How to Replace a Water Heater Thermocouple

Water heater thermocouple

If your gas water heater is not heating due to a bad thermocouple, don't worry. We will show you how to replace a water heater thermocouple, install, and test it for proper work.

Also, we will talk about common problems, troubleshooting tips, and how to repair it successfully.

How to replace a water heater thermocouple: Things to consider

• What is a thermocouple?
• How thermocouple works
• How to replace a water heater thermocouple
• Common thermocouple problems, troubleshooting and repair tips
• How to test a thermocouple for voltage output

What is a water heater thermocouple?

A thermocouple is an important water heater part - a heat-sensing safety device that is used on natural and propane gas appliances and installed in the pilot light burner assembly.

It is designed to send the signal to the gas valve to open gas so the pilot light and main burner can be ignited.

How thermocouple works

A gas water heater can use a pilot light with the thermocouple or electrodes (electronic ignition) to ignite the gas on the main burner and in the combustion chamber. The electrodes are also used as the safety elements to inform the gas valve/electric board that the gas burner is lit.

In the case of the standing pilot, it is always on, day and night, heating the thermocouple constantly, while the 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 consists of several essential elements:

• Main gas burner
• Main burner orifice
• Burner gas supply tube
• Pilot burner
• Pilot burner orifice
• Pilot burner supply tube
• Thermocouple

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

The 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 keeping this safety valve open and 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 anymore, 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 an electric current.

As you can see, the main purpose of the water heater thermocouple is to prevent the gas regulator from sending 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 the thermocouple to get hot and send the signal to the gas regulator to open.

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

There are hot water heaters with a TCO or thermal cutoff device that is integral to 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.

How to replace a water heater thermocouple

Note: A certified individual should be dealing with the gas appliances and their elements. A manufacturer user guide, including this article, should be used only as a reference.

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

• The main gas valve on the unit can be closed, but it is not mandatory. Gas from the mainline cannot pass the regulator and out. If the thermocouple is broken, there is no signal to open the gas port.
• At the bottom of your tank water heater, there is an access panel that needs to be removed so that you can clear the way to the broken element.
• Unscrew the thermocouple securing nut that is attached to the gas regulator and removes it away.
• Take the flame-sensing part using the pliers. Be careful not to damage the copper tube. Pull it out to dismantle it from the pilot assembly.
• To install the new water heater thermocouple back, follow the steps above but in reverse order. Once the replacement is done, the ignition process can be successfully finished.

A thermocouple is not expensive (for example, a 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? The recommendation is always to look for the genuine OEM part, and if the aftermarket product is your preference, try to match the original part.

Common thermocouple problems, troubleshooting and repair tips

If the pilot light doesn't stay ON when you release the red button, the thermocouple is loose or defective. The solution to this problem 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 the pilot flame is not enveloping the thermocouple hot junction, inspect the part and ensure that it is correctly installed.

If the burner doesn't stay lit, the problem might be with a thermocouple malfunction, so you would need to replace a pilot assembly.

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

How to test a thermocouple for voltage output

Closed-circuit testing is the recommended method for testing the hot water heater thermocouple. Light the pilot and keep it on for a 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 that value, replace it.