The present work aims to develop an integrated heat transfer and hydrodynamic model for the study of boiler dynamics in fluctuating load conditions. A transient heat transfer model has been developed to estimate water side heat transfer and it is directly integrated with the hydrodynamic model. Two independent natural circulation circuits for furnace water wall and convection tube bank have been considered due to differences in heat transfer, dryness fraction, and void fraction. The transient heat transfer model calculates the heat transfer and water wall temperature of the various sections to calculate total gas side heat transfer, gas temperature profile, and waterside heat transfer in the furnace water wall and convection tube bank. Mass momentum and energy conservation equations are generated for both natural circulation circuits using the estimated value of waterside heat transfer of the transient heat transfer model. These equations are integrated with mass and energy conservation equations for the drum below water level and drum above water level to generate a system of equations to estimate drum pressure and water level fluctuation. This model has been extended for boiler feedback control systems, including the load management and the water level control system. © 2022 Wiley Periodicals LLC.