The similarity between models and prototypes of glass tank furnaces is analyzed, considering both the hydrodynamic and thermodynamic aspects of the problem. By applying inspectional analysis to the Navier‐Stokes equations and to the first principle of thermodynamics, suftkiently similar conditions are obtained between model and prototype. Such conditions are used to choose the best liquid to simulate the melted glass. It is concluded that the selection of a liquid for the model requires that the hydrodynamic and thermodynamic aspects of the problem be considered, that a set of conditions may be established to ensure the similarity between model and prototype, and that such conditions may generally be fulfilled by real liquids.