Gas Water Heater Thermocouple Replacement

How to replace a thermocouple and gas pilot light assembly on tank type gas water heaters

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Hot water heater thermocouple is a heat-sensing safety device that is used on the natural and propane gas appliances and installed in the pilot light burner assembly. Thermocouple is the safety element, in contact with pilot light, so it can send the signal and inform that a gas water heater pilot light and the main burner are ignited.

In this article, we will show you how to test the thermocouple on the water heater, installation instructions, how to replace the thermocouple, problems and troubleshooting.

How it works

Gas water heater might use a pilot light with the thermocouple or electrodes (electronic ignition) to ignite the gas on the main burner and in the combustion chamber. Thermocouple or electrodes are used as safety elements to inform the gas valve that the pilot or main burner is lit.

The main difference is that the pilot is always on, day and night, heating the thermocouple constantly, while electronic ignition produces sparks only if there is a need for hot water. Electronic ignition is common on tankless water heaters and they are not using the thermocouple.

The gas burner assembly on the storage tank water heaters is consisted of several essential elements:

  • Main gas burner

  • Main burner orifice

  • Burner gas supply tube

  • Pilot burner

  • Pilot burner orifice

  • Pilot burner supply tube

  • Thermocouple

Hot water heater thermocouple has two ends. One end is connected to the gas regulator and the other end of this copper tube has a sensing probe that is immersed in the pilot flame.

water heater thermocouple pictureThe pilot flame heats one end of the thermocouple. As it gets hotter it generates a small electrical current that is transferred to the gas regulator. The current powers the electromagnet located in the gas valve and keeps this safety valve open, as long as the pilot flame is heating the thermocouple.

If the pilot light blows out, the contact is lost and the thermocouple cools off. Since the pilot flame doesn't generate the electricity any more, there is no signal going through the thermocouple and to the gas regulator, so it shuts down.

The thermocouple has the sensing probe and copper tube which are dissimilar metals, joined together. When the joint is heated it produces the electric current.

As you can see, the main purpose of the water heater thermocouple is to prevent the gas regulator to send a gas to the burner without being lit, so the regulator actually stays closed.

This is why you have to press the piezo button and keep it depressed for a while when trying to light the pilot. It takes some time for thermocouple to get hot and send the signal to the gas regulator to open.

In order to have an unobstructed water heater performance, thermocouple as a safety device has to work properly so it can transfer the impulse to the regulator constantly. If not, you have to test it for the proper voltage output and replace if it is faulty.

There are hot water heaters with a TCO or thermal cutoff device that is integral with the thermocouple. It is used to shut off the main and pilot burner gas flow when the temperature increases in the combustion chamber high above desired.

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Replacement - How To

Note: A certified individual should be dealing with gas appliances and its elements. Manufacturer user guide should be used as a reference only.

Thermocouple replacement is an easy job. How to do it yourself and change a broken thermocouple, read further.

  1. Main gas valve on the unit can be closed, but it is not mandatory. Gas from the main line cannot pass the regulator and out. If the thermocouple is broken, there is no signal to open the gas port.

  2. At the bottom of your tank water heater, there is an access panel that needs to be removed, so you can clear the way to the broken element.

  3. Unscrew the thermocouple securing nut that is attached to the gas regulator and remove it away.

  4. Using pliers take the flame-sensing part. Be careful not to damage the copper tube. Pull it out to dismantle it from the pilot assembly.

  5. To install the new water heater thermocouple back, follow the steps above in reverse order. Once the replacement is done and ignition process can be successfully finished.

Thermocouple is not expensive, (for example, full kit from Rheem might cost you around $30) and you can purchase it online, from many electrical or home improvement stores like Home Hardware, Home Depot, Lowe's...

Do you have an American water heater, Whirlpool or AO Smith? Recommendation is always to look for the genuine OEM part, and if the aftermarket product is your preference, try to match the original part.

Problems with the thermocouple and troubleshooting

If pilot light doesn't stay when you release the red button, thermocouple is loose or defective. Solution for these problems is to tighten the loose security nut or to change the whole part. If the thermocouple is not fully engaged in the pilot assembly bracket, adjust the tip of the element, and if pilot flame is not enveloping the thermocouple hot junction, inspect the part and ensure that is properly installed.

If the burner won't stay lit, the problem might be with a thermocouple malfunction and corrective action requires pilot assembly replacement.

If you experience the pilot outage, thermocouple malfunction might be one cause or tip of the tube might not be in contact with pilot flame, simply re-position it so pilot flame can touch it.

Testing the thermocouple voltage output

Closed circuit testing is the recommended method for testing the hot water heater thermocouple. Light the pilot and keep it on for few minutes. If the pilot light won't stay on, hold the pilot button and test the thermocouple.

Use the multimeter with the millivolts setting and measure the voltage output. Connect one lead of the multimeter to the copper sheath of the thermocouple and the second lead to probe the top terminal at the gas control valve. The reading should be 10 millivolts (13 millivolts on some gas valves) or higher when it is properly working. If it is below replace it.

(All pictures are courtesy of