontal
ducts, each opening shall have a minimum free area of 1
square inch (645 mm²) per 2,000 Btu (.56 kW) per total input
rating of all equipment in the enclosure. See Figure 10.
When ducts are used, they shall be of the same cross-
sectional area as the free area of the openings to which they
connect. The minimum dimension of rectangular air ducts
shall be no less than 3 inches (75 mm). In calculating free
area, the blocking effect of louvers, grilles, or screens must
be considered. If the design and free area of protective
covering is not known for calculating the size opening
required, it may be assumed that wood louvers will have
20 to 25 percent free area and metal louvers and grilles
will have 60 to 75 percent free area. Louvers and grilles
must be xed in the open position or interlocked with the
equipment so that they are opened automatically during
equipment operation.
Figure 6
Equipment in Conned Space - All Air from Outside
(Inlet Air from Crawl Space and Outlet Air to Ventilated Attic)
Figure 7
Equipment in Conned Space - All Air from Outside
(All Air through Ventilated Attic)
Figure 8
Equipment in Conned Space
(Inlet Air from Ventilated Attic & Outlet Air to Outside)
Figure 9
Equipment in Conned Space
(Inlet Air from Ventilated Drawlspace & Outlet Air to Outside)