Section 709 Louvers and Grills

The required size of openings for combustion, ventilation and dilution air shall be based on the free area of each opening. Where the free area through a design of louver, grill or screen is known, it shall be used in calculating the size opening required to provide the free area specified. Where the design and free area of louvers, grills or screens is not known, it shall be assumed that wood louvers will have 25-percent free area and metal louvers, grills and screens will have 50-percent free area. Screens shall have a mesh size not smaller than ¹/₄ inch. Nonmotorized louvers and grills shall be fixed in the open position. Motorized louvers shall be interlocked with the equipment so that they are proven to be in the full open position, by use of a blade position switch, prior to burner ignition and during burner operation. Means shall be provided to prevent the burner from igniting if the louvers fail to open during burner start-up and to shut down the burner if the louvers close during operation.