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506725-01Page 8 of 48 Issue 1109
Air from Outside
If air from outside is brought in for combustion and ventilation,
the confined 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 total
input rating of all equipment in the enclosure (See Figure 5).
If air from outside is brought in for combustion and ventilation,
the confined space must have two permanent openings. 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 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 (645 mm²) per 4,000 Btu (1.17 kW)
per hour of total input rating of all equipment in the enclosure.
See figures 5 and 6. 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 7.
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 fixed in the open position or interlocked with the equipment
so that they are opened automatically during equipment
operation.
FIGURE 5
EQUIPMENT IN CONFINED SPACE − ALL AIR FROM OUTSIDE
(Inlet Air from Crawl Space and Outlet Air to Ventilated Attic)
NOTE−The inlet and outlet air openings shall each have a free area
of at least one square inch per 4,000 Btu (645 mm
2
per 1.17 kW) per
hour of the total input rating of all equipment in the enclosure.
OUTLET
AIR
INLET
AIR
VENTILATION
LOUVERS
(For unheated
crawl space)
FURNACE
ROOF TERMINATED
EXHAUST PIPE
VENTILATION LOUVERS
(Each end of attic)
SIDE WALL
TERMINATED
EXHAUST PIPE
(ALTERNATE
LOCATION)
FIGURE 6
EQUIPMENT IN CONFINED SPACE − ALL AIR FROM OUTSIDE
(All Air Through Ventilated Attic)
NOTE−The inlet and outlet air openings shall each have a free area of
at least one square inch per 4,000 Btu (645 mm
2
per 1.17 kW) per hour
of the total input rating of all equipment in the enclosure.
OUTLET
AIR
VENTILATION LOUVERS
(Each end of attic)
INLET AIR
(Ends 12" above
bottom)
ROOF TERMINATED
EXHAUST PIPE
SIDE WALL
TERMINATED
EXHAUST PIPE
(ALTERNATE
LOCATION)
FURNACE
FIGURE 7
EQUIPMENT IN CONFINED SPACE −
ALL AIR FROM OUTSIDE
OUTLET AIR
INLET AIR
NOTE−Each air duct opening shall have a free area of at least one
square inch per 2,000 Btu (645 mm
2
per .59 kW) per hour of the total
input rating of all equipment in the enclosure. If the equipment room
is located against an outside wall and the air openings communi-
cate directly with the outdoors, each opening shall have a free area
of at least 1 square inch per 4,000 Btu (645 mm
2
per 1.17 kW) per
hour of the total input rating of all other equipment in the enclosure.
ROOF TERMINATED
EXHAUST PIPE
SIDE WALL
TERMINATED
EXHAUST PIPE
(ALTERNATE
LOCATION)
FURNACE