Common Boiler Faults And How To Identify Them (Updated)

features a modern boiler with some visible faults, such as leaking water and warning lights, set in a typical UK home utility room

Boiler malfunctions can be a source of annoyance and costly expenses, particularly during winter when we rely on boilers for warmth and comfort. Malfunctions such as loss of heating diminished heat output, and unanticipated shut-downs can greatly disrupt our daily routines and cause discomfort.

In this comprehensive article, we will analyze the most prevalent boiler faults in the UK, delving into the underlying causes and providing practical measures to prevent them from occurring. By familiarizing yourself with the most common problems and taking a preventative approach to boiler upkeep, you can save money on repairs, maintain a comfortable and warm home, and guarantee the safety of your loved ones.

Lack of Maintenance

The primary cause of boiler failures is inadequate maintenance. An annual examination by a certified Gas Safe engineer is essential to maintain the optimal performance of your boiler. The engineer will carry out a thorough examination of the boiler to detect any potential problems and prevent them from becoming severe.

During the boiler service, the engineer will evaluate the various parts of the boiler, such as the burner, heat exchanger, and flue, to guarantee they are functioning correctly. For instance, a faulty burner can result in incomplete combustion, increasing the threat of carbon monoxide poisoning. The heat exchanger is accountable for heating the water and can become clogged with mineral buildup if not regularly maintained. A clogged flue can also pose a risk as it can allow fumes to enter your home.

Along with reviewing the components, the engineer will also conduct a gas tightness test to ensure there are no leaks in the system. This is critical as even a minor leak can release toxic gases into your home. Finally, the engineer will assess the boiler’s safety features, such as the pressure gauge and overheat thermostat, to confirm they are functioning properly.

Regular boiler maintenance can keep it operating efficiently and prevent costly and inconvenient breakdowns. It also ensures the safety and longevity of your boiler, saving you money in the long run. To avoid the stress and frustration of a boiler failure, have your boiler serviced annually by a certified Gas Safe engineer.

A close-up photo of a person in a white shirt and black gloves using a wrench on a boiler's red and blue pipes, focusing on the hands and tools


Here are some of the most common types of boiler errors.  

Hot water but no heating 

No heating or hot water 



Noises coming from the boiler  

Low-pressure in the boiler

Radiators are cold 

Frozen condensate pipe 

Boiler is not responding to the thermostat 

Boiler keeps switching itself off

Faulty pilot light

Blocked Flue

1.Hot water but no heating 

 The boiler is designed to provide the home with heat as well as hot water for domestic use. If you have found that hot water is available from the taps but the central heating is not working, then you can bet there is a problem.  

If you notice there is a problem with your central heating, then you should begin by checking if the pressure in your boiler is too low.  

Thermostats allow you to adjust the desired temperature of your central heating as well as the timing for activation and deactivation. If the heating hasn’t turned on in the time you planned, it is time to check those thermostat settings again.  

Boiler pressure describes the pressure of the hot water making its way through your central heating system. If the pressure drops too low, the central heating will fail.  

Increasing the pressure of your boiler is a fairly simple procedure that can be done without the assistance of a heating engineer. But if you feel even slightly unsure about the process, it would probably be best to call in the professionals.  

Thermostat errors and boiler pressure are not the only reasons that you will have problems with your boiler and hot water supply. But, if your problems are not connected to boiler pressure or thermostat adjustments, things get a little more complex.  

If your thermostat is working fine and the pressure of your boiler is at the correct level the error may need professional boiler repair. Some of the more common reasons that can cause heating to fail while hot water seems to be working well is an issue with the diaphragms, airlocks or the motorised valves.  

If your home’s heating system utilizes a combi boiler, a potential issue you may encounter is with the diverter valve. A combi boiler provides both heating for the home and hot water on demand, and the diverter valve directs the hot water to the desired location, such as the hot water outlets or radiators. If the diverter valve is not functioning correctly, you may experience a lack of hot water or central heating.

In the event that your problem is not related to boiler pressure or thermostat settings, a Gas Safe registered engineer can help with fixing airlocks, diaphragms, diverter valves, and motorized valves. They can also perform necessary repairs and replace any damaged components.

2. No heat or hot water 

In an ideal scenario, your boiler should reliably warm your home and supply hot water without fail. However, if you encounter any malfunction, it is imperative to address the issue promptly.

To determine the source of the problem, it is crucial to verify that the boiler’s power source is not interrupted. If the power supply is uninterrupted, other potential issues can then be examined.

Most boilers come equipped with error codes that can provide insight into the location of the problem. Some common error codes to be aware of include:

fuel is not provided properly — if you are running a gas boiler, check that the fuel line is unobstructed. If your boiler is running on oil, make sure that the fuel in the tank is replenished. 

Thermostat — check that the thermostat settings are at the right temperature and higher than the current temperature of the room.  

Freezing — if the condensate pipe has frozen over, it will need to be thawed out properly, we will cover this point further in the article.  

Low boiler pressure — as mentioned, if the pressure of the boiler is low, it will need to be reset. You can find detailed instructions on how to do this in the owner’s manual.  

3.Boiler is leaking 

Leaking boilers are a widespread concern that can result in significant harm if neglected. The cause of the leak could stem from various reasons, including defective pipes, valves, or seals. Regardless of the origin, a leak can result in water damage to your home and lead to a loss of heat and hot water. When a leak occurs in your boiler, water can escape and result in harm to your home’s interior and possessions, including walls, flooring, furniture, and even the growth of harmful mold and mildew.

Furthermore, a leak can lead to a disruption of heating and hot water, depriving you of crucial comforts. To avoid these complications, it is imperative to have your boiler promptly inspected by a professional if you detect a leak. An experienced engineer can diagnose the issue and make necessary repairs to prevent further harm. In some cases, the solution may be a simple fix, such as tightening a loose valve or replacing a damaged seal. In other cases, however, the problem may require the replacement of pipes or other boiler components.

4.Boiler is kettling 

If you hear a strange sound of rumbling coming from your boiler, something like the sound of a kettle heating up, this is an indication of another common problem. Lime scale deposits can collect on your heat exchanger (the coils that transfers the heat from the source of fuel to the water) this problem is commonly referred to as kettling.  

When debris accumulates in your boiler, the water within the heat exchange can become obstructed. This can cause the water that does pass through the exchange to overheat, turn to steam and begin making funny noises.  

This is a more common problem in areas that receive hard water, but it is not uncommon in areas that receive soft water either. Not only does this place strain on the boiler but it can shorten the service life of the entire system. 

If you notice a kettling issue in your boiler, the best course of action will be to call in a professional Gas Safe Registered engineer. The engineer will remove the build-up and ensure the system is restored to full function.

5.Noisy Boiler

There are many various forms of Boiler noises. It can be banging noise, whistling, gurgling, and others. All these noise types originate through various reasons, and a sign of different faults.  For this reason, anytime you hear any kind of noise in the heating system or boiler, never ignore it – it could be a sign of fault. 

It is important to note that fault varies based on the type of sound produced and the place it is coming from. Below is a shortlist of common types of sounds produced by the boiler, which will help you determine the cause.  

  • Banging Noise – Often, a banging noise is caused by an internal component or the pipework. However, it can also be caused as a result of debris build up on the heat exchanger. It can also be an indication of a faulty pump that pumps hot water from the boiler to the heating system.  
  • Whistling Noise – This whistling sound closely resembles that of a kettle when water is boiling. The sound is caused by a build-up of debris in the heat exchanger.  
  • Buzzing Noise – This noise is often a clear indication that electrical components within the boiler are faulty. The noise could be originating from the thermostat, which is a sign that the conductor is either dirty or it needs replacing. 
  • Clanking Noise – This sound is closely similar to a banging noise, and it may be caused by loose pipework. It could also mean that the boiler fan is being obstructed by an object.  
  • Gurgling Noise – Did you know that too much air within the pipes can prevent water from circulating through the heating system? Whenever there is too much air in a system, you will hear a gurgling sound – especially when it is coming from the radiator. The solution is bleeding the system.  
  • Humming Noise – The truth is that a boiler tends to hum while it is in operation. Although the humming is slight, if you are looking for the most silent models, look for those with a Quiet Mark accreditation. Nevertheless, if the humming is loud, then it is a sign of loose parts –  mainly the central heating pump.  

6.Low Boiler Pressure 

Whenever water gets hot, it increases in pressure. As such, boiler pressure simply refers to the pressure of hot water passing through the central heating system. The central heating will fail to work if the boiler pressure is too low. 

Boilers always come with a pressure-measuring gauge that anyone can use to read the pressure in the boiler. Therefore, anytime you get a reading below the 1 mark, automatically know the pressure is too low. The boiler pressure should always sit somewhere between the 1 and 2. Nevertheless, it is wise to consult the boiler manufacturers guide to know the appropriate pressure for your system.

It is important to note that as the boiler ages, it gradually loses its pressure. However, a sudden pressure drop is a big problem and a sign of fault. For this reason, if you identify a pressure drop, automatically know that you have a water leak. 

The good news is that increasing boiler pressure is a task anyone can do for themselves. However, it is wise to consult a professional. 

7.Frozen Condensate Pipe 

Condensing boilers are featured with a condensate pipe whose primary function is to drain Acidic water away from the unit. It is important to note that as the boiler is working, it creates acidic water. Since these pipes drain acidic water away from the unit, they are susceptible to external elements such as freezing. 

It is paramount to thaw out the condensate pipe during winter or when you are living in frozen areas by pouring warm water on it. 

 To prevent future freezing, professionals recommend lagging the pipe. What does lagging mean? It simply means wrapping it with insulation to keep it warm. 

8.Cold Radiator 

shows a typical UK living room with the central focus on a radiator that's visibly cold, possibly indicated by frost or condensation.

When a radiator is not heating up, it means that it is either clogged with sludge or there is air in the system. If you start noticing the bottom of the radiator is getting warm/hot, and the top is not – then you should consider bleeding it. This is a straightforward procedure that does not need the help of a professional. However, if you do not know how to go about it, consult a professional. 

If you try balancing or bleeding the radiator; and you still notice no change, then the chances are that it has a sludge build-up, which is preventing the free flow of hot water to the radiator. A professional will help you clear the sludge through chemical cleaning and flushing the system. You should never try doing this yourself. 

9.Boiler Constantly Switching Itself Off 

There are several reasons why a boiler could switch itself regularly. Some of the common reasons are: – 

  • A build-up of sludge or debris in the system 
  • Too much air in the system 
  • Thermostat issues 
  • Low boiler pressure 
  • Lack of water flow (either due to a closed valve or pump issues) 

If the thermostat is working as required and the boiler has the required pressure as per the manufacturer’s guideline, then you should contact a Gas Safe Registered engineer.

10. Faulty Thermostat 

The thermostat is a crucial component in a boiler system as it controls the temperature of your home. A faulty thermostat can cause a range of problems, including preventing the boiler from heating up properly or turning off unexpectedly.

If the thermostat is not working correctly, it may not be able to maintain a consistent temperature in your home. This can result in fluctuations in the heat output, leading to periods of either too much or too little heat. For example, if the thermostat is not functioning properly, it may not trigger the boiler to heat up even though the temperature in your home has dropped below the desired level. On the other hand, it may also cause the boiler to overheat and shut off unexpectedly, leaving you without heat.

In addition to affecting your comfort, a faulty thermostat can also result in increased energy bills. If the boiler is not heating up efficiently, it will consume more energy, which will be reflected in your monthly bills.

It is therefore important to have your thermostat checked by a professional if you suspect that it is faulty. A trained engineer will be able to diagnose the problem and make any necessary repairs to restore your boiler to full working order. A properly functioning thermostat will ensure that your home remains at a comfortable temperature and that your boiler operates efficiently, reducing the risk of a breakdown and saving you money in the long run.

11. Faulty pilot light

A malfunctioning pilot light can result in various issues with your boiler, making it a vital component that requires routine maintenance and inspection. The pilot light, a small flame, ignites the boiler’s main burners, allowing it to efficiently heat your home.

If the pilot light is not operating correctly, your boiler may face difficulties in starting or may shut down unexpectedly, leading to a lack of heat and hot water in your home, causing discomfort and inconvenience. Furthermore, a defective pilot light can increase energy costs, as the boiler will have to work harder to heat your home, consuming more energy in the process.

In case you suspect that your pilot light is not working properly, it’s important to seek the help of a professional as soon as possible. A qualified gas-safe engineer can diagnose the issue and make the necessary repairs, guaranteeing that your boiler continues to operate efficiently. Some problems, such as relighting the pilot light or replacing a faulty thermocouple, may be a straightforward fix, while others, such as replacing the pilot light or other parts of the boiler system, may require a more complex solution.

12. Blocked flue

A blocked flue is a common issue that can cause a range of problems with your boiler, making it an important component that needs to be maintained and checked regularly. The flue is the pipe that carries the boiler’s exhaust gases outside of your home, and it needs to be functioning correctly to ensure the safety and health of your family.

If the flue becomes blocked, the boiler’s exhaust gases can escape into your home, creating a risk of carbon monoxide poisoning. Carbon monoxide is a poisonous gas that is produced by burning fossil fuels, such as gas, oil, and coal. It is odourless, colourless, and tasteless, making it impossible to detect without a carbon monoxide detector. Long-term exposure to carbon monoxide can lead to serious health problems, including headaches, dizziness, nausea, and in severe cases, death.

In addition to the increased risk of carbon monoxide poisoning, a blocked flue can also cause reduced heat output and increased emissions from your boiler. This can result in a reduction in comfort and an increase in energy bills, as the boiler will have to work harder to heat up your home, using more energy in the process.

If you suspect that your flue is blocked, it is important to have it checked by a professional as soon as possible. A trained engineer will be able to diagnose the problem and make any necessary repairs, ensuring that your flue continues to function properly and your home remains safe from carbon monoxide poisoning.

How Much Does A Boiler Repair Cost? 

The truth is that the cost of repairs will vary based on fault, professional rates, and the number of faults a boiler has. You can always save some money by doing some boiler maintenance by yourself. However, never try to tackle major boiler problems, it is wise to call a professional. Also, the cost of any part that needs replacing will affect the cost of repairing a boiler. To defiantly know the cost, consider talking to a professional.


To sum up, boiler malfunctions can bring significant inconvenience and extra expenses. Familiarizing yourself with the most common boiler issues, such as neglecting maintenance, faulty thermostats, kettling, leaking boilers, problematic pilot lights, and clogged flues, can help you keep your boiler running smoothly and prevent costly repairs or replacements. By scheduling regular maintenance with a Gas Safe certified technician and monitoring these faults, you can keep your boiler functioning efficiently and safely, avoiding any future breakdowns. By taking care of your boiler and being mindful of these common issues, you can ensure a long-lasting and reliable source of heat and hot water.

Also, check out

The 10 Most Common Boiler Problems in London Homes

Boiler Repair Services in London: What You Need to Know