The free area of each opening shall be used for determining combustion air. Unless otherwise specified by the manufacturer or determined by actual measurement, the free area shall be considered 50 percent of the gross area for metal louvers and 25 percent of the gross area for wood louvers. 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 burnr 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.