(a) Each boiler and unfired pressure vessel used or proposed to be used in the state except boilers or unfired pressure vessels exempt under AS 18.60.210 shall be inspected as to construction, installation, condition, and operation, as follows:

(1) power boilers and high pressure or high temperature water boilers shall be given a certificate inspection annually; the inspector may conduct an external inspection while the boiler is under pressure or an internal inspection if warranted;

(2) low pressure steam or vapor heating boilers shall be given a certificate inspection biennially with an internal inspection every four years if its construction permits an internal inspection;

(3) hot water heating, hot water supply, and coil heating boilers shall be given a certificate inspection biennially with an internal inspection at the discretion of the inspector;

(4) each unfired pressure vessel shall be inspected internally and externally at intervals set by the Department of Labor and Workforce Development; an internal inspection of an unfired pressure vessel is not required if its contents are known to be noncorrosive to the material of which the shell, head, or fittings are constructed, either from the chemical composition of the contents or from evidence that the contents are adequately treated with a corrosion inhibitor, and if the vessel is constructed in accordance with

(A) regulations of the Department of Labor and Workforce Development, if any, in effect at the time of manufacture;

(B) standards set out in the American Society of Mechanical Engineers Boiler and Pressure Vessel Code in effect at the time of manufacture until September 6, 1981; or

(C) if the department adopts subsequent revisions of that code, the standards set out in those revisions as adopted by the department and in effect at the time of manufacture.

(b) A grace period of two months beyond periods specified in (a)(1) - (4) of this section may elapse between certificate inspections.