If you need to detect water movement in a pipe (or air in a duct) from outside the pipe, this may help. This Arduino based device detects temperature changes and closes a relay contact. The pipe or duct has to change temperature when there is a flow for this to operate.
Use the software sketch provided in this step as a starting point.
The software will turn the relay on for ten seconds when a temperature difference is detected, then shut off. This timing (as well as the temperature change required to trigger the relay) can be adjusted in software.
Tape the thermistor to the pipe.