Lennox SL280DFNV Installation Instructions Manual page 7

Air from Outside
If air from outside is brought in for combustion and ventila-
tion, the confined space must have two permanent open-
ings. One opening shall be within 12 inches (305 mm) of
the top of the enclosure and one opening within 12 inches
(305 mm) of the bottom. These openings must communi-
cate directly or by ducts with the outdoors or spaces (crawl
or attic) that freely communicate with the outdoors or in-
directly through vertical ducts. Each opening shall have a
minimum free area of 1 square inch (645 mm2) per 4,000
Btu (1.17 kW) per hour of total input rating of all equipment
in the enclosure. See figures 4 and 5. When communi-
cating with the outdoors through horizontal ducts, each
opening shall have a minimum free area of 1 square inch
(645 mm2) per ,000 Btu (.56 kW) per total input rating of
all equipment in the enclosure. See figure 6.
NOTE-The inlet and outlet air openings shall each have a free area of at least one square inch (645
2
mm ) per 4,000 Btu (1.17 kW) per hour of the total input rating of all equipment in the enclosure.
CHIMNEY OR
GAS VENT
VENTILATION LOUVERS
(Each end of attic)
FURNACE
VENTILATION
LOUVERS
(For unheated crawl space)
When ducts are used, they shall be of the same cross-sec-
tional 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 protec-
tive 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 fixed in the open position or interlocked with the
equipment so that they are opened automatically during
equipment operation.
EQUIPMENT IN CONFINED SPACE
ALL AIR FROM OUTSIDE
(Inlet Air from Crawlspace and Outlet Air to Ventilated Attic)
OUTLET
AIR
AIR FLOW
FIGURE 4
Page 7
WATER
HEATER
INLET
AIR