Abstract: A detector system for detecting air flow wherein air flow is sensed by a first detector and air flow is sensed by a second detector which is spaced a distance from the first detector, the first and the second detectors each providing output indications indicating the respective sensed air flows. The detector system includes means for receiving the output indications provided by the first and the second detectors and for providing an output indication in response to an increase in one of the first and the second detector output indications, thereby providing an output indication indicating an increase in the air flow sensed by one of the first and the second detectors relative to the air flow sensed by the other one of the first and the second detectors.