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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. See chart on page 3 for combustion air minimum opening requirements.
FIGURE 3
FIGURE 4
