Abstract: The air velocity detector is used in combination with an air duct carrying heat, ventilation or air conditioned (HVAC) air therethrough. A rigid support subassembly is mounted in the HVAC air duct. At least a pair of ultrasonic transceivers are mounted in a spaced apart relationship facing each other on opposing end portions of the support subassembly. The acoustic, ultrasonic signal path from one transceiver to the other diagonally extends across a volume of duct covered by the rigid support subassembly. In one embodiment, the subassembly is a rigid sleeve mounted to the inboard, interior portions of the duct. In another embodiment, the subassembly is a rigid, substantially straight member diagonally extending through the duct volume. Electronic circuitry is connected to the transceivers which excites the transceivers, processes the received ultrasonic signal, and determines the phase difference or phase difference between the transmitted ultrasonic signal and the received ultrasonic signal.