How To Relight Your Water Heater’s Pilot Light

by Trevor Woods | Last Updated: May 10, 2020

Nothing ruins a hot shower faster than a snuffed out water heater pilot light, Brrrr! Standing pilot water heaters need this flame to stay lit at all times in order to heat up your water.

When this flame goes up in smoke, the pilot light needs to be relit so that the burner can ignite and bring your water back up to temperature. The number one cause of a pesky pilot light burning out is due to a faulty thermocouple.

No need to call a plumber, we will show you how to easily relight you water heater and troubleshoot your water heater’s thermocouple. Flame on!

How Do Thermocouples Work?

Before you relight the water heater pilot light, it’s helpful to understand the basic concept of thermocouples. A thermocouple is an electrical device that is comprised of two dissimilar electrical conductors forming an electrical junction.

The electrical conductors in thermocouples are two wires that are made from different metals that are joined at both ends inside the thermocouple. Where these wires are connected at the business end of the thermocouple is called the “hot junction”. Heating the end of a thermocouple at the hot junction leads to a flow of continuous current due to the thermoelectric effect. 

Water heaters and other gas-fed heating applicants are equipped with a pilot flame for igniting the main gas burner as needed. When this flame goes out, there is a risk of unburned gas being released and this can result in an explosion. Thermocouples are used in these appliances for the prevention of such accidents. The thermocouple acts as an essential safety device used for sensing when the pilot light is burning. 

The tip of the thermocouple is placed in the pilot flame, and a voltage is generated which flows into the gas valve and allows it to operate safely. When the pilot flame is lit the thermocouple remains hot, supplying a steady flow of electricity, ensuring that the gas valve stays open and the pilot continues to stay lit. When it goes out, the temperature of thermocouple falls, causing a drop in the voltage and the closure of the gas valve.

There are many different types of thermocouples and each has its own specific application. We will cover which style of thermocouples you are most likely to encounter.

Types Of Thermocouples 

You might be wondering whether every water heater thermocouple is the same. Various combinations of metals and calibrations are present in thermocouples. Base metal thermocouples are the most common ones, and they are referred to as Types J, K, T, E, and N. Noble metal thermocouple with high-temperature calibrations are named Types R, S, C, and GB.

Calibrations of different thermocouples have a varying environment and temperature range. And the diameter of the wire in the thermocouple determines its maximum temperature. Therefore, if the wire of the thermocouple is too thin, it would not be able to reach a full range of temperatures.

Type K thermocouples are used for most general applications like in your home because they are low cost and have an average temperature range. Thermocouples with a wider temperature range are usually seen in industrial settings. 

While selecting a thermocouple, one needs to pay attention to the temperature range, chemical and vibration resistance, installation requirements, and most importantly the appliance it is being used in.

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How To Light A Water Heater Pilot

Word Of Caution

Lighting a pilot light, especially on a hot water tank can seem like an easy task. However, if done incorrectly, it can be quite dangerous. Always be careful when working with gas appliances.

A lot of modern appliances now operate via electronic ignition. However, standing pilot systems are still widely used, that’s why it’s important to learn the procedures for lighting a pilot light. The basic principles can be applied to water heaters, furnaces, boilers, gas fireplaces, pool heaters, and so much more!

Before proceeding, we feel it prudent to mention that while relighting a pilot, you need to pay utmost attention to your safety. In case the pilot fails to ignite, or you feel something is amiss, do not hesitate to call a professional. Gas appliances can be dangerous if not handled properly.

Before attempting to light a standing pilot, it is crucial that you read the instructions that are written on the side of the hot water tank or in the owner’s manual. Every system can have slight changes to the usual methods, and thus it is key that you follow the manufacturer’s guidelines to make sure that no accidents occur. 

In case you are unable to see the combustion chamber while lighting the pilot, always use a flashlight instead of an open flame. There is no reason to take any chances. 

Sealed Combustion

First, are instructions for modern water heaters with a sealed combustion chamber. With these models of hot water tanks, you will not have to access the burner assembly to ignite the pilot with matches or a long-reach lighter.

You will find everything needed for lighting the pilot right to the burner control module or gas valve. A looking glass is present in the access panel, which can be used to make sure that the pilot is indeed ignited. 

1. Turn The Gas Valve To Pilot Position

The first step is to turn on the gas control knob to the PILOT position. This knob is found on the top of the gas valve.

2. Start The Flow Of Gas

You will then have to press and hold down the control knob to maintain gas flow.

3. Light The Pilot

You will find an electronic spark button mounted to the control module. This Piezo lighter will discharge an electrical spark every time it is depressed. You will hear an audible “click” when you are using the lighter.

While continuing to hold down the control knob use the Piezo lighter to ignite the gas. This may take multiple clicks if the pilot has been out for a while and the gas line is full of air.

You can look through the sight glass at the bottom of the combustion chamber to check to see if the pilot has been successfully lit.

4. Energize The Gas Valve

After you have confirmed that the pilot light is ignited visually, continue to hold the knob down for around 30 to 45 seconds. This will give the pilot adequate time to heat the thermocouple and confirm pilot light ignition to the gas valve. The pilot light will go out if the thermocouple is not been heated long enough.

Once you let go of the knob, ensure that the pilot remains lit. If the pilot light does fizzle out, start over and attempt to relight it again.

5. Turn The Burner On

With the standing pilot operating turn the gas control knob to the ON position. The burner should not take more than a few seconds to activate if the water temperature has cooled due to the pilot being out.

Depending on the size of your hot water tank your water should be back up to temperature in about 30 minutes. Done!

Open Combustion

Another system available on the market is an open combustion pilot system. In this case, the flame for lighting the pilot has to be provided by you. There is no electronic ignition involved.

1. Open The Combustion Chamber

The burner compartment panels have to be removed to open up the combustion chamber so you can light it. There is typically a metal cover panel that shields the gas lines and thermocouple and an inner door that can be removed or slid out of the way.

2. Turn The Gas Valve To Pilot Position

Next, make sure that the notch in the dial of the gas control knob is lined with the gas supply button. Turn the knob to PILOT position.

3. Start The Flow Of Gas

You will then have to press and hold down the control knob to maintain gas flow.

4. Light The Pilot

Now use a flame source like a lighter or long match and place it into the chamber so that the flame is directly beneath the orifice of the pilot supply line.

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5. Energize The Gas Valve

Confirm that the pilot is lit visually. Continue holding the gas supply button for around 45 seconds to heat the thermocouple. Let the button go, and if the pilot stays lit, you are ready to turn ON the gas control knob. 

If the pilot light goes out after you release the pilot button try lighting the pilot again, making sure to keep the pilot button depressed long enough to thoroughly heat the thermocouple.

6. Turn The Burner On

With the standing pilot operating reinstall the hot water tank’s cover panels and turn the gas control knob to the ON position. The burner should not take more than a few seconds to activate if the water temperature has cooled due to the pilot being out.

Depending on the size of your hot water tank your water should be back up to temperature in about 30 minutes. Done!

Extend Your Reach

Use a long flexible lighter to reach into the combustion chamber to light the pilot. No need so singe your fingertips!

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How To Test A Thermocouple

If your water heater pilot fails to stay lit, it’s time to test your gas system and see if something is worn out and needs to be replaced. Bad thermocouples are the number one reason why pilot lights fail to stay lit.

Testing Thermocouples Manually

To test for a faulty thermocouple, start by lighting the pilot light and holding down the pilot button for about 1 minute. This should give the thermocouple plenty of time to heat up.

Now release the button and check if the light is still there. If the light goes out or it has failed multiple lighting attempts it may have to be repaired or replaced.

Testing Thermocouples With A Voltmeter

A thermocouple can also be tested using a multimeter to confirm if it is worn or needs replacement. To do this, the thermocouple has to be removed from the appliance using a wrench. A metal nut holds the thermocouple into the gas valve body, which has to be rotated counterclockwise to be removed.

Now turn on the multimeter by flipping on the power switch and adjust the settings dial to DC millivolts (mV). Connect the leads of the multimeter to the thermocouple with the Red lead clamped to the thermocouple tubing and the Black lead clamped to the top terminal that inserts into the gas valve.

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You can now commence the test by starting the pilot light. Light the pilot as you normally would and continue to hold down the pilot button. Allow the thermocouple 30 seconds to 1 minute to reach its maximum voltage displayed on the meter.

The thermocouple should reach about 25 millivolts with a range between 17 and 32 mV being acceptable. If the thermocouple tests below this range, it needs to be replaced as it will fail to produce enough millivolts to keep the pilot lit. 

How To Maintain A Thermocouple

Thermocouples are pretty low maintenance but sometimes they need to be cleaned to make sure they generate the millivolts needed to maintain the pilot light.

Whether you have a sealed or open combustion chamber you need to get access to the thermocouple underneath the water heater. Use steel wool or emery cloth to gently clean the tip of the thermocouple of any soot or carbon buildup.

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Keep It Consistent

If you have removed your thermocouple to clean it, when you reinsert a thermocouple make sure that the depth of insertion remains unchanged. The same amount of thermocouple has to be exposed to the pilot flame.

Keeping your thermocouple properly maintained and ensuring that your thermocouple is not exposed to excessively high temperatures or corrosive environments can extend its lifespan and keep your pilot lit for years!

How To Replace A Thermocouple

If your thermocouple is not functioning correctly or tests poorly with your voltmeter, there is no reason to panic. It can easily be replaced by following the steps below.

1. Turn Off The Gas

The first thing that you need to do is turn OFF the shutoff valve on the gas line to your water heater. Turn the handle of the valve so that it is perpendicular to the pipe.

2. Remove The Burner Assembly

Next, unscrew the nuts at the control valve that secures the gas line, pilot tube, and thermocouple in place. Now you can slide the burner assembly out of the combustion chamber so it is easier to work on.

Handle With Care

Be careful not to kink or excessively bend the gas supply line or the pilot tubing. These are made of soft metals and can deform easily.

3. Remove The Thermocouple

Unscrew or pull out the old thermocouple from the burner assembly. Once you have the old thermocouple removed, you can measure it to find the correct replacement length.

When In Doubt

If you are unsure of the length of thermocouple you need, purchase a longer model. The excess length can always be coiled up and installed out of the way.

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4. Install The New Thermocouple

Insert the replacement thermocouple in the holder that the old one was mounted in. Make sure the depth of the thermocouple matches the previous thermocouple so that the tip of the thermocouple is positioned properly in the pilot flame.

5. Reinstall The Burner Assembly

Gently replace the burner assembly in the combustion chamber of your water heater. The burner assembly will typically have a bracket or mount that the assembly has to be placed on.

Brighten Up Your Workspace

Use a flashlight to look inside the combustion chamber to make sure the burner assembly is inserted properly into its mounting location.

6. Reconnect The Gas Valve

You will now have to reattach the gas line, pilot line, and thermocouple back into the gas valve. You can tighten the nuts with your fingers after threading them into place. They can be snugged up with the help of a small adjustable wrench. Make sure that you do not over-tighten them as these metals are soft.

7. Turn On The Gas

Once the burner assembly is reconnected, turn ON the gas valve that supplies your hot water tank.

8. Light The Pilot

Follow the pilot lighting instructions posted on the side of your water heater and visually confirm that the new thermocouple allows the standing pilot to stay lit.

Turn the water heaters gas valve to the ON position to ignite the main burner.

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9. Test For Leaks

Once you are done, watch out for gas leaks. Use a leak detector solution or electronic sensor to make sure your connections are airtight. Tighten as needed. Your water heater should be back up to operating temperature within an hour. Done!

Conclusion

Knowing how to relight your water heater and replacing the thermocouple is a great way to keep your hot water tank running when you need it and you can save a ton of money if you fix it yourself. So put on your tool belt and start checking your water heater tank today, so you can enjoy an extra warm shower tomorrow.

Happy Plumbing!

Trevor Woods is the founder of Plumb University® and he started in the plumbing and construction industry in 1997. Since then, his mission is to make plumbing repair and maintenance easy for everyone. And each year, he continues to help more people with their plumbing installation, care, and troubleshooting.