This paper presents a study of the potential for the use of natural ventilation systems in Portuguese multi-family residential buildings under winter climatic conditions. The behaviour of various natural ventilation systems is tested in a standard residential dwelling, using the TRNSYS 15 and COMIS 3.1 software programs. This study leads to the conclusion that the use of hybrid ventilation systems can save a considerable amount of the energy normally spent on continuously operating mechanical ventilation systems. Hybrid ventilation systems will not be adequate if there are no ventilation inlets in the façade and air intake is only by means of infiltration, especially in the case of certain modern windows with better seals; in such situations, the use of continuous mechanical ventilation systems is recommended. Therefore, it is essential to equip buildings with air inlets in the façades, preferably self-adjustable grilles, to ensure the best use of hybrid systems.