The following shall be exceptions to the provisions of Section 43-2:

A.    When cleaning the boiler or stack.

B.    When a fire box is cleaned out or a new fire is being built therein, smoke shall be permitted of a density equal to but not in excess of No. 2 on such Ringelmann Chart for a period or periods aggregating not more than nine (9) minutes in any one (1) hour or of a density in excess of No. 2 on such chart for a period or periods aggregating not more than six (6) minutes in any one (1) hour.

C.    Blowing of boiler tubes.

D.    In the event of a breakdown of the fuel-burning equipment or other failure, which is not reasonably preventable and which causes the emission of smoke of a density greater than No. 2 on the Ringelmann Chart, the emission of such smoke shall be permitted for a reasonable time during the period of such breakdown or failure.  The person responsible for such equipment or failure shall forthwith notify the Air Pollution Commission or its representatives of such breakdown or failure, shall give the reasons therefor, shall exercise every reasonable effort to prevent without undue delay the emission of smoke of a density greater than No. 2 on the Ringelmann Chart, shall state when such breakdown or failure will be corrected and shall further notify the Air Pollution Commission or its representatives when such breakdown or failure will be corrected.