District heating effectively meets the heating needs of multiple buildings while consuming less resources compared to individual heating at each building. In U.S. district heating systems, steam is the most common heat transport medium. Simulation of large steam district heating systems requires a computationally efficient and accurate steam model. However, the commonly adopted IF97 water model is not fast enough for district-scale simulations, and its discontinuous thermodynamic property functions have shown to cause simulation problems and sometimes failure. To address these issues, this work introduces a new steam medium model for heating applications with invertible polynomial approximations for specific enthalpy and entropy, which simplify the calculations. Further, we adopt a novel split-medium approach in components for district energy systems, dividing liquid and vapor phases of water into two separate models. This avoids the common numerical challenges at the phase change boundary. We implemented the model in the equation-based Modelica language and evaluated the accuracy and numerical performance across multiple scales: from fundamental thermodynamic properties to complete heating districts of several sizes. The results show that the new model can calculate specific enthalpy within 2.04% of CVRMSE, but with a 39% reduction in computing time. For complete districts, the new implementation has similar accuracy as the IF97 model for evaluating the energy consumption, but at a speed that is 5.6 – 9.3 times faster. Moreover, for the IF97 model, in our district models the dimension of the nonlinear system of equations of the piping network increases linearly in the problem size, but it stays constant with the new model; this is critically important for large scale system simulations. The new steam medium model is available open-source in the Modelica IBPSA and Buildings Libraries.