Cast iron boilers shall be inspected externally at least biennially. Steel boilers shall be inspected at least biennially. When the type of construction of the boiler permits, such inspection shall be an internal inspection at least every three years for steam boilers and an internal inspection at least once every five years for hot water boilers, in addition to the biennial external inspection. A grace period beyond the period specified above may be permitted between inspections at the discretion of the commissioner.