Low water pressure sensors / switches as the name suggests are used to tell the boiler if the system water presure is too low. There are two types: First one is one that breaks an electrical contact when the water pressure is low and usually has 2 wires connected to it. The second type has an electronic sensor chip in it which sends a voltage to the pcb telling it what the pressure is. These usuallay have 3 wires connected to it, a power wire to provide power to the sensor and an ground wire. The third wire is a variable voltage dependent upon tje system pressure.