There are a number of different types of flow sensing devices which fall within the general heading of flow switches. These devices are used to inform the boiler of the demand for domestic hot water. (Although they can be used to check for the operation of a pump in a similar way i.e. checking for water flow.)
The reed switch usually consists of two permanent magnets one of which is connected to a sensing lever and the other to a microswitch. The magnets are usually like pole (both north poles or both south poles).
Several boilers use the Sifan flow switch this consists of a paddle
connected to a lever and a reed switch.
Another variation on a flow switch is the ball reed type. This is basically a magnetic ball that moves with the flow of water and operates a reed switch. A similar switch can be found in use with certain power showers.
This is a diaphragm that moves up when water flows. On the top side is a reed switch.
Membrane Type + microswitch
A thermistor can be used as a flow sensor by sensing slight changes in temperature as water flown to the hot tap.