Flame Sensor vs. Thermocouple: What Is the Difference?


Flame sensors and thermocouples are similar in many ways, including their purpose. Both are safety devices that help monitor flames inside equipment such as gas furnaces and boilers. However, several differences set them apart from each other, especially in terms of how they function. In fact, you can’t replace a flame sensor with a thermocouple or vice versa.

If you plan to buy any domestic or industrial gas furnace, you should learn more about them and their crucial components. Top heating service companies like Hurliman heating try to keep their customers aware of these details as they are extremely important.  

So, let’s get to it and check out how these two safety devices differ. However, before we discuss the differences, you need a solid understanding of what exactly flame sensors and thermocouples are and how they work. 

What is a flame sensor?

A flame sensor is a device that detects the existence of a flame and responds accordingly. The response may vary depending on the purpose of the system that the flame sensor is a part of. One of the most common uses of flame sensors is in industrial boilers and gas furnaces. Here, the flame sensor helps detect whether the equipment is functioning properly.

Other uses of flame sensors include fire alarms, natural gas lines, propane systems, domestic heating systems, etc. In case the purpose of the flame sensor relates to fire safety, the device will trigger an alarm or even a fire suppression system.

Flame sensors take less than a second to detect a fire or a flame in the area it covers. There are primarily three types of flame sensors based on their working principle:

  • Infrared (IR) flame sensors: As the name suggests, these sensors work by detecting infrared radiations. Hot gasses from flames emit specific infrared radiations that the flame sensors can detect. However, an IR flame sensor won’t work unless the flame has the usual flickering motion. 
  • UV sensors: Besides infrared wavelengths, flames also generate ultraviolet radiation. UV flame sensors monitor the UV readings from the point of ignition and produce electric pulses in case it detects a fire. Other components within the device then convert these pulses into alarms.
  • UV-IR flame sensors:  These sensors combine the two flame detection technologies we discussed above. They can detect both UV and IR waves and are therefore more reliable. UV-IR flame sensors are also less likely to generate false alarms. 

What is a thermocouple?                                                                                

Like a flame sensor, a thermocouple is a safety device used to detect the occurrence of fire or flames too. These versatile devices are suitable for pretty much any application that involves flame detection, which makes them quite popular. However, their working principle is different from that of a flame sensor.

The most important part of the device is a sensor that consists of two metallic wires made of two different metals. The two wires connect by being joined together on one end. Electric current flows in this thermocouple circuit constantly.

Now, when the junction of the two wires faces a temperature change, it results in a change in voltage. The device can now measure the voltage, calculate it back to the temperature that caused it, and detect whether the heat was from a flame.

Thermocouples are used in a wide range of equipment due to their compact style, simple working principle, and accuracy. 

Flame sensor vs. thermocouple

Well, now that you have a better idea of the two flame detection devices, we can move on to check out their differences. As we mentioned earlier, the two devices differ in several ways despite their similarities.

1. Detection times

Although both flame sensors and thermocouples have very quick detection times, the former is the winner in this aspect. A flame sensor detects flames instantly as all it has to do is measure the UV or IR radiation that the flame has already emitted. A thermocouple, on the other hand, has to heat up enough first.

2. Functions

Indeed, the two devices have a very similar purpose – detecting temperature changes. However, they do not always serve the same function. A thermocouple usually stays near the pilot light to ensure it’s functional. On the other hand, the flame sensors stay in the burner assembly and detect it once the burners are lit.

3. Versatility

A thermocouple is much more versatile than a flame sensor. In fact, you can use a thermocouple as a flame sensor, but not vice versa. This is because a flame sensor only detects flames, while a thermocouple can produce different amounts of current for different purposes.

4. Application in gas furnaces

Whether your gas furnace needs a thermocouple or a flame sensor depends on the ignition system. Gas furnaces with pilot lights need thermocouples, while electronic ignition systems require flame sensors.

5. Appearance

Not sure whether your gas furnace has a flame sensor or a thermocouple? Well, you can just check it out yourself. A thermocouple consists of a copper tubing, wires, and a bracket. A flame sensor, on the other hand, looks like a thin metallic rod (usually curved). Besides appearance, the device’s position should give away whether it’s a flame sensor or a thermocouple. 

What’s better – a thermocouple or a flame sensor?

Honestly, we can’t declare either of the two as a better option. While a flame sensor does better at detecting flames fast, a thermocouple is more versatile. Whether you need a thermocouple or a flame sensor depends on the heating unit and its technology. While furnaces with electric controllers use flame sensors, those with standing pilot lights use thermocouples.

In case your furnace or home heating system has developed a fault in its flame detector and you need a replacement, it’s best to consult a professional. Leading companies like Hurliman Heating not only know exactly what your heating system needs but can also provide you with the best components. 

Also Read: Health Benefits Of Artichoke Powder.

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