This paper deals with the issue of how to properly model fluid flow in pipe junctions. In particular we investigate the numerical results from three alternative network models, all three based on the isothermal Euler equations. Using two different test cases, we focus on the physical validity of simulation results from each of the models. Unphysical solutions are characterised by the presence of energy production in junctions. Our results are in accordance with previous conclusions; that only one of the network models yields physical solutions for all subsonic initial conditions. The last test case shows in addition how the three models may predict fundamentally different waves for a given set of initial data.