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Last updated: 4th April, 2024


Unusual Boiler Smells: Causes and Fixes

Unusual Boiler Smells: Causes and Fixes

Key takeaways

  • Learn to identify unusual boiler smells and their sources.
  • Understand the importance of boiler engineers in diagnosing and fixing the problems.
  • Discover ways to prevent and address odour-related issues to ensure boiler efficiency and safety.

Unusual boiler smells can be a sign of a serious problem, such as a gas leak or bacterial growth, and should be addressed promptly. Our guide explores the causes of unusual boiler smells and provides practical fixes to keep your boiler running safely and efficiently.

Unusual boiler smells can be both concerning and perplexing for homeowners. Recognising these smells can help in identifying underlying issues that might require immediate attention. Delving into the world of boiler odours, this article aims to educate readers on common causes for these smells and how to address them with proper fixes.

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Understanding the sources of these unusual smells enables readers to take appropriate actions to ensure the safety and efficiency of their boiler system. As boilers play a crucial role in daily household activities, it is essential for homeowners to be familiar with the tell-tale signs of complications. 

By highlighting the role of boiler engineers, prevention techniques, and potential costs, readers can be better equipped to handle situations involving unpleasant odours from their boilers.

Understanding Boiler Smells

Boiler smells can certainly be unpleasant and concerning. Identifying the cause of these odours can help you address the issue and maintain a safe environment in your home. In this section, we will discuss some of the common causes of boiler smells and their potential fixes.

Natural gas is commonly used in boilers, and because this gas is naturally odourless, a chemical called mercaptan is added to give it a distinctive smell. This rotten egg-like odour helps in detecting any leaks easily. 

Should you notice a smell resembling rotten eggs coming from your boiler, it is highly likely that there is a gas leak. In this case, immediately shut off the gas supply, open windows for ventilation, and contact a professional gas engineer to assess and repair the issue.

Moreover, bacterial growth in water heaters can sometimes produce a foul smell similar to sewage. This typically occurs if the boiler's water temperature is set too low, allowing bacteria to thrive. Raising the water temperature to around 60°C should help eliminate any bacteria and prevent further growth.

Another cause of unusual boiler smells could be a faulty flue or damaged wiring. If your flue isn't functioning correctly or the wiring is damaged, this could lead to a gas leak or combustion byproducts not being efficiently removed from your home. 

A regular boiler check-up by a qualified service technician will help identify any potential issues with the flue or wiring and provide necessary repairs.

In some cases, boiler smells may stem from the use of poor-quality gas or fuel components or the need for proper maintenance. Ensure you're using high-quality gas or fuel components and perform routine boiler maintenance to minimise odour issues.

Proper understanding of common boiler smells and their causes is crucial in maintaining a safe and comfortable living environment. While some fixes can be implemented quickly, always consult a professional if you are unsure or find the problem persists despite efforts to rectify it.

Signs of Unusual Boiler Smells

Boiler smells can be a sign of a variety of problems with your boiler. Early identification of these warning signs can help prevent severe issues, ensuring the safety of your household. While some odours may be harmless, others could indicate dangerous situations such as gas leaks or other malfunctions.

The most common and concerning odour is one reminiscent of rotten eggs or sulphur. This smell is usually associated with a gas leak. When you encounter a pungent scent like this, it is of utmost importance to immediately shut off the gas and boiler supply and call a qualified engineer to inspect and rectify the situation.

Aside from rotten egg smells, other unusual odours from your boiler may be indicative of a problem. For example, an oily or plastic-like scent could be caused by an overheating pump or motor, which can pose a fire hazard. In such cases, it's wise to switch off the boiler, unplug it if possible and consult a heating professional.

Moreover, a musty odour might point to mould or mildew growth inside the boiler's components or water damage in the surrounding area. This issue might not be as immediately hazardous as a gas leak but should still be addressed by a technician to prevent long-term damage and health risks.

Recognising the warning signs of these unusual smells is crucial. However, also pay attention to other indicators that your boiler might be having difficulties. Unusual noises, leaks, inconsistent heating, or frequently fluctuating pressure can all serve as red flags for problems that require attention.

Maintaining your boiler may sometimes involve addressing these minor issues before they escalate. Regular servicing by a certified professional can help with early identification of issues, ensuring that your boiler runs smoothly and continues to provide a comfortable and safe environment for your home.

Common Causes of Boiler Smells

Gas Leaks

A gas leak might be the cause if you smell something similar to rotten eggs near your boiler. In this case, it's essential to act fast: start by turning off the gas supply at the meter, opening windows for ventilation, and avoiding any potential sources of ignition, such as electrical appliances or naked flames. 

What To Do If You Think you Have A Gas Leak

If you suspect your property has a gas leak it is crucial to carry out these steps:

  • Ensure all people and pets are evacuated from the house.

  • Leave doors and windows open.

  • Turn off the gas at the mains tap- this can usually be found near the gas meter.

  • Immediately call the National Gas Emergency Service number on 0800 111 999 as soon as you have evacuated the house.

  • Do not turn any power or light switches on or off.

  • Do not light any sort of flame within the property.

  • Do not use any appliances that could cause a spark.

Anaerobic Bacteria

Anaerobic bacteria can sometimes cause a boiler's water supply to emit a foul odour. These microorganisms break down certain compounds present in water, like sulphur, and releasea foul gas, resulting in a nasty smell similar to rotten eggs. To fix this issue, a plumber can flush the system, treat the water with a biocide, or replace components like anodes or valves, which may have become affected by the bacteria.

Faulty Parts

Often, boilers will develop unusual smells if parts like burners, valves, or the heat exchanger have become faulty. In these instances, the boiler may not be operating at its full capacity, leading to issues with combustion and incomplete burning of gases. This situation can result in smells such as burning, smoke, or even a metallic scent. A qualified technician must inspect and replace the faulty components promptly.

Overheating and Kettling

Overheating can occur when limescale or mineral deposits build up inside the heat exchanger and on other internal parts of the boiler, causing it to work harder to maintain the desired temperature. This buildup can result in a phenomenon known as kettling, which is characterised by gurgling, banging, or whistling noises coming from your boiler. In some cases, overheating can even produce a burning or melting plastic smell. A certified professional can address overheating by cleaning the boiler's components and adding inhibitor chemicals for preventing further limescale buildup.

Blockages and Moisture

If you notice a musty, damp smell around your boiler, it could be due to blockages or moisture in the pipes or radiators. These issues might cause your system to be less efficient, and can even lead to corrosion and rust inside the heating system. Regular maintenance, including bleeding the radiators, can help identify and prevent blockages and moisture problems. However, for more severe cases, you may need a professional plumber to flush the system or replace damaged parts.

The Role of Boiler Engineers

Boiler engineers play a vital role in maintaining, repairing, and installing boilers to ensure efficient and safe operation. They possess in-depth knowledge about the functioning, potential issues, and preventative measures of boilers, including dealing with unusual smells. 

One of the key aspects of our iHeat boiler engineers is their accreditation as Gas Safe Registered Engineers.

Gas Safe Registered Engineers

Gas Safe Registered Engineers are certified professionals who have undergone extensive training and assessment to handle gas appliances, such as boilers, safely and competently. It's vital to have a Gas Safe Registered engineer carry out any work on your boiler to ensure the highest standards of safety and quality.

These engineers are experienced in identifying the root cause of unusual boiler smells and implementing effective fixes. For instance, they can address issues such as:

  • Rotten smells caused by dead animals in the flue or near the boiler

  • Gas leaks due to faulty appliances or wiring

  • Bacteria in water heaters leading to unpleasant odours

When faced with an unusual smell coming from your boiler, it's crucial to seek the expertise of a Gas Safe Registered engineer to inspect and resolve the issue promptly. They will assess your boiler, identify the problem, and proceed with the necessary repairs or replacements, ensuring the safety of your home and its occupants.

In conclusion, Gas Safe Registered boiler engineers are essential in diagnosing and resolving unusual boiler smells and other issues. Their expertise, knowledge, and professional certification guarantee safe and efficient boiler operation, providing comfort and peace of mind within your home.

Prevention and Fixes for Unusual Boiler Smells

Regular Check-ups and Services

It is crucial to schedule regular check-ups and servicing for your boiler by a professional engineer. These inspections can detect problems early on and help maintain the efficiency and reliability of your boiler system. Regular services will also ensure that your boiler stays in good condition, preventing unusual smells and potential hazards.

Replacement of Faulty Parts

Boiler smells can be caused by faulty components, such as valves, burners, thermostats, and flues. If any parts are found to be damaged or worn, they should be replaced as soon as possible. Prompt replacement helps prevent further damage to your boiler system and eliminates the source of any unusual smells.

Installation of a New Radiator or Boiler

In some cases, the best solution for unusual boiler smells may be the installation of a new radiator or boiler. Older radiators and boilers can develop problems over time, and replacing them with newer, more efficient models will help improve the overall performance of your heating system while reducing the likelihood of unpleasant odours.

Proper Ventilation

Ensuring proper ventilation in the room housing your boiler is essential. Adequate airflow helps to dissipate any smells and keeps your boiler running efficiently. If your existing boiler space lacks proper ventilation, it is important to address this by adding or improving the ventilation system.

By following these measures, you can help prevent and fix unusual boiler smells, ensuring a safe and comfortable environment in your home. Regular services with a qualified engineer, replacing faulty parts, installing new radiators or boilers, and maintaining proper ventilation are all essential steps to keeping your boiler.

New Boiler Costs

Boilers can be a daunting purchase for many people as they’re an appliance with a lot of responsibility, providing heat for you and your family is something you want to get right. 

Boilers aren’t exactly a quickly disposable item either, potentially lasting you a decade.

Boiler costs can be split into two segments: the first is the actual boiler itself (unit price), and the second is the cost of the boiler being installed (set up) in your property by an expert engineer. 

Here at iHeat, we want to remove all of this undue stress and make the decision making process of upgrading to a new central heating system, as easy as possible.

Boiler costs can vary depending on a number of factors including their brand, model, fuel, output, warranty, labour and installation type.

Typically a new boiler will cost between £1,845 and £3,500, below is a list of average boiler installations offered by iHeat (intended as a guide only).

Installation Type

Price (inc VAT)

Combi to combi swap


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System to combi conversion


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New boiler install


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Back boiler to a combi


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System to system


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Some location factors that can influence the price of your new boiler

You will be given a specific quote from the boiler installation company based on:

The type of home you have (i.e. terraced, semi-detached, etc.)

  • How many bedrooms you have

  • How many bathrooms you have

  • Your postcode and specific location

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In summary, unusual boiler smells can be attributed to various causes, and prompt action is crucial to prevent more significant issues from arising. Generally, these odours can stem from gas leaks, water leaks, bacteria, or obstructed pipes.

The moment an unpleasant smell is detected, it is advised to tighten valves with a wrench or investigate any possible leaks. If the situation remains unresolved or worsens, it's best to contact a professional boiler technician for assistance.

Implementing regular maintenance and inspections is a proactive approach to minimising unusual odours and ensuring the boiler system runs efficiently. By addressing causes early on, households can avoid expensive and potentially dangerous consequences.

Remember, the safety of the residents is of the utmost importance. Therefore, when it comes to unusual boiler smells, it pays to be vigilant and address any concerns without delay.

Frequently Asked Questions

What to do when your boiler smells?

If your boiler smells, first, try identifying the specific smell. If it resembles a rotten egg or sulphuric odour, it might be a gas leak. In that case, immediately turn off the gas supply, open windows and doors, and do not use any electrical appliances. Call a professional gas engineer to assess and fix the problem. If it is a metallic or chemical smell, it could be due to overheating or internal issues; seek professional help in this case as well.

Is it normal for a boiler to smell?

No, it is not normal for a boiler to smell. If you notice an unusual odour, it could indicate a problem with your boiler or the surrounding components. Some common causes include gas leaks, faulty appliances, bacteria in water heaters, or wiring issues. It is important to investigate and address any unusual odours to ensure the safe and efficient operation of your boiler.

Why does my combi boiler smell?

A combi boiler may smell due to various reasons such as gas leaks, faulty components, bacterial growth in the water heater, or electrical issues. It is essential to determine the specific cause and address it promptly to ensure your boiler operates safely and efficiently.

What to do if you can smell gas near the boiler?

If you smell gas near your boiler, immediately turn off the gas supply, open windows and doors to ventilate the area, avoid using electrical appliances, and call a professional gas engineer. Gas leaks can be dangerous and need to be addressed immediately.

What does a boiler gas leak smell like?

A boiler gas leak usually smells like rotten eggs or sulphur. This is because gas suppliers add an odorant called mercaptan to the natural gas, which is typically odourless, to make leaks noticeable and easier to detect.

How do you know if your boiler is leaking gas?

Signs of a gas leak from your boiler include a rotten egg or sulphuric smell, hissing or whistling sounds near the boiler, visible damage or corrosion on the boiler or its components, and higher gas bills than usual. If you suspect a gas leak, it is essential to act promptly and call a professional to inspect and fix the problem.

Is it normal to smell a little gas?

It is not normal to smell gas in your home. Even a faint smell can indicate a potential problem with your boiler or gas appliances. It is important to treat any gas smell seriously and contact a professional gas engineer to investigate the issue and ensure your safety.

Stephen Day profile photo
Article by
Stephen Day | Co-founder
Gas Safe registered and FGAS certified engineer with over 20 years experience in the heating and cooling industry.