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top of the enclosure and one opening within 305 mm (12
inches) 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 indirect
ly through vertical ducts. Each opening shall have a mini
mum free area of 645 mm
2
(1 square inch) per 1.17 kW
(4,000 Btu) per hour of total input rating of all equipment in
the enclosure. See
figures 2 and 3. When communicating
with the outdoors through horizontal ducts, each opening
shall have a minimum free area of 645 mm
2
(1 square inch)
per .56 kW (2,000 Btu) per total input rating of all equipment
in the enclosure. See
figure 4.
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 75 mm (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 cov
ering is not known for calculating the size opening required,
it may be assumed that wood louvers will have 20 to 25 per
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 opera
tion.
EQUIPMENT IN CONFINED SPACE
ALL AIR FROM OUTSIDE
(Inlet Air from Crawlspace and Outlet Air to Ventilated Attic)
NOTE-The inlet and outlet air openings shall each have a free area of at least
645 mm
2
(one
square inch) per 1.17 kW (4,000 Btu) per hour of the total input rating of all equipment in the enclo
sure.
VENTILATION LOUVERS
(Each end of attic)
OUTLET
AIR
WATER
HEATER
INLET
AIR
CHIMNEY OR
GAS VENT
FURNACE
VENTILATION
LOUVERS
(For unheated crawl space)
FIGURE 2
