How to Replace a Water Heater Thermocouple or Flame Sensor

  • 01 of 08


    Water Heater Thermocouple

    A thermocouple or flame sensor is used to control gas flow in gas water heaters. On older-style heaters with standing pilot lights, the device is typically called a thermocouple, while on newer units with electronic ignition, the same (or virtually the same) device is called a flame sensor. Here it is simply called a thermocouple.

    The thermocouple is part of the pilot burner assembly and connects to the gas control valve on the outside of the water heater. It is a simple device that converts the...MORE heat from a burning flame into a small electrical current, allowing it to work like a switch to control the flow of gas from the gas supply valve. In simple terms, when the thermocouple does not sense flame, it prevents the gas valve from delivering gas to the burner. It is, therefore, a key safety element of your water heater system. Often a bad thermocouple is the reason a water heater pilot does not ignite or a burner does not stay lit.

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  • 02 of 08

    Standing Pilot vs. Electronic Ignition

    Thermocouples with standing pilot and electronic ignition

    The type of ignition system you have in your water heater will determine how you replace the thermocouple. With both types of ignition—standing pilot or electronic—the thermocouple is part of the burner assembly. With a standing pilot, there is just a pilot and thermocouple attached to the burner assembly. With an electronic ignition, there is a pilot, thermocouple, and electronic igniter attached to the burner assembly.

    Tip: If you don't know which kind of water heater you have, look for a...MORE pilot flame: standing-pilot water heaters have a small pilot flame burning at all times. With electronic ignition heaters, the pilot flame comes on only when the thermostat calls for the burner to ignite, and the pilot goes out after the heating cycle.

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  • 03 of 08

    Disconnecting the Control Valve

    Disconnecting the connections to the gas control valve is the first step when servicing either an electronic-ignition or a standing-pilot water heater:

    1. Turn the gas control valve on the water heater to the OFF position. 
    2. Close the shutoff valve on the gas supply piping to the water heater.
    3. Remove the cover panel exposing the burner assembly (standing pilot) or burner manifold cover plate (electronic ignition).
    4. Use an adjustable wrench or open-end wrench to loosen and remove the thermocouple...MORE connection, the main burner supply tube, and the pilot supply tube from the control valve. For electronic ignition heaters, remove the wire to the Piezo igniter.

    Note: If you have an electronic-ignition water heater, proceed to the next slide. If you have a standing-pilot water heater, skip ahead to slide 7: Standing Pilot Water Heater—Replacing the Thermocouple.

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  • 04 of 08

    Electronic-Ignition Water Heater—Removing Burner Assembly Manifold Cover

    Electronic ignition water heaters have a burner assembly manifold cover to seal the combustion chamber.

    The combustion chamber of water heaters with electronic ignition is sealed tight and gasketed. To access the combustion chamber, you must remove the burner assembly manifold cover, which contains the burner supply tube, burner, electronic ignition, pilot, and thermocouple. Follow these steps to remove the manifold cover:

    1. Remove the fasteners securing the burner assembly manifold cover plate. Depending on the brand and model of water heater, the panel may be secured by nuts, screws, or specialty...MORE tamper-resistant fasteners such as #20 Torx-head screws.
    2. Grab the burner supply tube and push down slightly to free the manifold, pilot tube, and thermocouple connections.
    3. Gently remove the manifold/ burner assembly from the burner combustion compartment. Take care not to damage the internal components. If the burner does not move easily, check around the base: there may be additional screws holding it down.
    4. Carefully remove the old gasket from around the manifold cover plate if it is in good condition and you would like to reuse it. Otherwise, remove the gasket and plan to buy a replacement.
    5. Clean the metal surfaces between the cover plate and the water heater.
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  • 05 of 08

    Electronic-Ignition Water Heater—View of Burner Assembly Manifold

    Burner assembly manifold.

    Once removed, the burner assembly for an electronic ignition water heater should look like the unit in this photo.

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  • 06 of 08

    Electronic-Ignition Water Heater—Replacing the Thermocouple

    Thermocouple is fastened to the burner/pilot assembly.

    Replace the thermocouple as follows:

    1. Slowly and firmly pull and twist the old thermocouple from its mounting bracket. It should come out easily if you use firm pressure. Make sure you do not bend or deform the connection between the pilot and the thermocouple bracket.
    2. Remove the old thermocouple completely by pulling it through (from inside to outside) the rubber grommet seal, or by opening in the burner assembly manifold cover. Cut the old thermocouple, if needed.
    3. Take the old thermocouple to the...MORE hardware, home improvement, or plumbing supply store to buy an exact replacement. Also buy a new gasket for the manifold cover plate, if you are replacing the old one. 
    4. Insert the new thermocouple from the outside through the grommet or opening in the burner assembly manifold cover.
    5. Insert the thermocouple into its mounting bracket until it is fully seated or clicks into place.
    6. Position the thermocouple tip so that the pilot flame will envelope the top 3/8 to 1/2 inch of the thermocouple tip.
    7. Install a new burner manifold cover gasket, or reinstall the old one if it is in good condition.
    8. Place the burner in the combustion chamber and properly seat the unit.
    9. Reinstall the cover assembly. Do not over-tighten the screws or fasteners.
    10. Skip to slide 8: Reconnecting To the Gas Control Valve to complete the project. 
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  • 07 of 08

    Standing-Pilot Water Heater—Replacing the Thermocouple

    For older-style water heaters with standing pilot lights, removing and replacing of the thermocouple is very simple: 

    1. Once the pilot supply tube, burner supply tube, and thermocouple are disconnected from the gas control valve, simply lift and remove the burner assembly from the bottom of the water heater.
    2. Slowly and firmly pull and twist the old thermocouple from the thermocouple bracket. It should come out easily if you use firm pressure. Be careful not to deform any of the parts.
    3. Take the old...MORE thermocouple to the hardware, home improvement, or plumbing supply store to buy an exact replacement.
    4. Insert the thermocouple into the bracket from which you removed it, until it is fully seated or clicks into place.
    5. Position the thermocouple tip so that the pilot flame will envelope the top 3/8 to 1/2 inch of the thermocouple tip.
    6. Reinsert the burner in the combustion chamber and properly seat the unit.
    7. Proceed to the next slide for the final project steps. 
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  • 08 of 08

    Reconnecting To the Gas Control Valve

    For both electronic-ignition and standing-pilot water heaters: Once the new thermocouple is installed, reconnect the pilot, burner, and thermocouple (and electronic igniter for electronic-ignition unit) to the gas control valve as follows:

    1. Reconnect the burner supply tube, pilot supply tube, and thermocouple to the gas control valve. The thermocouple should be no more than 1/4 turn beyond hand-tight. Also reconnect the Piezo igniter on electronic-ignition units.
    2. Open the shutoff valve on the gas...MORE supply piping. 
    3. Turn the gas control valve to the ON position. For standing-pilot units, relight the the pilot, per the manufacturer's instructions before setting the valve to ON.
    4. Set the water heater thermostat to the desired temperature.
    5. Check the main supply tube and pilot supply tube at the gas control valve for gas leaks, using a soap-and-water solution. Coat the fittings with the soapy water, and look for leaks. Bubbles indicate a leak. If a leak is found, tighten all gas connections and retest them with the soapy water until no leaks are detected.