Discover what a thermocouple is, how it works, and why it’s a crucial safety feature in boilers. Learn when to replace one and common signs of failure.
Want to know what a thermocouple is?
You’ve come to the right place – we’re going to explain everything you need to know about them here.
We’ll cover what a thermocouple is exactly, how it functions, what the different types are, whether all boilers use them, and what you can do if a thermocouple stops working.
Key Points:
Thermocouples are critical boiler components that control gas flow
Boilers need thermocouples to prevent gas leaks
Consider replacing a thermocouple every few years or so
Regular boiler servicing keeps a thermocouple running for longer
Unless you’re a boiler engineer, don’t try to fix a broken thermocouple yourself
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What Is a Thermocouple?
A thermocouple is your MVP when it comes to boiler safety, making sure your boiler doesn’t leak gas when the pilot light goes out.
The thermocouple produces an electrical current when the pilot light (a small flame that ignites your boiler) is on, which keeps the gas valve open. When the gas valve is open, your boiler burns the fuel and keeps you warm and toasty.
When the pilot light isn’t lit, the thermocouple stops producing an electrical current, which closes the gas valve and prevents gas from leaking out of your boiler.
This is a critical part of boiler safety because if the thermocouple stops working, gas can leak, leading to a risk of explosion or fire.
How Does the Thermocouple Work?
The thermocouple is a temperature sensor created using two different types of metal. When the pilot light is activated, the thermocouple is heated and the two metals generate an electrical current. This in turn tells the boiler’s control system that the pilot light is on, and that it’s okay to keep the gas valve open.
A thermocouple works because it follows the Seebeck effect, which is where a voltage is created when there is a difference in temperature – in this case, the temperature at the joined end of the metal is different to the other parts, creating a voltage.
So the pilot light heats the joined end of the metal, which starts the Seebeck effect, sending a voltage to the boiler’s control system, signalling it to keep the gas valve open.
Pretty smart right? And pretty essential to the safety of a gas boiler.
Advantages and Disadvantages of Thermocouples
Thermocouples have become a staple in measuring temperatures, but they have pros and cons to be aware of.
Pros
Wide temperature range – thermocouples can measure a huge range of different temperatures, from extreme lows to extreme highs.
They’re very versatile, coming in several types which can be used for different applications.
Thermocouples are generally cheap, especially compared to other temperature sensors.
They have a simple design – thermocouples are essentially just two wires held together.
Cons
Poor accuracy – thermocouples might have a wide temperature range, but they’re not great at providing very accurate readings.
Prone to corrosion – some thermocouples are particularly susceptible to corrosion, such as those that use iron wires.
They don’t last as long as other temperature sensors.
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Are There Different Types of Thermocouples?
Yes, there are quite a few different types of thermocouples and each type differs in materials used, temperature range, and other qualities. Here are some of the most popular types of thermocouples:
Type K
This is the most popular type of thermocouple and is made from a positive nickel or chromium alloy leg, and a negative nickel or aluminium leg (the positive and negative aspect is what allows the thermocouple to generate a voltage).
It works across a temperature range of -200°C to 1250°C, so it’s ideal for all sorts of applications, including boilers.
Type J
Another common type of thermocouple, Type J has a smaller temperature range than Type K (0°C to 750°C), but is cheaper. It uses a positive iron leg and a negative copper/nickel alloy, with the iron making it susceptible to moisture damage. Because of this, it’s exclusively used in dry applications indoors.
Type T
A type T thermocouple is normally used in fridges and freezers because of its capabilities at lower temperatures, but also in sensitive laboratory experiments.
Do All Boilers Use a Thermocouple?
A thermocouple is a must in a boiler with a pilot light… but what about boilers without pilot lights? This is exactly the case for modern condensing boilers, which don’t need pilot lights to work.
And with no pilot light, there’s no need for a thermocouple. Instead, modern boilers use advanced electrical ignition systems to work, and complex sensors such as Resistance Temperature Detectors (RTDs) and thermistors to detect when to stop the flow of gas.
Simply put, boiler technology has advanced beyond the need for a thermocouple, but there are still hundreds of thousands of UK homes using boilers with thermocouples.
Thermocouples vs RTDs
Thermocouples and RTDs function much the same way in that they are both designed to detect changes in temperatures, but there are differences between them.
RTDs actually have a smaller potential temperature range than thermocouples, but are much more accurate and capable of providing consistent temperature readings. They’re a lot more stable too, meaning they last longer than thermocouples and don’t need to be repaired/replaced anywhere near as often.
Generally, most boilers never need to worry about the extreme temperatures some thermocouples are capable of measuring.
Thermocouples vs Thermistors
Thermistors are similar to thermocouples in that they measure changes in temperature, but just like RTDs, thermistors are more accurate.
Also like RTDs, thermistors have a much smaller temperature range, which makes them more suitable for domestic applications where huge temperature swings are less likely.
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How To Tell When the Thermocouple Isn’t Working
There are some obvious signs to look for that’ll tell you your boiler’s thermocouple isn’t working properly. First off, check to see if your pilot light keeps going out – if so, a faulty thermocouple is likely to blame.
A pilot light going out can mean the thermocouple isn’t generating enough voltage to keep the gas valve open. This can be because the thermocouple is damaged, or has worn to a point over the years it’s started to malfunction.
Secondly, you might not be able to light your boiler at all, as some boilers have safety features that prevent them from igniting if the thermocouple is broken or not working properly.
A more serious issue to keep a lookout for is a gas leak, which a faulty thermocouple can cause. It’s more accurate to say keep a nose out for, because you’ll be able to smell a strong odour like a rotten egg, and if you do, evacuate your home and call a gas engineer immediately.
Fixing a Faulty Thermocouple
Right off the bat, we’ll say if your thermocouple is broken, don’t attempt to fix it yourself because not doing it properly could lead to bigger issues (such as the aforementioned gas leak!). It’s always best in this situation to contact a qualified gas engineer for your boiler repairs, as they’ll know exactly what to do to get your boiler back up and running again.
If you are a gas engineer, you can attempt to fix or replace a broken thermocouple. Here’s how:
Switch off the gas and make sure it’s turned off – you don’t want any gas to be flowing when you start repairing/replacing the thermocouple.
Remove the faulty thermocouple – use a screwdriver to remove the thermocouple, which you’ll find near the pilot light.
Install the replacement thermocouple – be sure to check the thermocouple is placed correctly to detect the pilot light flame.
Fire up the boiler – if installed correctly, the boiler will start as usual. Job done!
Should I Call a Gas Engineer?
Call a gas engineer if you spot any of the signs of a faulty thermocouple mentioned above, especially any signs of gas leaks.
In general, it’s always best to turn to the professionals if you have any inklings your boiler might not be working as it should. Boilers are complex machines juggling explosive gas after all, so always err on the side of caution and get a gas safe registered engineer to take a look.
Consider calling an engineer if:
You’re getting no hot water or central heating and you’re 100% certain it’s not because your boiler is losing pressure.
You’ve not had your boiler serviced in over a year. Regularly maintaining your boiler is the best way of preventing it from breaking down in the first place.
It’s been around five years since your boiler had a power flush. Think of this like a much-needed spa day for your boiler, as a reward for all the hard work it does keeping you warm.
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