Boiler Overheating – Signs, Causes & How to Fix It

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Most property owners worry that their overheating boiler may explode, causing damage and injury.

Thankfully modern boilers are equipped with a handy safety feature that kicks in and shuts the boiler down when the boiler becomes overheated.

However, while this may seem the answer to your ailing boilers’ worries, it’s far from the solution.

An overheating boiler can result in significant damage to the appliance itself, resulting in the boiler requiring costly repairs or, worse still, replacement.

If you suspect your boiler is on the blink and suddenly running hotter than usual, there are some basic steps you should follow.

But, first, let’s run through why boilers tend to overheat, the potential risks, and what to do to prevent this situation from occurring.

Boiler overheating causes

Boilers are meant to be hot, right? In fact, they are specifically designed to heat water and keep our homes nice and cosy. However, when they become too hot, that’s when the problems start.

Generally, overheating occurs when the hot water in the central heating system is restricted from circulating correctly.

This leads to a build-up of hot water, which triggers the safety feature in the boiler. This is called a boiler lockout. Usually, this indicates a system failure that could lead to the boiler dangerously overheating if not resolved.

The lockout prevents this danger, but it still leaves you ‘out in the cold.’ If your boiler has gone into lockout mode, you should see an error code on the display panel.

Is an overheating boiler dangerous?

When dealing with gas, electricity, and water, there is always a risk of things going wrong. After all, the three don’t have a good relationship when mixed!

However, today’s modern boilers are designed with intricate safety features and shut-off mechanisms; therefore, it’s unlikely that your boiler would explode if overheated.

That said, if the safety features on your boiler fail for some obscure reason, this is called a runaway boiler, which can be very dangerous.

Possible risks associated with overheating boilers:


Excessive heat and pressure can cause damage to the boiler’s internal components. As a result, the boiler may function inefficiently, or if the damage is significant, need to be replaced.

Toxic fumes

The internal components are primarily plastic which, if exposed to high heat, may melt and release toxic fumes.


While boiler explosions are rare due to their many safety features, unresolved issues such as a build-up of pressure or gas can result in an explosion. Should you suspect a gas leak, contact emergency services immediately.

Why do boilers overheat?

While boilers are meant to get hot and are designed to withstand large amounts of heat and pressure, certain factors can cause them to overheat. These factors include:

Limescale build-up

A limescale build-up on the boiler’s heat exchanger can result in restricted water flow resulting in hotspots.

This causes the restricted water to get hotter than normal, which then leads to an overheating boiler.

Limescale build-up is often identified by noisy boiler sounds or noises in the central heating system.

Related boiler problems:

High pressure

This is a primary reason for overheating boilers and is usually a sign of other system failures, such as faulty pressure relief valves.

Thermistor failure

The thermistor regulates the temperature in the boiler by signalling to the circuit board to increase the temperature.

When the thermistor fails, it may signal an increase in temperature when it’s not needed resulting in an excessive temperature increase.

Blocked system

Sludge is a common cause of blockages in the central heating system. If allowed to build up, it can lead to blockages in the pipes, radiators, or gas valves.

Blocked or frozen condensate pipe

This pipe drains condensation vapours from the boiler. In low temperatures, this pipe can freeze, causing a blockage in the pipe.

Faulty pump

The boilers pump circulates the hot water around the central heating system. If the pump fails or is not operating correctly, the water can remain in the boiler for an extended period and become overheated.

Often this results in a boiler lockout or system failure.

Safety feature failure

Should the boiler’s safety feature ever fail, it may cause the boiler to become overheated.

However, the high temperatures will probably result in the boiler shutting down entirely and an error code displayed on the panel.

Can you fix an overheating boiler?

If you suspect your boiler is overheating, you should follow the below steps immediately.

Switch off the water supply

Preventing additional cold-water intake will stop the cold water from making contact with the hot internal components of the boiler. Thus, preventing steam production and, therefore, internal pressure.

Do not switch off the electricity

Leave the electricity on as the boiler will need access to this to cool itself down.

Contact a gas safe registered engineer

Let’s be clear on this. You should not attempt to fix an overheating boiler yourself. Not only is this dangerous, but it can also void the warranty you have on the boiler.

Instead, you should contact a gas safe registered engineer who can assess the boiler and take the necessary steps to make your boiler safe again.

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How to prevent a boiler from overheating?

As with any appliance, your boiler requires regular servicing and maintenance to keep it functioning efficiently. Below are some ways to keep your boiler in mint condition and avoid overheating episodes.

  • Book an annual service for your boiler.
  • Request a system flush to remove limescale and sludge with each service.
  • Install a magnetic filter to catch limescale and prevent the build-up of sludge.
  • Consider upgrading your boiler if it is ready for retirement.

Boiler Overheating Final Thoughts

Unfortunately, boilers age with time, and due to the amount of pressure and heat they withstand each day, they can begin to struggle to function.

In addition, older boilers are more susceptible to overheating issues, and in most cases, it’s probably safer to have them replaced.

If you are unsure how old your boiler is, look for the serial number on the boiler or in the user manual. Once you have located it, note the numbers down and contact the manufacturer, who can confirm its age.

Boiler brand faults: 

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