A passive ventilation system has been installed in four new houses: it comprises simple ducts which lead up from the kitchen and bathroom to outside near the house ridge and utilise the wind and the temperature difference between inside and outside (the stack effect) as driving forces. During occupation the system provided a consistent background ventilation rate: the flows dropped only when it was warm and calm outside (when other ventilation measures might be taken by the occupier); when it was very cold and windy outside the system did not over extract but appeared to self-throttle. Window vents gave the occupiers the option of an increase in ventilation. The occupiers made many favourable comments about the system. Cooking smells and steam had cleared quickly, there were no musty smells in the bathroom, nor stale tobacco smells in the living room after being closed up over night, and there were no significant annoying side effects: they had been able to 'forgett about the system. The fitting of the passive system and associated window vents is recommended in new tightly built houses as a means of providing a continuous controlled level of ventilation, and thus reducing condensation risk. For older less tight houses draughtproofing would be necessary in addition to the system, to prevent overventilation.