The main problem in natural ventilation is that its efficiency depends very tightly on the meteorological conditions : high wind velocity and outside temperature lower than inside are optimal conditions for efficient ventilation. Consequently, air renewal inside buildings is very fluctuating from one moment to another, and extreme comportments can be reached from one season to another : in winter, ventilation is usually very satisfying, whereas in summer unwanted reverse airflows can hardly be avoided. One solution is to use a fan that should work at least on the periods of bad efficiency, so that an acceptable ventilation rate can be ensured all the time. One crucial point is that this fan - integrated in an already existing ventilation net - should not decrease at all the performance of this original net in case of no working. Such an electrical device has been developed by the AERECO and CERGA Companies, with such a low energetic consumption that only 1 to 3 watts per dwellings are enough to ensure all the year a ventilation efficiency at least equal to the best winter's one. Thus, such a fan - that can be electrically supplied thanks to photovoltaic panels - puts forward a full natural and performing solution to the problems of traditional natural ventilation, taking into account the saving of energetic expenditures.