If your gas hot water heater won't heat or stay lit, the thermocouple may need replacing. The thermocouple is a safety device that cuts off gas from the water heater in case the pilot goes out. The sensor helps it detect when the pilot light is out. When the thermocouple malfunctions, it will transmit incorrect signals to the gas valve which the gas valve misinterprets as the pilot light is out. Here are some tips to replace the thermocouple on a gas hot water heater.

Turn Off Gas Connections

Look for the gas supply line usually located on the right or left of the red valve. Turn the knob to the 'Off' setting or keep turning right until it stops perpendicular to the gas line and valve length and set the temperature dial on the gas valve on the lowest number.

Be sure to raise windows to help keep access gas fumes out of the room, and do not smoke near the heater when you are working.

Remove the Burner

Access the burner by prying off the cover at bottom of the water heater. You should be able to do this by hand. Some hot water heaters have two doors that open at the base. Remove the thermocouple lead and pilot tube located on one side of the red knob with a 7/16 wrench.

The thermocouple lead is opposite the pilot tube. Remove the burner assembly with a ¾ inch wrench and take off the burner supply tube below the knob with a ¾ wrench which should release the burner assembly.

Replace the Old Thermocouple

Loosen the screw to release the bracket that secures the old thermocouple, and then insert the new thermocouple into the burner assembly taking care not to bend the soft metal.

One-half of the thermocouple should set in the pilot flame. Set the burner back on the hot water heater and reconnect the gas supply line, thermocouple connection, and pilot supply. Avoid tightening the connections more than ¼ inch past hand tight.

Turn the gas back on and test the valves for leaks by using soapy water on the connections. If the valves are leaking, bubbles will form in the soap. Tighten connections until you see no more bubbles.

Your hot water should now work properly, but keep watch on it for two or three days. If your gas hot water heater is old, you may consider installing a new one. New hot water heaters are designed to be energy-efficient. Continue here to learn more about this topic.