Recently a 15-year old gas water heater stopped working; the pilot could be lit and would not stay on. It can found on the internet a diagram of the gas water heater controller. It becomes clear that the thermocouple is the issue. Commonly, we use thermocouples as temperature measuring devices, but here it is actually a fail-safe device, which seems ingenious. The thermocouple is an electricity generating device. The thermocouple is in contact with the flame of the pilot and generates electricity (rated for 30mV) for a solenoid coil to open a gas valve. So in case the flame is out, the gas value is automatically closed. When we light the pilot, we push down a switch to bypass the solenoid to get gas flow and once the pilot flame is lit and the thermocouple is heated sufficiently to open the valve we can release the switch.
The whole thermocouple assembly costs less than $10. It can be appreciated the simplistic yet robust and rugged solution. It is self powered and the very thing energizes it is controlled by it. When the thermocouple fails, it closes the valve to keep the system in a safe state. It is amazing that it has lasted for 15 years bathing in the flame constantly. The life time is longer than the average life of water heaters. Ultimately, oxidation probably claimed its life. It is relatively easy and inexpensive to replace.
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