In this paper, the methods of heat transfer are presented with the associated mathematical models for each method. Natural convection and the mathematical models necessary for the calculation of the heat transfer coefficient and dimensionless features such as the Reynolds, Rayleigh, Grashof, Prandtl and Boussinesq features are also described in detail. The method of finite volumes is described, and the program and working principle in the Ansys program are described in detail. The program first solved the "benchmark" problem of natural convection in a square cavity, in order to better understand the simulation of natural convection using numerical methods, and then simulated natural convection in the room. Two cases are shown: a room with given wall layers and a window, and a case without a window with the aim of determining the effect of the window on the temperature field and the velocity field in the room. After the numerical simulations, the temperature in the room without a window was ≈4 °C higher than in the room with a window, and the speed of the air flow was lower by ≈0.2 m/s, which indicates that the greatest influence on the temperature and speed of the flow air in the room, has a window, and using a window with a lower heat transfer coefficient will provide greater thermal comfort.