Gas Water Heater Pilot Light Won’t Stay Lit: Thermocouple Replacement

Updated March 20, 2019

The pilot light is a component of a gas water heater that functions to ignite the gas burner. If you are getting no hot water at the moment and you’ve noticed that the pilot light in your gas water heater won’t stay lit, the most likely scenario is that the thermocouple is the issue.

So how can you fix the thermocouple if the gas water pilot light won’t stay lit? This article will talk you through a thermocouple replacement, in simple steps.

In general, a thermocouple is an electrical device used to measure temperature and they are used quite widely for a variety of applications such as in power generation, industry and domestic appliances.

There are many different types of thermocouple which are determined by the types of metals used to make them and this determines how and where they are used.

The thermocouple consists of two different metal plates which are separated at one end and joined together where the thermocouple connects to the metal tube. In a gas water heater, the thermocouple generates a current when it gets hot which travels along the metal tube and activates a spring mechanism in the gas valve, enabling the pilot light.

If the current generated by the thermocouple is not strong enough it will disengage the spring mechanism which closes off the gas valve and does not allow fuel to flow to the pilot light. This keeps the pilot light from staying lit.

By having a faulty thermocouple you will still be able to light the pilot light but it will not stay lit. This is why we need to first check if indeed the pilot light won’t stay lit, and then test if it is the thermocouple that is causing the issue.

Still not entirely sure what a thermocouple is? Take a look at the following video, it might help you!

Need help with a repair or install? Get a free estimate online from top local home service pros in your area.

How to Light a Pilot Light

The first step is making sure your gas water heater pilot light won’t stay lit when you have tried to relight it yourself. If you already know how to relight a pilot light and have already tried, feel free to skip ahead.

Newer water heater models have a self ignition function which makes this slightly easier and safer. Either way, these instructions are just general and it’s important that you refer to the manual provided by the manufacturer of your water heater (the instructions for how to light the pilot light may be printed on your water heater).

  1. Find the regulator and turn it to the “off” position.
  2. Wait at least 5 minutes for the gas to disperse.
  3. Turn the regulator to “pilot”
  4. If Self Ignition: Hold the ignition button or switch for a minute and then turn regulator to “on”
    If manual Ignition: Using a long reach lighter (such as a BBQ lighter) and ignite where the gas supply pipe is.

Now wait to see if your pilot light won’t stay lit. Most of the itme it will go out as soon as you let go of the control knob or ignition button and if it does not then it is time to check your thermocouple.

Need help with a repair or install? Get a free estimate online from top local home service pros in your area.

How to Test a Thermocouple on a Water Heater

For the thermocouple replacement test you are going to need a multimeter. Multimeters are affordable and useful pieces of kit and can be easily obtained such as the Fieldpiece HS26 found on Amazon. A thermocouple sends a current of between 25- and 35- millivolts to activate the spring mechanism in the gas valve and you will be using your multimeter to detect that current.

  1. Set your Multimeter to read millivolts (mv)
  2. Connect one lead with an alligator clip to the copper tube
  3. Disconnect the nut of the gas valve
  4. Connect the other lead to the nut of the gas valve
  5. Start the pilot light and keep the gas control held down
  6. If the mv reading is less than 20mv, the thermocouple will need replacing

If the reading if giving you more than 20mv, it is likely that the thermocouple is not the problem and there may be some issue with the gas valve supplying fuel to the pilot light. In this situation, the gas pipe is blocked and insufficient fuel is the reason the gas water heater pilot light won’t stay lit.

How to Replace a Thermocouple on a Gas Water Heater

Replacing a thermocouple is relatively easy, you only need to know how to use a wrench and have a new thermocouple to replace your old one. A thermocouple is very cheap and easy to buy on Amazon such as the Honeywell 24-Inch Replacement Thermocouple which should work just fine but check the size, length and specifications of your old thermocouple before buying a replacement!

  1. Turn the gas shut off valve to the water heater to the “off” position before starting
  2. Turn off the control and shutoff valves on the water heater
  3. Remove the burner access covers
  4. Loosen the nuts that secure the gas, pilot and thermocouple pipes
  5. Remove the pilot light assembly and replace the old thermocouple with a new one
  6. Re-insert the assembly, reattach the pipes and replace everything as it was

This replacement can be difficult to perform on some gas water heaters as they have an enclosed burner chamber. If you find that this is the case with yours, it’s better to call a pro to handle the task.

You also should not smell gas during this operation except briefly when you have removed the gas pipe. You should also use soapy water around the nuts when you have put everything back together so that the bubbles produced give away any leaks.

If you do smell continuous gas or are suspicious of a leak you should immediately call your utility company. Even though replacing the thermocouple is a simple procedure, if you are unsure of your ability to do this at all, it’s still better to have a service professional to do it just to be safe.

After the thermocouple replacement is complete you should find that your pilot light stays lit now and you should have no problems with your hot water unless there are other underlying issues with your water heater.

Need help with a repair or install? Get a free estimate online from top local home service pros in your area.

About the Author

Dave Miller is a HVAC technician with over 10 years in the industry. Dave created HeatTalk with the ambition for it to become a resource for individuals looking for answers, whether they be a layman, student or a professional.

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