Boiler Overheating: Causes, Signs & How to Fix It

Last Updated on February 11, 2025

An overheating boiler can cause a range of issues, including inefficient heating, inconsistent hot water, and potential damage to your system. Understanding the causes, recognising the signs, and knowing the appropriate fixes is essential for maintaining a safe and efficient heating system in your home.

Boiler overheating can arise from several factors, and it’s important to act quickly if you notice any signs of trouble. If you experience an erratic hot water supply, poor heating performance, or strange noises coming from your boiler, these could be indicators of overheating.

In such cases, it’s vital to turn off your boiler immediately and contact a Gas Safe registered engineer or heating specialist.

In this guide, we will discuss the common causes and effective solutions for overheating boilers, so you can restore your system’s efficiency and avoid future issues.

Why Is My 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.

Here are reasons why your boiler is overheating:

1. Printed circuit board failure

The printed circuit board (PCB) in your boiler is the central hub where all the electronic components communicate to ensure the safe operation of your heating system. It plays a critical role in regulating the boiler’s temperature, pressure, and overall functionality.

Essentially, the PCB acts as the “brain” of the boiler, controlling and directing the electrical signals that power various components.

If the PCB fails or malfunctions, it can disrupt the boiler’s safety mechanisms. In this case, the boiler may overheat due to a failure in the temperature sensors, pressure switches, or electronic components designed to monitor and control the system.

Without these sensors functioning properly, the boiler may continue to operate beyond safe limits, leading to excessive heat buildup.

Common signs of a faulty PCB

Common signs of a faulty PCB include irregular boiler cycling, sudden temperature changes, or the boiler failing to respond to temperature adjustments. In more severe cases, you may notice boiler error codes related to PCB malfunctions, which can be found in your boiler’s manual or via the display screen.

A damaged PCB may require replacement by a professional. While some homeowners might attempt repairs themselves, it’s always recommended to consult a qualified heating engineer to avoid further damage or voiding any warranties.

Important tip: Regular boiler maintenance, such as annual servicing, can help identify potential issues with the PCB before they lead to a full failure. Boiler overheating caused by PCB faults is often preventable with proactive care.

2. Failed thermistor

A thermistor is a crucial component of your boiler’s temperature regulation system. It is a type of thermal resistor that adjusts its resistance based on the temperature, helping control the boiler’s water temperature and ensure it stays at an optimal, stable level.

Typically, thermistors are housed within boiler thermostats and play a key role in the safe operation of your heating system.

The PCB in your boiler depends on accurate temperature data from the thermistor. This information allows the PCB to determine whether to increase or decrease the heating output. When the thermistor malfunctions or fails, it can provide inaccurate readings to the PCB.

As a result, the boiler may continue to heat the water beyond the desired temperature, leading to the boiler overheating.

Common signs of a failed thermistor

Common signs of a failed thermistor include erratic boiler temperature fluctuations, failure to maintain consistent heat, or a boiler displaying error codes related to temperature regulation.

If left unchecked, a faulty thermistor can cause excessive strain on other components, potentially leading to further damage and higher repair costs.

To prevent overheating issues caused by a failed thermistor, it is essential to regularly maintain your boiler. Boiler servicing can detect problems with temperature sensors, including thermistors, before they escalate into more severe issues.

In cases where the thermistor is found to be faulty, it will need to be replaced by a qualified heating engineer to restore the proper function of your boiler.

Pro Tip: If you notice that your boiler is overheating or inconsistencies with your heating, it’s advisable to seek professional help promptly. Replacing a faulty thermistor before it leads to more significant damage can save both time and money in the long run.

3. Faulty circulation pump

The circulation pump in your boiler system plays a crucial role in circulating hot water throughout the system, ensuring it reaches your radiators and taps at the right temperature. If the pump becomes faulty or malfunctions, it can cause a range of issues that affect the performance of your heating system.

A common problem associated with a faulty pump is inadequate water circulation. If the pump isn’t working properly, it can restrict the flow of water through the system. This disruption means that hot water may not be able to flow through the heat exchanger as intended. As a result, the heat exchanger can overheat, which leads to boiler overheating.

Common signs of a faulty circulation pump

Several symptoms can indicate that your pump is malfunctioning. These may include:

• Strange boiler noises (like whistling or grinding)
• The boiler not reaching the desired temperature
• The boiler pressure dropping.
• In some cases, you may even notice water leaks around the pump or a complete lack of hot water.

If the pump isn’t circulating water efficiently, it can also cause a low water pressure issue, further contributing to overheating.

Insufficient water levels or drainage can increase the risk of overheating, as the boiler struggles to maintain the correct temperature, often resulting in system shutdowns or damage to the boiler’s internal components.

To prevent boiler overheating caused by a faulty pump, it is important to schedule regular boiler servicing with a professional heating engineer. They can inspect and clean the pump to ensure proper function and detect any signs of wear before they result in more significant issues.

In cases where the pump is deemed faulty, a replacement may be necessary to restore your boiler’s efficiency and prevent overheating.

4. Blockages

Blockages within your boiler system can be caused by either air blockages or physical blockages due to rust, debris, or scale buildup. These issues can interfere with the normal flow of water or air, leading to various problems, including boiler overheating.

Air Blockages

An air blockage typically occurs within the central heating system or radiators. Over time, air can get trapped in the pipes, reducing the flow of water and causing cold spots in the radiator, which limits the system’s ability to efficiently heat your home.

In this case, you will need to bleed the radiators to release the trapped air and restore normal water flow.

Airlock signs include:

• Radiators not heating properly
• Hot water being inconsistent or absent
• Gurgling sounds coming from the system

Bleeding the radiators helps to resolve the issue, allowing the water to flow freely and preventing unnecessary overheating in the boiler due to insufficient heat exchange.

Physical Blockages

In addition to airlocks, physical blockages, such as rust or debris accumulation, can occur within the boiler pipes or other components like the heat exchanger. Over time, sediment and sludge can build up and restrict the flow of water, making it harder for the boiler to operate efficiently.

This can lead to boiler overheating as the system struggles to maintain the required temperature.

Diverter Valve Blockage (Combi Boilers)

If you have a combi boiler, another common cause of blockages is a diverter valve obstruction. The diverter valve is responsible for directing the flow of water between the heating system and the hot water supply.

If the valve becomes clogged with debris, it may prevent hot water from being distributed properly, causing a boiler overheating situation. Regular servicing and cleaning of the diverter valve can help prevent this problem.

Expansion Vessel Blockage (System Boilers)

For system boilers, a blocked or damaged expansion vessel can lead to overheating. The expansion vessel helps regulate the system’s pressure by absorbing excess water volume when the system heats up. If the vessel is compromised or blocked, the boiler’s internal pressure increases, potentially causing the boiler to overheat and shut down.

Common signs of a pressure-related blockage include:

• High pressure on the boiler gauge
• Boiler lockout or system failure
• Leaking water from the pressure relief valve

If you notice these signs, it’s essential to contact a qualified heating engineer to inspect and rectify the issue. Neglecting these blockages could lead to severe damage to the boiler and other system components.

5. Limescale buildup

Limescale buildup is one of the most common causes of boiler overheating, especially in areas with hard water. Hard water contains a higher concentration of minerals, particularly calcium and magnesium, which can accumulate over time, forming a scale inside your heating system.

This gradual buildup of limescale on the heat exchanger can lead to several issues, ultimately causing your boiler to overheat.

How limescale affects your boiler

Limescale deposits restrict the flow of water through the heat exchanger by forming an insulating layer. This means the water cannot transfer heat efficiently, causing the boiler to work harder than usual. As a result, the boiler may overheat as it struggles to achieve the desired temperature due to insufficient heat transfer.

Common signs of limescale buildup:

• Reduced heating efficiency: You may notice your radiators are not heating up as quickly or evenly.
• Increased energy consumption: The boiler uses more energy to heat the water, leading to higher heating bills.
• Strange noises: Limescale buildup can cause kettling or a banging noise in the system, which is caused by steam pockets forming due to poor heat transfer.

Preventing and treating limescale

The good news is that limescale buildup is both preventable and treatable. Regular boiler servicing can help identify early signs of limescale and allow for professional descaling. In addition, magnetic filters and water softeners can be installed to reduce the risk of limescale accumulation in the future.

By tackling limescale early, you can protect your boiler, improve its efficiency, and prevent costly overheating issues. If you notice any of the signs of limescale buildup, such as strange noises or decreased heating efficiency, contact a heating engineer to assess and resolve the problem.

6. Frozen condensate pipe

A frozen condensate pipe is a common issue during the cold winter months in the UK, particularly when temperatures drop below freezing. The condensate pipe is responsible for carrying acidic wastewater away from the boiler and into an external drain.

When temperatures are low, this moisture can freeze, leading to a blockage.

How a frozen condensate pipe causes boiler overheating

• When the pipe is frozen, the boiler cannot safely dispose of condensate, causing a backup of water within the system.
• If the boiler’s safety mechanisms fail, this backup can cause a pressure increase, forcing the boiler to work harder and potentially overheat.
• In some cases, a frozen condensate pipe can cause the boiler to shut down entirely, displaying an error code (common on combi boilers).

Common signs your condensate pipe is frozen

• Gurgling or bubbling noises coming from the boiler
• Boiler lockout or fault codes on the boiler display (e.g. F1, F2, L2, or similar depending on the manufacturer)
• No hot water or heating, despite the system being switched on
• Ice or frost visible on the external condensate pipe

Important: If your boiler has already overheated due to a frozen condensate pipe, attempting DIY fixes may not be effective. Turn off the boiler immediately and contact a Gas Safe registered engineer to inspect the system and prevent further damage.

7. Insufficient water flow

Insufficient water flow is a leading cause of boiler overheating, as it prevents the heat exchanger from effectively transferring heat away from the system. If water circulation is restricted, heat accumulates within the boiler, increasing the risk of overheating, pressure build-up, and potential system failure.

How insufficient water flow causes boiler overheating

• Pump Malfunction – The circulating pump is responsible for moving hot water through the system. A faulty, blocked, or airlocked pump can reduce flow, causing the heat exchanger to overheat.
• Limescale or Sludge Build-up – Over time, limescale deposits and sludge accumulate inside pipes and the heat exchanger, restricting water movement and trapping heat inside the boiler.
• Airlocks in the System – Trapped air within the heating system can prevent water circulation, leading to uneven heating and potential overheating of the boiler.
• Blocked or Partially Closed Valves – If the radiator valves, zone valves, or isolation valves are partially closed or blocked, water flow becomes restricted, leading to increased internal temperature.

Common signs of insufficient water flow in a boiler

• Radiators not heating up properly or heating unevenly
• Boiler making banging, kettling, or whistling noises (due to trapped heat and steam build-up)
• Error codes on the boiler display indicating water circulation issues
• Frequent boiler shutdowns or overheating warnings

How to fix insufficient water flow issues

Check & Bleed Radiators – If radiators have cold spots or are not heating properly, bleeding them can release trapped air and improve circulation.
Inspect & Clean the Pump – A blocked or malfunctioning pump should be checked and, if necessary, repaired or replaced by a Gas Safe registered engineer.
Power Flush the System – If sludge or limescale is restricting water flow, a power flush can help remove these obstructions and restore efficiency.
Check for Valve Blockages – Ensure all radiator and isolation valves are fully open to allow unrestricted water flow.
Install a Magnetic Filter – A magnetic sludge filter helps trap debris and metal particles, preventing future blockages and improving long-term boiler health.

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.

Explosion

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.

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.

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.

Is Your Boiler Overheating Too Often? It Might Be Time for an Upgrade!

If your boiler keeps overheating, frequent repairs can quickly become costly—not to mention the inconvenience and potential safety risks. Instead of pouring money into repeated fixes, investing in a new boiler can save you money in the long run while keeping your home safe and warm.

At Eco Happy, we make upgrading your boiler quick, hassle-free, and budget-friendly. Simply answer a few quick questions, and we’ll provide you with a free, no-obligation quote tailored to your home and budget—within minutes!

Reduce your energy bills with a more efficient boiler
Say goodbye to costly repairs and emergency breakdowns
Enjoy reliable heating and hot water all year round

Request your FREE boiler quote today! It only takes a few minutes, and we’ll match you with the best boiler for your needs at the best possible price.

FAQs

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.

Conclusion

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.