Since 1974 the french Authorities have insisted on energy being saved in all buildings. There was very strong pressure on manufacturers to obtain better sealed window frames . In practise less than two or three meters cubed per hour at a pressure of ten pascals penetrates. Also television campaigns have insisted on weather stripping all windows and window frames in all old buildings. The result of these campaigns is that all buildings with no or natural ventilation systems actually have indoor condensation problems. The use of controlled mechanical ventilation systems is well established in France and is present in 80 per cent of multistorey flats and 40 per cent of single family housing. A new regulation is in preparation and will probably require the fitting of mechanical ventilation in all new buildings. In the meantime we are to have a new regulation concerning the heat losses in house and flats . This means greater interest in planning of buildings and houses with centralised controlled mechanical ventilation systems with heat exchangers. Because of this above new regulation we are sure of great developments of our systems in the coming years.