A building ventilation system (1) comprising a central unit (2) and at least one electrical operating unit (3) for adjusting a movable member of a ventilation device, the central control unit (2) is connected to the at least one electrical operating unit (3) by two power supply wires (4) and configured to provide an operating voltage for operation of the at least one electrical operating unit (3) via the two power supply wires (4), wherein the power supply lines (4) are connected to an end module (5) and the central control unit (2) further is configured to provide a test voltage being lower than the operating voltage, wherein the end module (5) is configured to temporarily vary its load impedance with a predetermined pattern, and the central control unit (2) is configured to detect the temporarily varying load impedance of the end module.