6
B. When communicating with the outdoors by
means of vertical ducts, each opening shall have
a minimum free area 1 square inch per 4,000 BTU
per hour of total input rating of all appliances in the
enclosed space.
(Figure 3)
FIGURE 2
FIGURE 3
FIGURE 4
C. If horizontal ducts are used, each opening shall
have a minimum free area 1 square inch per
2,000 BTU per hour total input rating of all
appliances in the enclosed space.
(Figure 4)
D. When ducts are used, they shall be of the same
cross sectional area as the free area of the area
of the openings to which they connect. The
minimum dimension of rectangular air ducts shall
not be less than 3 inches.
6. In calculating free area using louvers, grills or
screens for the above, consideration shall be given
to their blocking effect. Screens used shall not be
smaller than 1/4 inch mesh. If the free area through
a design of louver or grill is known, it should be used
in calculating the size opening required to provide
the free area specified. If the design and free area is
not known, it may be assumed that wood louvers will
have 20-25% free area and metal louvers and grills
will have 60-75% free area. Louvers and grills shall
be fixed in the open position or interlocked with the
boiler so that they are opened automatically during
boiler operation.
