Boiler Overheating: Causes, Signs & How to Fix It

Boiler Overheating: Causes, Signs & How to Fix It

Electric Heating Questions

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 and know how to recognise the signs of an overheating boiler.


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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.

Potential risks of an overheating boiler

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.

Nonetheless, 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.

Boiler overheating causes

If an overheating boiler isn’t dealt with early on the unit becomes a risk to everyone in the property and the surrounding area.

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

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 to find a Gas Safe registered engineer, make sure that they’re listed on the Gas Safe register and when they arrive at your property remember to ask to see their Gas Safe ID.


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How to fix an overheating boiler

First and foremost, if you’ve noticed signs of the boiler overheating, turn the water supply off straight away. Not turning the water supply off will cause cold water to enter the boiler and turn into steam once it reaches the overheating unit which, in turn, will cause the pressure to rise to dangerous levels.

Once the water supply has been turned off, a Gas Safe registered engineer will be needed to diagnose and fix the problem that has caused the boiler to overheat.

Limescale build-up on the heat exchanger

The heat exchanger uses hot gas to heat cooler water that can then be circulated around the radiators to heat up the rooms. Over time, limescale can build up on the heat exchanger and restrict the flow of water.

This not only puts the boiler at risk of overheating but will make the system more expensive to run and potentially 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

A build-up of limescale, sludge and other debris in the central heating system can be detected before the boiler will begin to overheat.

If you’ve noticed that any strange sounds from your central heating system or that any radiators haven’t been heating up properly, then they’re strong indications of a blockage. Radiators not heating up properly 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 so that the hot water can circulate freely around the radiators to heat the space efficiently and effectively.

Faulty 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 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 in many cases it might need replacing.

Can a 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.

How to prevent your boiler overheating

A build-up of sludge, limescale and other debris in the heating system is one of the most common causes of boiler overheating but this can be managed.

Installing a magnetic filter will catch any debris circulating around the system before it has a chance to cause a blockage. Many heating engineers will recommend having a magnetic filter installed as standard along with a new central heating system.

For any properties with an older central heating system, the best way to prevent the unit from overheating is to simply have a new boiler installed.

Consider a new boiler

If you have an old boiler, any unit that was installed more than 8 years ago, you should have it replaced with a modern boiler. Modern boilers are able to withstand higher pressures and will also alert you to any issues so that you can hire a heating engineer to repair them before they become hazardous.

An additional benefit of replacing an old boiler with a modern unit is the higher efficiency. Modern boilers can achieve efficiencies of around 93%, compared to roughly 70% for older heating systems.

To put these percentages into context, for every £1 you spend on heating your home with a boiler that’s 93% efficient, only 7p is wasted. That’s a considerable saving over boilers with a 70% efficiency.

It’s also worth knowing that the older boilers get, the more likely they are to experience problems and the harder it becomes for heating engineers to source replacement parts. A modern central heating system, on the other hand, will be much less likely to break down and in the unlikely event that there is a problem, any necessary parts will be much easier to source.

Whether you’re looking for a new boiler or need your current heating system repaired, we can match you with Gas Safe registered engineers in your local area.

Simply take a minute to complete our online enquiry form and you’ll get free quotes from up to 3 heating engineers for you to compare. Comparing multiple quotes gives you the greatest chance of finding the most competitive price for the work.


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