Modern heating, ventilation, and air conditioning (HVAC) systems are complex, interconnected systems optimised to be energy efficient. Damper-optimised demand-controlled ventilation systems (DCV) minimise energy consumption by using a dedicated control unit that calculates the optimal fan speed based on room sensors and the feedback from all DCV dampers, which each measures the airflow rate and adjusts its damper angle accordingly. In buildings that do not use a compartmentation strategy in the event of a fire, it is crucial that the ventilation system is pressurised and provides balanced ventilation in order to prevent smoke from spreading via the ventilation system and to avoid creating pressure imbalances, which may impair evacuation. In the present study, two full-scale fire tests from a series of 14 tests in a mock-up building equipped with a damper-optimised DCV system are presented, and the ventilation system’s performance during the fire is assessed. The tests revealed various failure mechanisms caused by heat exposure, leading to individual damper uncontrolled opening or closing or the building management system losing contact with all dampers. Furthermore, it was shown that the failure of individual dampers and the gradual clogging of the extraction filter can affect the pressure balance in other parts of the building outside the fire room and increase the risk of smoke spreading through the ventilation ducts.