What Is a Thermocouple and What Does it Do in a Boiler?

Gas-powered home appliances require a reliable safety mechanism to prevent gas leaks. These appliances often use a pilot flame, which remains lit permanently to ignite the burner whenever needed. This mechanism is mostly used in gas fires, heaters, and boilers. If the gas supply is suddenly disrupted, the burner and the pilot flame would both extinguish. On resumption of gas supply, there would be no flame available to ignite the burner. Hence, the gas would continue to leak from the burner and the pilot, posing a serious threat of fire and explosion. That's why all such gas appliances are fitted with a safety device that forces-closes the gas inlet valve, preventing leaks. While it mostly goes unnoticed, this very important device is known as a "thermocouple".

A thermocouple, made of just two metallic wires joined at one end, plays a critical role in many home appliances. It is also used for many industrial purposes, as a safety device. Its role is not restricted to safety; it is also used for other purposes, such as temperature measurement and as a temperature sensor in various devices. But a very common question is: what a thermocouple actually is, how it works, and why it is so important for home appliances, such as boilers and heaters.

This blog explains everything about a thermocouple, so keep reading.   

What is a Thermocouple?

A thermocouple is primarily a heat sensor that converts heat into an electrical voltage. It comprises two dissimilar electric conductors. These conductors are made of different types of metallic wire, and both are joined at one end, with the other ends remaining open and separated from each other. When these wires are heated, a small electrical current is produced, known as "voltage", which is normally in terms of millivolts. This voltage can then be used to measure heat as temperature, since it is proportional to the difference between the temperatures of the thermocouple's hot and cold ends. Scientifically, this principle is known as the "Seebeck Effect”.

A thermocouple is frequently used in home and industrial appliances as a "heat or temperature sensor". The principle of the Seebeck effect is practically highly useful for home appliances like boilers and heaters, where a pilot flame is used to ignite the burner.

Irrespective of its scientific interpretation, a thermocouple can be defined simply as a device that generates a very small electrical current when heated. The generation of this very small current is a confirmation of the existence of a flame. When this flame goes off, for any reason, that tiny current disappears, which is an indication that there is no flame available. When it happens, the gas inlet valve automatically turns off, disconnecting the supply of gas to the burner, which ultimately prevents leakage of gas. 

A thermocouple is a simple yet highly effective safety device, which makes it one of the best options to prevent gas leaks in industrial and home appliances. Hence, these are widely used in home appliances, such as boilers, gas fires, ovens, and heaters, as an essential component of the gas safety control system.

Understanding How a Thermocouple Works

The working of thermocouples is quite simple:

• Most of the gas appliances are fitted with a thermocouple just adjacent to the pilot flame.
• The tip of the thermocouple is adjusted, right above the pilot. 
• When the pilot is ignited, its flame starts heating the tip of the thermocouple. 
• After a few seconds, when the tip of the thermocouple is heated to a certain level, it starts generating a tiny electrical current, in terms of millivolts.
• This tiny current is transmitted to the other end of the thermocouple, which is connected and integrated into the thermostatic device. 
• The current received at the thermostatic device is a confirmation that flame is available and live. Hence, the gas valve remains open, supplying the gas to the burner, and the gas appliance, such as a boiler, keeps running. 
• If, for any reason, the gas supply is not available, the pilot flame extinguishes. Resultantly, the current generation through the thermocouple is also disconnected. 
• When there is no current reaching the other end of the thermocouple, it is a confirmation that there is no flame. Hence, the gas inlet valve closes and the gas supply to the burner is disconnected. 
• This is the most critical step in the working of a thermocouple because when the gas supply is restored, the gas inlet valve still remains closed until the pilot flame is ignited again. 

What Would Happen If There were No Thermocouple?

If the gas appliance does not have a thermocouple and the pilot flame goes off due to the non-availability of gas, the burner will also turn off. But when the gas is restored and still no flame, the burner will not automatically be ignited. In this scenario, gas will keep coming out of the burner, which is a serious case of gas leakage. In such an event, a continuous gas leak may result in fire or explosion. On the other hand, continuous gas leakage may result in serious health issues. 

So, a thermocouple prevents all these possible consequences simply by cutting the gas supply to the burner. 

In simple words, the working of thermocouples can be explained as follows:

• If Pilot Flame is live, the thermocouple remains heated and the gas supply continues.
• When Pilot Flame is extinguished, the thermocouple cools down, and the gas supply is discontinued. 

Role of Thermocouples in Gas Safety 

All the gas appliances and allied installations are governed by the Gas Safety (Installation and Use) Regulations 1998. Under these regulations, all gas appliances must be gas safe, including "Flame Detection Safety Devices". The thermocouple is the cheapest and most effective flame-detecting safety device for home appliances. 

Hence, the majority of the boilers, especially the older ones, are fitted with this safety device. It fulfills the following basic gas safety protocols:

1. Prevention of Gas Leaks
2. Prevention and safety against possible explosions
3. Prevention and safety against possible health issues

However, the use of thermocouples has been discontinued with the introduction of “Resistance Temperature Detectors” (RTD) and “Thermistors” to detect when to stop the flow of gas. Despite that, the thermocouples are still widely used for gas safety in many other appliances.

How to Know If the Thermocouple is Not Working? 

When the following are noticed, especially in the case of boilers, it is a clear indication that the thermocouple is faulty.

• The pilot flame frequently goes off, despite repeatedly igniting it. 
• The boiler struggles to ignite even after repeated attempts.

Possible Reasons?

• The thermocouple has outlived its useful life.
• The thermocouple is physically damaged due to bending or corrosion. 
• Soot or dirt has accumulated on the thermocouple.

Important

1. The useful life of a thermocouple is normally around 5 years, after which it must be replaced, even if still working. 
2. Only a Gas Safe Registered engineer is authorized to replace the thermocouple. 
3. Always include checking and servicing of the thermocouple in the annual gas safety inspection of boilers. 

Other Uses of Thermocouples in Homes

In addition to the conventional utility of thermocouples in boilers, ovens, gas fires, and heaters, these are also quite useful for the following household purposes:

• In Electric Ovens and toasters, a thermocouple is used to control heat.
• Cooling temperature is effectively maintained in Refrigerators and Freezers. 
• In Air Conditioners, thermocouples are used to control the indoor air temperature.
• Thermocouples are quite effective in monitoring the floor temperature in underfloor heating systems.
• It is also frequently used in kitchen appliances, such as Coffee Makers, Microwave Ovens, and Deep Fryers.
• It is being effectively used in fireplaces and outdoor heaters, and Water Heating Systems. 

Disadvantages of Thermocouples

1. Their accuracy in temperature measurements is poor. 
2. Being made up of metal, these are likely to be affected by corrosion.
3. Their useful life is usually less than that of other modern temperature sensors. 

Conclusion

Although in modern boilers, thermocouples are now less commonly used, these can still be found in the majority of the old boilers, as well as other home appliances, because they are cheap and highly effective for gas safety. A thermocouple effectively prevents gas leaks in the event of a sudden or occasional gas outage. If the pilot light turns off due to any reason, such as a gas outage or any other fault, the gas inlet valve to that particular burner automatically closes, preventing gas leaks. If there is no thermocouple, the gas will start leaking out of that appliance, causing serious accidents like a fire or explosion, or serious health and medical issues. Hence, the thermocouples are essentially installed in home appliances, including boilers, ovens, heaters, and gas fires.

However, modern boilers are fitted with the alternative of thermocouples such as Thermistors and Resistance Temperature Detectors (RTDs). The reason behind that is the only drawback of thermocouples, i.e., inaccuracy in temperature measurements.