Annually for power boilers, bi-annually for heating and hot water supply boilers and service water heaters; and every three years for unfired pressure vessels. A shorter period of time may be determined as appropriate by the inspector, but the maximum period between certificates is as stated above,except that power boilers exceeding 300,000 pounds of steam per hour may be inspected internally at a frequency that may not exceed 24 months, providing that all of the additional requirements spelled out at 680 IAC 2-3-9 (A) (3) have been met to the satisfaction of the BPVSS Director / Chief Inspector. http://www.in.gov/legislative/iac/iac_title?iact=680&iaca=2