A flow switch, also reffered to as a flow sensor or flow turbine is used to detect the flow of water in a boiler. Flow switches are used to detect when a hot water tap is opened. Older boilers used a diaphram to detect the flow of water and then either operate an electrical switch or hydraulically operate the appliance. A flow switch operates by moving a magnet with the flow of water or by rotating a turbine attched to a magnet. The movement of the magnet is picked up by a hall sensor which sends the signal to the pcb. Older flow swtches simply switched a microswitch via a paddle which let the pcb know that a tap was running.