The operation of steam boilers with high efficiency requires control of thermo-hydraulic and strength parameters in the on-line mode. During operation under transient conditions, there are significant temperature differences over the circumference of the horizontal pressure vessels. This occurrence is caused by the different values of heat transfer coefficients on the inner surface of the boiler drum in the water and steam region. Large thermal stresses can occur in partially filled horizontal vessels. This takes place in large steam generator drums, superheater headers, and steam pipelines. The paper presents a method for determination of thermo-flow parameters for steam boilers. This method allows performing the calculations of the boiler furnace chamber and heating flow rates absorbed by superheater stages. These parameters are important for monitoring the performance of the power unit and determining the degree of the furnace chamber slagging. Knowledge of these parameters allows determining the degree of the furnace chamber slagging. The calculation can be performed in online mode and use to monitoring of steam boiler. The presented method allows to the operation of steam boiler with high efficiency.