Page 5
Air from Inside
If the confined space that houses the furnace adjoins a
space categorized as unconfined, air can be brought in by
providing two permanent openings between the two
spaces. Each opening must have a minimum free area of
22cm
2
per 1 kW (1 square inch per 1000 Btuh) of total input
rating of all gas-fired equipment in the confined space. Each
opening must be at least 646cm
2 (
100 square inches). One
opening shall be within 305mm (12 inches) of the top of the
enclosure and one opening within 305mm (12 inches) of the
bottom. See figure 1.
EQUIPMENT IN CONFINED SPACE
ALL AIR FROM INSIDE
WATER
FURĆ
NACE
OPENINGS
(To Adjacent
Room)
NOTE-Each opening shall have a free area of at least
22cm
2
per 1 kW (1 square inch per 1,000 Btuh)
of the total input rating
of all equipment in the enclosure, but not less than
646cm
2
(100 square inches)
.
G24MCE
FURNACE
CHIMNEY
OR GAS
VENT
FIGURE 1
Air from Outside
If air from outside is brought in for combustion and ventilation,
the confined space must have two permanent openings.
One opening shall be within 305mm (12 inches) of the top
of the enclosure and one within 305mm (12 inches) of the
bottom. These openings must communicate directly or by
ducts with the outdoors or spaces (underfloor or attic) that
freely communicate with the outdoors or indirectly through
vertical ducts. Each opening shall have a minimum free area
of 5.5cm
2
per 1 kW (1 square inch per 4000 Btuh) of total
input rating of all equipment in the enclosure. See
figures 2
and 3. When communicating with the outdoors through horiĆ
zontal ducts, each opening shall have a minimum free area
of 3cm
2
per 1 kW (1 square inch per 2000 Btuh) of total inĆ
put rating of all equipment in the enclosure. See
figure 4.
When ducts are used, they shall be of the same cross-secĆ
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 75mm (3 inches). In calculating free
area, the blocking effect of louvers, grilles, or screens must
be considered. If the design and free area of protective covĆ
ering is not known for calculating the size opening required,
it may be assumed that wood louvers will have 20 to 25 perĆ
cent 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 Underfloor Space and Outlet Air to Ventilated Attic)
NOTE-The inlet and outlet air openings shall each have a free
area of at least 5.5cm
2
per 1 kW (1 square inch per 4000 Btuh)
of the total input rating of all equipment in the enclosure.
VENTILATION LOUVERS
(Each end of attic)
OUTLET
AIR
WATER
FURĆ
NACE
INLET
AIR
CHIMNEY
OR GAS
VENT
G24MCE
FURNACE
VENTILATION
LOUVERS
(For unheated underfloor
space)
FIGURE 2
