In the case of power boilers a grace period of not more than two months longer than the period established by the board under RCW 70.79.240(1) may elapse between internal inspections of a boiler while not under pressure or between external inspections of a boiler while under pressure; in the case of low pressure heating boilers not more than twenty-six months shall elapse between inspections, and in the case of unfired pressure vessels not more than two months longer than the period between inspections prescribed by the board shall elapse between internal inspections.