Abstract: A method of detecting an operating condition of a controllable flow regulator in a fluid flow channel of an HVAC system, comprising transmitting a sonic signal, from a sonic transmitter, directly or indirectly to the flow regulator, the sonic signal being distinguished from background noise by being at least one of: (i) modulated according to a modulation schema, (ii) an ultrasonic signal, (iii) a frequency selected to be away from background noise, receiving a signal from a sonic receiver for detecting the transmitted signal after interacting with the flow regulator; and determining, in an electronic signal processor, the operating condition of the flow regulator on the basis of the signal received.