Thermocouples
This part is fitted to the older standard efficiency boilers that have a permanent pilot light, they work in a way by making a small electrical charge which activates an electromagnet to open a valve to allow gas or water to go through the system. they are an early way of detecting if there is a flame present. They are made from 2 dissimilar metals that when heated generates a small electrical current that is then sent down to the gas valve to hold in the pilot solenoid on the gas valve allowing the user to let go of the knob.
If the pilot light goes out for any reason the thermocouple cools down, the electrical current decreases allowing the pilot solenoid to drop out. Once the pilot solenoid has dropped out the gas to the burner is extinguished and the boiler is switched off for safety.
