EN12952-4 provides a method for calculation of in-service low cycle fatigue damage. The procedure describes the calculation of low cycle fatigue damage of boiler components during operation. Temperature, pressure, temperature differences, strain, etc. are considered in the determination of actual damage level. The EN12952-4 also includes creep calculation and creep–fatigue combined calculation, but this part, essential for Remaining Life Assessment, it is not in the scope of this paper. The European standard provides also calculation examples, even if only by computer data logging system it is possible to apply the procedure. Fatigue calculation, as well as creep and creep–fatigue calculation, is essential for modern HRSG plants, where the changing of operational conditions is quite often applied.