Air change efficiency evaluates the ability of a system to achieve the change of the air contained in a confined interior space. It is conditioned by the geometry of the room, the different openings and the properties of the air among others. Rectangular rooms do not offer impediments when mixing the interior air; however, in rooms with non-regular shapes, stagnation phenomenon can occur in the cers. This paper proposes the study of the air change efficiency in an L-shaped room in which the position of the inlet and outlet openings is sequentially variated. This study is based on the execution of successive numerical simulations, validated by experimental studies. The results show that there are differences on the ventilation efficiency of up to 13.49% between the most dissimilar cases, which occur when one of the openings is located in a cer. If the other opening is placed nearby, the results are very poor; whereas if it is placed at the opposite cer, the efficiency value improves due to the circulation of the air through the room. When these openings are placed near the centre of the room, the efficiencies that are obtained are more similar and this analysis is less relevant.