Hybrid ventilation systems have become increasingly attractive, because they are considered to provide good indoor air quality (IAQ) with low energy consumption. In this study, we evaluate the performance of a hybrid ventilation system (operating on both mechanical and natural ventilation mode) by monitoring the IAQ (e.g. temperature, humidity, CO2 levels) and the energy consumption of a multifamily building. We then investigate occupants' window-opening behavior during the heating season by means of window sensors. The results show that the hybrid ventilation system maintains a good level of IAQ while being energy efficient. We also find that occupants' window-opening behavior is significantly affected by indoor temperature and CO2 concentration. We therefore recommend adopting them as input parameters when designing ventilation control strategies to enhance the performance.