Page 7
Air from Outside
If air from outside is brought in for combustion and ventila-
tion, the confined space shall be provided with two perma
-
nent openings. One opening shall be within 12” (305mm)
of the top of the enclosure and one within 12” (305mm) 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 indirectl through
vertical ducts. Each opening shall have a minimum free
area of 1 square inch per 4,000 Btu (645mm2 per 1.17kW)
per hour of total input rating of all equipment in the en-
closure. When communicating with the outdoors through
horizontal ducts, each opening shall have a minimum free
area of 1 square inch per 2,000 Btu (645mm2 per .59kW)
per total input rating of all equipment in the enclosure (See
FIGURE 5). It is also permissible to bring in air for com-
bustion from a ventilated attic (FIGURE 6) or ventilated
crawl space (FIGURE 7).
FIGURE 5
EQUIPMENT IN CONFINED SPACE
(Inlet Air from Ventilated Attic and Outlet Air to Outside)
NOTE-The inlet and outlet air openings shall each have a free area
of at least one square inch per 4,000 Btu (645mm
2
per 1.17kW) per
hour of the total input rating of all equipment in the enclosure.
Ventilation Louvers
Inlet Air
(Minimum
12 in.(305mm) Above
attic floor)
Roof Terminated
Exhaust Pipe
Furnace
*Intake Debris
Screen
(Provided)
FIGURE 6
NOTE-The inlet and outlet air openings shall each have a free area
of at least one square inch per 4,000 Btu (645mm
2
per 1.17kW) per
hour of the total input rating of all equipment in the enclosure.
EQUIPMENT IN CONFINED SPACE
(Inlet Air from Ventilated Crawlspace and Outlet Air to Outside)
Roof Terminated
Exhaust Pipe
Furnace
Ventilation
Louvers
(Crawl space)
*Intake Debris Screen Provided)
Inlet Air
(Minimum
12 in.(305mm)
Above crawl
space floor)
Coupling or
3 in. to 2 in.
Transition
(Field Provided)
FIGURE 7
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 commu-
nicate 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 FIGURE 5 and FIG-
URE 8. 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 9.
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 Crawl Space and Outlet Air to Outside)
NOTE-The inlet and outlet air openings shall each have a free area
of at least one square inch per 4,000 Btu (645mm
2
per 1.17kW) 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)
