Cal. Lab. Code § 7682
The division shall inspect or cause to be inspected each installed fired boiler internally and externally at least every year, except that the division may grant extensions to permit the interval between internal inspections to be increased to a maximum interval of 36 months where operating experience and design of the boiler has demonstrated to the satisfaction of the division that equivalent safety will be maintained.
For other classes of boilers, the division shall establish internal inspection intervals which will ensure the safety of people working in the vicinity of the boiler. In determining the intervals, the division shall consider such factors as the design and construction of the boilers and the conditions under which they operate.
External inspection shall be made of all boilers at the time of the internal inspection and at any other intervals as are deemed necessary by the division acting through qualified safety engineers and certified inspectors.
Ca. Lab. Code § 7682