In this paper a comparison is made between Open Modelica and Dymola for a simulation model of a power boiler. The similarities and differences are pre-sented. Dymola has the advantage of having a more elaborate user interface and solver, but the Open Modelica user interface and solver has improved very much during last few years. The advantage of Open Modelica is that anyone can use the models without having to pay high license fees, something that is of significant interest when installations are made in industries. In many ways a combination of the two is advisable, where Dymola can be used for application developments and later Open Modelica can be used in the actual installations. It has been seen in this application for a CFB boiler that it is easy to use the same model in both environments with-out any modifications. Still, the solver for Open Modelica is not as powerful as for Dymola, which may be a problem for on-line applications for larger models, while no problem for small models.