In order to explain the effect of ventilation systems, the article presents the results of investigations made on a simulation program to calculate the temperature, heat loads, ventilation rates and indoor air quality. The study considered the Japanese daily schedule and the residents behaviour in regard to maintaining comfort. The indicators of indoor air quality were taken to be carbon dioxide, carbon monoxide and formaldehyde concentrations. Three systems were analysed: basic passive ventilation system, a passive stack ventilation system and a mechanical ventilation system.