In recent years, natural ventilation system have attracted greater attention as an alternative to reduce building energy consumption. And natural ventilated buildings have been increasingly applied even in large office buildings. In Japan, outdoor air of middle season as natural ventilation season has wide variation range of temperature and humidity. Therefore, determining the criteria of outdoor air to introduce natural ventilation is an important factor to achieve energy saving and comfortable indoor thermal environment. In this paper, switching conditions to natural ventilation in 42 buildings with natural ventilation system were investigated and classified in six groups. Each type was evaluated with the reference weather data of 2000yr versions from three aspects: 1) total operating hours, 2) continuity, defined as the daily average hours of operating natural ventilation, 3) the quality of supply outdoor air. The total operating hours of the six types in basic setpoints ranged from 972h to l,278h. Continuity was between 6.2 and 8.3.
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