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Thermocouples: Everything You Need to Know (and then some!)

Thermocouple. It sounds like a complicated piece of equipment. And it no doubt, plays an important role in your fire feature. When it isn’t working properly, your gas fire pit will not stay lit. So, taking a few minutes to learn a little bit about this device will save you a lot of headaches down the road. You’ll be able to identify problems, troubleshoot them, and fix them yourself. 

What Is a Thermocouple?

A thermocouple is a safety device, which will turn off the flow of gas if a flame is not sensed.  While it’s an important safety feature for your gas fire pit, it can often be the cause of many problems.

A simpler name for a thermocouple could easily be a “flame sensor.”  It measures temperature and creates electricity, which keeps a solenoid opens and allows gas to flow. (a solenoid resembles a miniature, spring-loaded plunger.)

The thermocouple looks like a small nub, but inside the tubing, it is comprised of two different metals, usually wires. They are welded together at one end, referred to as the “hot junction,” while the other end is the “cold junction.”

BEYOND the normal range of measurement applications, the thermocouple can be designed to go to the extremes. Those designed for high heat are used in applications like nuclear reactors, while those on the other end of the spectrum are used for things like pharmaceuticals. 

Thermocouples can be made from a variety of different pairings, depending on the intended purpose. For instance, some metal pairings work best at subzero temperatures, while others work well in ultra-high heat situations. However, the rare metals used to make them are very expensive. For general applications, less expensive metals can supply reliable results.

How Does a Thermocouple Work?

The voltage generated by the thermocouple creates a magnetic force that keeps the gas valve open and flowing.

  • How does a thermocouple convert heat to electricity?

When sufficient heat is applied to the welded end (hot junction) of a thermocouple, the two metals in close proximity to each other produce a small amount of electricity across the opposite end. The amount of voltage created is in direct relation to the amount of heat applied. This is what is known as the “Seebeck effect.”

THOMAS JOHANN SEEBECK was a German scientist credited with discovering the thermoelectric effect in 1822 that is now referred to as the “Seebeck effect.” 

  • How does a thermocouple gas valve work?

The gas valve connected to the thermocouple works using a magnetic solenoid which is held open when electricity is produced by the thermocouple.  Simply applied to your gas fire pit, the magnetic surface keeps the solenoid open.  The solenoid acts as a safety valve allowing gas to flow to the burner as long as there’s a flame on the thermocouple. When the flame goes out (and therefore, the heat), the valve will shut off automatically. Below is an illustration showing how the thermocouple controls the flow of gas to your fire pit.

How a Gas Fire Pit Thermocouple Works - a look at the inner workings of a themocouple.

Why Do I Need a Thermocouple?

A thermocouple is a good safety feature to have.  If your gas fire pit will be used in a commercial space, such as a rental (perhaps an Airbnb), or in another publicly-accessible space – you should definitely have one.  

In fact, for any public access location, you should have a CSA-certified system. The CSA is a well-known certifying body for gas-fueled appliances. CSA certification is recognized in the U.S. and is often required by building inspectors.

If you’re building a DIY gas fire pit in your backyard for your personal use, a thermocouple is not a “must-have” component.  Ultimately it will be your decision on whether you need one for your fire pit.

What Does a Thermocouple Look Like?

The thermocouple looks similar to the tip of a thin ballpoint pen attached to a bolt with thin, copper tubing that connects to the back of the gas control knob. The thermocouple will most likely be hidden within a protective cage in your fire feature.  If you see a metal cage in your burner pan, look inside to locate the thermocouple.  This metal box keeps fire glass and lava rocks from interfering with the flame.

The thermocouple must always be in direct contact with the flame so it can quickly detect changes in temperature. This sensor provides protection against gas leakage and accidental combustion.

How to Test and Troubleshoot a Thermocouple

There are a number of things that can cause problems with your fire pit, but the thermocouple should be the first thing to look at if your flame won’t stay lit.  Understanding how they work makes testing and troubleshooting them quite easy.  

Anything that interferes with the flame coming into direct contact with the thermocouple is a problem. That heat is what activates the thermocouple and allows gas to flow. 

Thermocouple Testing Checklist

  1. Locate the metal cage and make sure there is no glass on top of it. There should never be glass on the top of the cage. This obstructs the airflow which diminishes the flame, which must be in contact with the thermocouple.  If there is no glass on top, push the fire glass away from all sides of the cage. If you can get the flame to stay lit, then gradually move the glass back toward the sides of the cage until it causes a problem. 
  2. While you are eyeballing the cage, look to see if any bits of glass have worked their way into the cage. This happens quite often, especially when using fire glass in the smallest size category. (That’s why we recommend using the 1/2-inch size.)
  3. If you’re still having the problem, look inside the cage while the fire is on.  Locate the thermocouple and check to see if it’s in contact with the flame.  You may need to do this in the evening when the light is low so you can see the flame.  If the flame is missing, proceed to step 4.
  4. If the problem is not above the surface, you’ll need to take look underneath. Examine the copper tubing connected to the thermocouple to see if there are any major kinks or severe bends in the line or to see if it’s damaged.  Even though it’s metal, it can be fragile and sometimes gets damaged in shipping or installation. 
  5. If that looks fine, next try pushing in the knob and turning it, do you hear gas flowing? (If not, there may be a problem with your gas feed.)
  6. If you can hear the flow, try to ignite the flame. If it lights, keep the knob pressed in for 15 to 20 seconds, then let go of the knob. If the flame goes out immediately, the problem is the thermocouple. If it goes out gradually, it’s likely something else.

Unless damaged, thermocouples should last for the lifetime of your kit. But if you determine that yours needs to be replaced, we carry replacement parts for the kits we sell at Celestial Fire Glass. 

Replacing the thermocouple is something most people can do with minor effort. And as always, we’ll be happy to walk you through it. Don’t hesitate to ask for help.

A thermocouple helps to ensure the safety of your fire pit so you can gather around it with family and friends and enjoy it with confidence. Using our handy checklist and the expertise of our Tech Team, you will likely have it working again in no time.  

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