The supply-air window is a passive system of heat recovery contributing to the building ventilation. It allows the air renewal to circulate between glasses before entering the inside environment. Based on this principle, a part of heat transfer through the glasses is recovered by the airflow. In order to study the thermal performance of this window, Computational Fluid Dynamics (CFD) simulations are firstly conducted. Then, this article proposes simplified models which can be implemented easily in building simulation codes. The models are based on analytical solution of the problem of air circulating between isothermal panes differentially heated. It has been carried out in Modelica language using the Buildings library. A special attention is paid to the convective heat transfer coefficient between air space and glasses. The results obtained from these simplified models are compared to those obtained from the CFD model.