We have developed a new coupling method between ES (energy simulation) and CFD (computational fluid dynamics). First, the validity of the coupling method in the stationary part is verified, and the temperature distribution of the space is predicted. At this time, we compare whether it is better to use a temperature boundary for the high boundary conditions or a heat flow boundary. An ES cannot consider the spatial temperature distribution, but it is possible to divide the space into any number of divisions. Since the amount of advection of the cross section of the space of the divided analysis model is not known, the temperature distribution of the space can be reproduced, even by ES, by integrating the values calculated by CFD. In this study, we clarify that the temperature distribution in an environment where natural convection by floor heating is dominant can be reproduced in detail by a combination of ES and CFD. We also conduct a fundamental study of a method for predicting the temperature of an arbitrarily divided zone in a large space. As a result, the temperature distribution when the amount of advection is coupled is clarified.