A thermocouple functions as a safety feature of gas water heaters that don't have automatic electronic ignitors. It's a heat-sensitive device that sends an electric signal to the gas valve to keep the valve open when the pilot is lit. As soon as the pilot goes out and the gas valve stops receiving the electric signal, the valve closes, preventing the release of unburned gas. Thermocouples wear out, and when that happens, you usually have trouble keeping the pilot lit. You can easily replace a bad one.
Principle and Operation
The thermocouple relies on the Seebeck effect, discovered in 1821 by German-Estonian physicist Thomas Johann Seebeck. When two metals are placed in close proximity, they generate a voltage between them that is directly proportional to temperature. The thermocouple in a water heater consists of a long copper capillary tube with a bulb at one end and a threaded contact connection at the other. Two metals -- copel, an alloy of copper and nickel, and stainless steel -- are fused in the tip of the bulb. The voltage difference produced between the tip of the bulb and the other end when the pilot is burning becomes a current that is carried by a copper wire through the capillary tube to the contact connection where it powers an electromagnet in the gas valve.
Identifying a Faulty Thermocouple
When a thermocouple wears out, the pilot won't stay lit, but a bad thermocouple isn't the only potential cause for this problem. Excessive drafts can also make the pilot go out.
You can test the thermocouple with a multimeter set to measure millivolts by attaching one lead to the thermocouple connector and the other to the back of the gas valve. When you light the pilot and hold the button down to keep it lit, a multimeter reading should be at least 10 millivolts. A reading less than that either means the thermocouple is bad, the tip of the sensing bulb is positioned too far from the pilot flame or the pilot isn't burning strongly enough.
Troubleshooting Thermocouple/Pilot Problems
When the thermocouple is bad, the pilot won't stay lit after you light it, no matter how long you hold in the button in before releasing it. Before you suspect a faulty thermocouple, though, look at the pilot flame -- it should be blue and about an inch in height. If it's orange and weak, it probably means the pilot tube is blocked and needs to be cleaned with compressed air.
It's also possible the thermocouple is set too far away from the flame. When the flame is burning, it should be touching -- or very nearly touching -- the tip of the thermocouple bulb. If it isn't, you can reposition the thermocouple by sliding it.
When Replacement Becomes Necessary
The thermocouple isn't a difficult part to replace. After turning off the gas valves on the water heater and the gas line feed, unscrew the connector from the gas valve and pull the bulb to unhook it from the bracket to which it's attached. Generic replacement thermocouples exist that can fit any gas water heater, and you shouldn't run into any problems if you use one. You can also buy one from the manufacturer of your water heater to get extra assurance that the tip is the proper length and thickness for your model and the tube is the correct length.