Two approaches, which analyse the interrelationships between the occurrence of pressure oscillations and the geometrical arrangement of flue gas passes in a boiler, are presented. Firstly a complex mathematical description of the process by means of partial differential equations is solved analytically. Then the compartment approach is presented. Using this approach, the complexity of the model can easily be selected to attain the desired accuracy, on the basis of experience. Both approaches have been verified by experimental measurements. The proposed modelling of boiler pressure pulsations can be a useful tool in boiler design.