Installing a Thermocouple

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The thermocouple keeps the pilot lit, which in turns keeps the home fires burning.
The thermocouple keeps the pilot lit, which in turns keeps the home fires burning. (Image: Jupiterimages/Photos.com/Getty Images)

The thermocouple on a gas appliance such as a furnace, boiler, water heater and fireplace keeps the pilot lit by transferring a small electric signal back to either the gas valve or a circuit board, depending on the equipment. Over time, the signal can degrade and weaken to the point where the thermocouple is no longer able to keep the pilot lit. Replacing a thermocouple can be accomplished in a few steps.

Things You'll Need

  • Long screwdriver
  • 7/16-inch wrench
  • 3/8-inch wrench
  • Pliers

Turn off the gas and electricity to the appliance. Follow the gas line out of the unit until you hit a elongated valve. Turn the valve in either direction 90 degrees to shut off the gas. A switch similar to a standard household light switch is usually mounted on or near the appliance. Flip the switch to "Off."

Trace the thin aluminum gas pilot tube and the thinner copper thermocouple lines from the pilot to the gas valve. Use the 7/16-inch wrench to loosen and remove the aluminum gas line from the gas valve. Loosen and remove the thermocouple from the gas valve with the 3/8-inch wrench.

Loosen and remove the two screws down at the pilot assembly. The pilot assembly is most often mounted between two of the appliance's burners. Two small screws hold the pilot assembly to a bracket, usually welded to one of the burners.

Remove the entire pilot assembly from the appliance. Loosen the thermocouple at the pilot with the pliers. The thermocouple screws into the pilot assembly bracket with a large, knurled nut. Use the pliers if the thermocouple cannot be removed with your fingers. Discard the old thermocouple.

Blow out and clean the pilot assembly by using compressed air and a rag. Slide the tip of the new thermocouple into the pilot assembly and secure it in place with the thumb screw. Snug with pliers, if desired.

Reinstall the pilot assembly in the reverse order you removed it. Reinstall the two screws removed in Step 3 and secure the pilot in place on the burner. Tighten the pilot gas line and the thermocouple with the appropriate wrench.

Restore the electricity and gas to the appliance. Light the pilot according to the instructions for your specific unit or gas valve.

Tips & Warnings

  • Clean the pilot assembly when you replace the thermocouple. The pilot assembly is the device to which one end of the thermocouple connects. The small, pilot gas tube feeds a pilot assembly containing a tiny orifice, or small hole, through which the pilot gas escapes. Small dust particles and even insects can partially plug up the pilot and prevent the appliance from operating properly.
  • Turn the gas and electricity off at the unit you are servicing. Gas components must be removed and reinstalled carefully. Improperly replaced gas combustion parts can ignite improperly and cause gas buildup inside of the combustion area. A buildup of unburned gas is a fire hazard, can release carbon monoxide into the home and can damage components of your furnace, water heater, fireplace or boiler.

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