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Boiler Overheating: Causes, Signs & How to Fix It

Boiler Overheating

Boiler overheating is a serious problem that in the most severe cases can put the unit at risk of explosion. Fortunately, modern boilers are fitted with safety features that mean the unit will lockout before the boiler overheats.

However, in the unlikely event that these safety measures fail, it's important to be prepared. That’s why it’s good to know how to recognise the signs of an overheating boiler.

Why do boilers overheat?

A boiler overheating is often the result of the safety features failing.

Modern boilers are designed to lockout when the system detects signs of overheating to prevent serious damage. More often than not, an error code will be displayed on the digital display to help you find out what's wrong with the unit.

Boilers will overheat when the hot water that should be sent to the central heating system, taps or a hot water cylinder is being prevented from circulating. This is normally due to a failed part, such as the pump, or a blockage in the system.

Why is an overheating boiler a problem?

Blink Boiler Overheating
In the event of a boiler overheating, components within the boiler could melt and release toxic smoke into the property. In the very worst case scenario, the unit could even explode.

Thanks to the safety features of modern boilers and the high pressures they can still operate under, the risks that can come from a boiler overheating aren't as concerning as they once were.

Can an overheated boiler explode?

In the most extreme circumstances, where boiler overheating isn't dealt with at the earliest opportunity, a boiler can catch fire, blow up or even explode.

However, it's important to stress that this is extremely unlikely. Particularly when it comes to modern boilers which will lockout at the first sign of an issue.

To ensure that your boiler is running safely you should arrange for it to be serviced annually by a Gas Safe registered engineer. Also, if your boiler was installed more than 10 years ago then you should seek a replacement as this will give you the opportunity to install a boiler with the latest safety features.

Signs of an overheating boiler

If your boiler begins to overheat then it will lockout. At this point, the boiler will display an error code. If the boiler is overheating, that's what the error code will tell you.

Causes of an overheating boiler

If an overheating boiler isn't dealt with early on the unit becomes a risk to everyone in the property and the surrounding area. Common causes of a boiler overheating include:

  • Build-up of limescale
  • Blockage in the system
  • Fault with the pump
  • Thermistor issue

Now let's take a look at each of these causes in more detail.

Build-up of limescale

Limescale build-up on the heat exchanger can restrict the flow of water and lead to your boiler overheating. The heat exchanger's role within the heating system is to heat cooler water before it's then circulated around the radiators. Not only putting the boiler at risk of overheating but could also reduce the lifetime of the boiler.

This build up of limescale is known as kettling as it can cause sounds resembling a kettle's whistle. So, if you hear the sound of a kettle, contact a heating engineer who will be able to flush out the system and clear it of any limescale.

Blockage in the system

Blink Boiler Replacement
If you've noticed that any strange sounds from your central heating system then that’s strong indications of a blockage. Another sign is if any of your radiators haven't been heating up properly. Radiators not heating up could also be a sign of excess air in the radiator, which can be released by bleeding the radiator.

At the point of a significant blockage, a heating engineer will be needed to perform a powerflush. A powerflush is a cleansing process that cleans the central heating system of all limescale, rust and various other bits of debris. This then allows the hot water to circulate freely around the radiators to heat the space efficiently and effectively.

Fault with the pump

The job of the pump is to circulate water around the central heating system. If it develops a fault then the hot water won't be able to leave the boiler which could cause the unit to overheat. A heating engineer might be able to repair the pump but it's most likely that a new pump will be needed.

Thermistor issue

The thermistor monitors the temperature of the water so that the PCB (printed circuit board) knows whether to increase or decrease the temperature depending on the demand. Should the thermistor fail then that could lead to the temperature of the water getting too high and result in the boiler overheating as a result. A fully-qualified heating engineer is the only person who can work on a thermistor and it will likely need replacing.

How to fix an overheating boiler

Fixing the issue that's resulting in the boiler overheating depends on the problem. All of the common causes that we've listed above will require a professional.

It's important to always hire a fully qualified engineer when it comes to boiler installation, repairs and servicing. Hiring someone without the appropriate qualifications could itself lead to your boiler overheating and worse.

By law, the only person able to work on a gas boiler is a Gas Safe registered engineer. So, when looking for a heating engineer, make sure they're listed on the Gas Safe register. Then when they arrive at your property remember to ask to see their Gas Safe ID.

How to prevent a boiler from overheating

Blink and Blaze
When it comes to a boiler overheating, prevention is better than the cure.

Many of the common causes are the result of a build-up of debris in the system – whether it's on the heat exchanger or in the pipes. Fortunately, a magnetic filter can be fitted to collect any rust and debris in the heating system before it has a chance to build-up and form a blockage.

You could also have central heating inhibitor added to the heating system. It's worth noting some manufacturers state that inhibitors must be added to the heating system on a regular basis to keep the warranty valid.

Finally, if you live in a hard water area then it's worth considering a scale reducer. A scale reducer will help to prevent the high amounts of limescale in hard water from building up in the heating system.

Taking these steps will help to keep your heating system running safely, reliably and efficiently.

For any properties with an older central heating system (installed more than 8 years ago), the best way to prevent the unit from overheating is to simply have a new boiler installed.

Get quotes for a new boiler

Time for a new boiler? You can get free quotes for a replacement boiler right here on Boiler Guide. Simply take a few moments to pass through our simple online form and you’ll soon be contacted by up to 3 local heating engineers. Each will offer a free no-obligation quote for you to then compare.


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Boiler Guide Team

About the author

Boiler Guide Team

Boiler Guide has been home to a host of heating experts who have helped millions of UK homeowners searching for advice and guidance. If you’ve got heating questions, we’ll have the answers.

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