What Is A Thermocouple & How Does It Work? (Find Out Now!)

Ryan Womeldorf
by Ryan Womeldorf

In order to heat your home properly, your furnace has several components that work in tandem with one another. When those components do their jobs, the air that gets sucked into your furnace is heated and then blown back out into the home.

The thermocouple is an important safety device within your furnace system. It detects whether or not the pilot is lit. The pilot is essential as it sends voltage into the thermocouple, keeping the gas line open to produce warm air.

What is a Thermocouple and How Does it Work?

The main responsibility of the furnace’s thermocouple is to monitor the pilot light and make sure it is performing. One end of the thermocouple is attached to the gas line that supplies gas to the pilot light. The thermocouple is what controls the supply of gas to the pilot light at all times.

The thermocouple’s other end, a metal rod, gets pointed right over the flame of the pilot light. The rod reads the flame’s temperature accordingly. The temperature then gets registered by the rod, providing a small amount of voltage to keep the gas supply open. When the pilot light goes out, there is no more heat to keep said voltage going, so the gas supply will eventually shut off.

How the Thermocouple Improves the Safety of Your Furnace

The various components of your furnace are not solely about creating heat. While that is the main objective, there are components that regulate the safety of the furnace as well. The thermocouple is one of those tools. More importantly, it helps to improve the safety of your furnace in two different ways.

  • Prevents gas buildup. When the pilot light goes out, there is no flame available to burn gas that gets delivered to it. With no thermocouple, gas can potentially build up inside of the furnace. A gas buildup can lead to small explosions within the furnace that can impact the burners. The thermocouple works to shut off the gas supply going into the pilot light when that flame goes out, preventing those explosions from taking place.
  • Prevents gas leaks. Another issue pertaining to the pilot going out is potential gas leaks. Remember, when the flame goes out, there can be gas lingering. In some cases, that gas can leak out into the air in your home. That gas can be a potential health issue and can be a fire hazard as well. The thermocouple works by shutting off that gas supply the moment it realizes that the pilot light is no longer on.

Common Issues with a Furnace’s Thermocouple

There are a few common issues that can impact the performance of your thermocouple, thereby impacting the rest of the system. Some are relatively simple to fix and can even be done by a do-it-yourselfer. Others are more serious and require professional intervention.

  • Dirty thermocouple. Just like any other component in your furnace, there are times where the thermocouple simply becomes dirty and clogged. Cleaning it is simple enough if you know how to locate the thermocouple. Gently cleaning it off should leave it working optimally once again.
  • Dirty air filter. How did the thermocouple get to be so dirty? The most likely reason is a dirty air filter. Dirty air filters can cause several issues within your furnace. When they get clogged up, they stop catching dirt and debris. Those things then get into your HVAC system, collecting on the other components within. Clean or change your air filter when this happens and consider changing the filter on a monthly basis to ensure that it stays clean and efficient.
  • Regular tune-ups. Some thermocouples are simply not rated to last for the life of a furnace. Schedule a tune-up on a yearly basis. The tune-up will take a closer look at all of the components within your furnace, pinpointing which ones are working fine and which need replacing. When the thermocouple is old or worn down, it may need to be replaced. The tune-up will catch that and handle the issue.

How to Tell if a Thermocouple is Going Bad

While there are plenty of reasons that can lead to your thermocouple losing efficiency, there are also signs that it is going bad altogether. These signs are essential so that you can replace the thermocouple, improving the efficiency and safety of your furnace.

The pilot won’t stay lit. Generally speaking, you can tell when the thermocouple is going bad when the pilot light won’t stay lit. If the flame won’t light at all and you are certain that the gas is on, there are two causes. One is an obstruction somewhere in the process. The other is a bad thermocouple.

If the flame lights but immediately goes off when you let go of the gas control knob, that is a clear indicator that the thermocouple is bad. You can also look right at the pilot flame. If it is orangeish-yellow, it may be too weak to properly heat the thermocouple.

Thermocouple Probe Too Far Away

If you want to troubleshoot the issue on your own, try one more thing before blaming the thermocouple. Keep in mind that the pilot flame has to be both hot enough and big enough to heat the thermocouple so that it generates enough voltage.

From time to time, the problem is that the thermocouple’s probe is just too far away from the flame. You can simply reach in and reposition the probe so that it is closer to the flame. Make sure that there are no obstructions between it and the pilot tube as well. That can prevent the pilot from staying lit, too.

Testing the Thermocouple

For most gas appliances, furnaces included, the thermocouples will have a standard design. The thermocouple generally uses a probe that is attached to a copper tube that screws into a gas valve port. When the thermocouple is working, you will be able to measure around 30-40 millivolts of voltage when the pilot flame is on.

Test this yourself using a multimeter. The tricky part is that you may need someone to keep the pilot lit as you test the thermocouple. Unscrew the thermocouple, set your multimeter, and start the pilot. After a minute or so, place a lead on the shaft of the thermocouple with the other lead going to the connector. Any reading less than 25 millivolts means that the thermocouple is bad. Replace it immediately.

How Do I Light the Pilot on my Automatic Furnace?

Older furnaces have pilot lights that need to be lit by hand. Newer furnaces have automatic pilot lights, meaning they are controlled by the thermostat and only ignite when they are needed. Older pilots burn constantly; automatic furnaces shut off the gas supply when the pilot is lit.

Automatic furnaces can’t be lit by hand. That means there is a different way to perform the relighting process compared to the traditional way that you would light an older gas furnace. There are several steps required to relight your automatic furnace’s pilot light.

Lighting an Automatic Furnace Pilot Light

  • Set the thermostat. Start off by setting your thermostat to the lowest setting. Kill the power to the furnace, either through the off switch or by flipping the breaker that controls the furnace. Always turn off the power when working on your furnace.
  • Remove the access panel. Locate the access panel on your furnace and remove it. For most automatic furnaces, they are at the bottom. You need this to locate the gas control knob.
  • Gas control knob. Locate the gas control knob and turn it off. Depending on the model of furnace that you have, there may be two knobs. If there are two knobs, turn them both off. Give the furnace 5-10 minutes to allow for any residual gasses to clear out. Then turn the gas control knob(s) back on.
  • Restore power. Return the access panel to your furnace. When everything has been secured, you can turn the electricity back on. Set your thermostat to whatever temperature that you want. In a few seconds, the pilot should light itself, starting the burners up as well.

Does Gas Still Flow When the Pilot Light Goes Out?

The thermocouple detects the flame from the pilot light, waiting for it to hit the appropriate temperature before closing the gas valve. It is possible, however, for gas to still flow even when the pilot light goes out.

When the flame sensor fails, gas can continue to flow out even when the pilot light has gone out. Gas leaks or a buildup of gas can be a major safety hazard. Call your local HVAC technician immediately so that they can troubleshoot the issue and make your home safe once again.

Should the Flame Touch the Thermocouple?

We know that the thermocouple needs to be close enough to the pilot light to appropriately detect the flame. But should the flame touch the thermocouple? The answer is yes and it has to be at a specific level.

The flame from the pilot should cover the top of the thermocouple and burn steadily. If it sputters or stalls out, there is an issue at play preventing the pilot from staying lit.

Ryan Womeldorf
Ryan Womeldorf

Ryan Womeldorf has more than a decade of experience writing. He loves to blog about construction, plumbing, and other home topics. Ryan also loves hockey and a lifelong Buffalo sports fan.

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