The water flow sensor is mainly composed of a brass valve body, a water flow rotor assembly, a steady flow assembly and a Hall element. It is installed at the water inlet end of the water heater to detect the flow rate of the inlet water. When the water passes through the water flow rotor assembly, the magnetic rotor rotates, and the speed changes linearly with the flow rate. The Hall sensor outputs the corresponding pulse signal and feeds it back to the controller. The controller judges the size of the water flow and regulates it.