James Elston

Published April 9, 2024

If you suspect your boiler may be overheating, but aren’t yet convinced that you need to make a fuss, have a look at the reasons, warning signs, dangers, and fixes listed below. When dealing with an overheating boiler (even a potential one), knowing exactly what to watch out for and acting promptly can make all the difference.

Furthermore, Eco Happy can guide you in the prevention of such an occurrence, ensuring the safety of your home, your family and yourself. If any of the warning signs are present, such as an inconsistent hot water supply or inadequate performance from your central heating system, be sure to shut off your water immediately and call aGas Safe registered engineer or heating engineer.

Now, let’s see if your boiler’s behaviour is any cause for concern and what can be done to fix it.

Reasons For Boiler Overheating

There are many reasons that boilers overheat, but ultimately, a safety feature has failed somewhere along the line or the boiler is old. The majority of modern boilers are equipped with safety features that ensure any kind of fault ceases further operation from your boiler.

Boilers go through a lot of wear and tear and require proper maintenance to deal with their workload. While the safety features may have failed several other underlying reasons could cause your boiler to overheat.

1. Printed circuit board failure

The printed circuit board (PCB) in your boiler is where all the electronic components receive and deliver messages to keep your boiler functioning safely and soundly. This board is responsible for the control and running of the boiler.

If there is a failure or fault with the PCB, the components responsible for keeping the boiler running safely will fail too, likely resulting in an overheating boiler.

2. Failed thermistor

A thermistor is a thermal resistor that responds to temperature and controls the temperature of your boiler to keep it at a stable level. These are placed inside thermostats. The PCB inside the boiler relies on accurate information from the thermistor reading of the water temperature to determine whether to increase or decrease the temperature.

If the thermistor is faulty, the water temperature may be inclined to rise, leading to the boiler overheating.

3. Faulty pump

The pump is responsible for the water circulation within your boiler system. A faulty pump could result in inadequate water supply or prevent the hot water from leaving the boiler. If there is not enough water passing through the heat exchanger, you will end up with an overheating boiler. Either way, a low water level or insufficient drainage could lead to your boiler overheating.

4. Blockages

Blockages relating to boilers can refer to either an air or physical blockage of rust or other debris. If your central heating system or radiator has an air blockage, the system would need to be bled.

If you have a combi boiler, the blockage may be occurring in the diverter valve.

If you have a system boiler, a blocked or damaged expansion vessel may be the cause of the overheating. These valves are responsible for maintaining correct pressure, and when it is compromised, the pressure inside the boiler increases, resulting in an overheating boiler.

5. Limescale buildup

The build-up of limescale is a result of hard water running through your heating system. This accumulation can happen quite gradually. The limescale buildup on the heat exchanger is what prevents a sufficient amount of water from flowing and circulating and thus leads to the boiler overheating.

Limescale is the most common cause of an overheating boiler. Luckily this is something that is easily treatable and preventable.

6. Frozen condensate pipe

The colder months in the UK can cause your boiler’s condensate pipe to freeze. If the safety features have failed or the pipe has not been thawed out in time, water will go back up the pipe and into the boiler, resulting in overheating.

This problem is easy to deal with on your own by using things such as hot water bottles to thaw the pipe. However, if your boiler has reached the point of overheating, DIY thawing measures won’t help and it’s best to call in a professional.

7. Insufficient water flow

If water flow is restricted due to a pump malfunction or scale build-up, the heat exchanger in the boiler cannot adequately dissipate the heat. This is caused by an insufficient amount of water flowing through the heat exchanger which results in an overheated boiler.

Boiler Safety Features To Prevent Overheating

Modern boilers come readily equipped with safety features and devices to ensure that overheating is prevented or avoided. Each of these components plays a vital role in the safe functioning of a boiler.

  • Pressure relief valves and safety valve: The pressure relief valve allows the build-up of pressurised water within the boiler to be expelled safely. A safety valve performs a similar function, but it ceases any further operation of the system until the optimum level of pressure has been achieved.
  • Thermistor: The thermistor will shut down the boiler and trigger an alarm if any detection of a short circuit, fault, or increase in water temperature occurs.
  • Boiler lockout: Boiler lockout is a feature of modern boilers that causes the system to shut down in the event of overheating to prevent the further accumulation of pressure. If you find that your boiler has been switching on and off, it’s likely that it has been overheating and will require further investigation.

Warning Signs Of An Overheating Boiler

The most obvious sign of overheating on modern gas boilers is an error code. If the boiler has made it this far, then the system should shut down or lock out automatically. If the boiler has not displayed an error code, there are other things to look out for that signal overheating. If your boiler does not have this feature, warning signs from your heating system may clue you in.

1. Leaking or dripping

Water leaks near the boiler and components such as the pressure relief valve will leak when the build-up of pressure within the system has caused the boiler to overheat.

2. Strange noises

Strange noises emitted from your boiler could be indicative of overheating. The types of sounds you hear may give you a more precise reason for this. Banging or knocking sounds are a result of the rapid expansion and contraction of the heat exchangers and pipes.

A hissing or whistling noise (similar to a kettle) can be the result of an excess of steam or pressure from the hot water escaping through leaks or cracks in the system. This is common with limescale accumulation.

Rumbling and gurgling sounds are a result of boiling water inside the system which is not being released efficiently. This is likely due to a faulty pump or a frozen condensate pipe. A boiler making noises is usually a cause for concern, and it should definitely be looked at.

3. Unusual odours

The smell of burning plastic or metal is a sign that your boiler may be overheating. If your boiler has overheated or broken down completely, it could result in other problems such as gas leaks. The smell of rotten eggs would be a sign of a gas leak.

4. Higher-than-usual energy bill

An overheated boiler consumes more fuel, resulting in your energy or utility bill being higher than usual.

5. Inconsistent or excessive heating

When a boiler has overheated, the hot water flow may seem inconsistent due to the boiler’s inability to maintain a stable temperature. It’s also likely that rooms near the location of the boiler seem warmer than usual, or your central heating system temperature is inconsistent.

6. Scorching or discolouring of the boiler

The excess heat from within a boiler may cause physical scorch marks or discolouration on the boiler’s exterior.

Dangers Of An Overheating Boiler

When a boiler has overheated, it should be dealt with immediately by a certified Gas Safe registered engineer or heating engineer.

Toxic fumes

As mentioned above, the smell of rotten eggs or burning plastic or metal is an indication of overheating. The fumes from the melted components of a boiler are toxic and can spread throughout your property. A gas leak as a result of an overheated gas boiler can lead to carbon monoxide poisoning, which can be fatal.


Probably the number one question on everyone’s mind is can a boiler explode when it has overheated? Simply put, yes it can. But this is the most dire outcome of an overheating boiler. If a boiler is overheating and is not fixed in time, it may explode and/or cause a fire.

You can take comfort in the fact that boilers are built to withstand pressure and, according to an article by The Guardian, the UK has only 31 gas explosions on average per year. Additionally, only a small amount of these are attributed to boilers overheating. If your boiler has reached the point of rupturing, exploding or catching fire, call the emergency services immediately.

What To Do If Your Boiler Is Overheating

If you have deduced that your boiler is overheating, you must stay calm but act fast. The very first thing you need to do is switch the water off. Once that is done, it’s highly recommended that you call a skilled and qualified engineer to assess further and assist with the overheated boiler.

Consult with a professional

When a boiler overheats, the situation can quickly become dangerous and needs to be handled by a qualified heating engineer or Gas Safe registered engineer.

If you’re concerned about the steam accumulation and pressure build-up inside of the boiler, remove yourself and any pets or loved ones from the property until help has arrived.

Common fixes performed by heating and gas engineers

Once the heating engineers or Gas Safe engineers have identified the cause of the overheating, internal components such as valves, heat exchangers and thermistors may be repaired or replaced on modern boilers.

If your boiler is on the older side, a new boiler would need to be installed to prevent any further danger. Other problems may require power flushing or radiator bleeding.

Ways To Prevent Boiler Overheating

Now that you’re aware of the many things that cause a boiler to overheat, you can take further precautions to ensure that your boiler runs smoothly. Taking steps to prevent overheating not only ensures your safety but may also prevent unforeseen costs that come with replacing a boiler or damaged components.

Power flushing

A power flush is the most effective way of clearing out any blockages in your boiler or pipes. It removes limescale buildup, sludge, and debris caused by rust. A newer boiler system should only require a power flush every five years.

Insulating or thawing condensate pipes in winter

If your condensate pipes are predisposed to freezing in winter, providing insulation covering or thawing them out regularly should prevent any complications that may lead to your boiler overheating.

Get a central heating inhibitor added to your boiler

A central heating inhibitor or boiler inhibitor is a chemical substance that is added to boilers to prevent the problems caused by build-up and corrosion. It’s important to note that these inhibitors merely break down the build-up but other measures need to be in place to remove any existing sludge. Boiler inhibitors do not compromise or change the composition of the water coming out of the boiler.

Get a magnetic filter fitted

Magnetic filters are a great way to ensure that debris and rust don’t collect and cause blockages in the pipes. These can be fitted to existing boiler systems and last up to 20 years.

Add a water softener

Water softeners can be added to any type of boiler. These act as a scale reducer. Unlike the boiler inhibitor, a water softener is a system that is added to the boiler.

Replace your old boiler

If your boiler is on the older side (older than 15 years), you might want to consider replacing it with a new boiler to avoid overheating and the complications, hazards and costs that come with it.

Ideally, you should also have your new boiler serviced once a year. Following these boiler maintenance tips will point you in the right direction.

If the idea of a new boiler is filling you with financial anxiety you could apply for the UK Government’s Boiler Upgrade Scheme. If you’re eligible, you will be provided with a grant to cover part of the cost of replacing your boiler system.


How do I know if I need to replace my boiler?

If your boiler is exhibiting any of the signs mentioned above (in any capacity), it’s probably time to replace it. The age of your boiler is another indicator, as the lifespan of a boiler is 15 years. If you’ve moved into a house and don’t know if you need a new boiler, have a Gas Safe engineer or heating engineer take a look at it.

Which type of boiler is more prone to overheating?

All boiler types are prone to overheating, but older, unmaintained boilers are more likely to overheat.

Which type of boiler is less prone to overheating?

While all boilers are prone to overheating, certain models have a lower susceptibility to overheating when compared to others. Water tube boilers have a higher safety rating and are less likely to overheat.

How important is it that I use a Gas Safe registered engineer?

It is very important. Legally, gas boilers may only be worked on by Gas Safe registered engineers or by heating engineers. You are well within your right to ask to see your engineer’s Gas Safe ID.

What actually happens when a boiler overheats?

When boilers overheat, there is a hazardous rise in pressure within the boiler which is the main cause for concern. If the boiler has not gone into lockout mode, the pressure will continue to increase which can result in bursting, exploding, or catching fire. Before this happens, you will notice changes in your heating system.


An overheating boiler is a serious concern and should not be dealt with lightly. Your safety is the number one priority in this situation and any problems relating to your boiler should be handled swiftly and by registered professionals.  

There are many ways to identify, fix and prevent overheating from taking place. If you suspect that your boiler is overheating, contact Eco Happy. We have an abundance of information regarding boilers at your disposal. So, there is no need to worry about your boiler overheating ever again.

James Elston

Boiler Expert

James Elston is the top boiler replacement and heating expert at Eco Happy. He has over 20 years of experience in the industry, focusing on Gas Safe boiler installations and offering home-heating and energy-saving solutions to homeowners across the UK. From sourcing the most energy-efficient combi boiler to providing specialist heating advice, James ensures that Eco Happy maintains the highest standards and best customer service.

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