Category - Boilers
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Will Scholfield, Engineer

18 May : Updated 17 Apr ● 4 min read

Why does my boiler smell like rotten eggs?

This might sound like a peculiar topic to address. However, a strange smelling boiler is probably one of the most crucial warning signs to be aware of.

It’s often the case that if something becomes a little smelly, there's likely something wrong with it, and this also applies to your boiler. You may already associate the smell of gas with your boiler if you use a gas-fueled boiler. But did you know that sulphuric-smelling odours (or the smell of rotten eggs) could be a severe warning sign?

This article reveals what smells to look out for with your boiler, and offers solutions to remedy them.

As always, when it comes to boilers - gas boilers in particular - we recommend calling an expert to take a look and refrain from trying to fix the problem without professional assistance.

What boiler smells should I look out for?

Rotten eggs smell 

The smell of gas is often associated with an ‘eggy’ aroma. 

Gas itself is odourless, which is undetectable to the human nose. Due to this, domestic gas supplied from the National Gas Grid is treated with a chemical called methyl mercaptan. This chemical is mixed with the odourless gas as a safety precaution to help early detection of leaking gas. It has a distinctive sulphurous smell, which, when we catch a smell of it, makes us associate it with gas. 

If you can detect a rotten egg-like smell coming from your boiler, don’t ignore it.

What to do if:

1. There’s a rotten egg smell coming from your boiler

If you notice a rotten egg smell which seems to be emerging from your boiler or any other gas appliance in your home, it could mean that you have a gas leak. 

The recommended course of action is to locate your gas shut-off or Emergency Control Valve (usually next to your gas meter or at the entry point of your flat) and isolate your gas supply to your home. 

If shutting off your supply is not an option, open windows and doors to allow ventilation and reduce the risk of gas pressure building up in your home. Call the Emergency Gas Helpline immediately on 0800 111 999.

2. Your hot water smells like rotten eggs

Suppose you are running the hot tap, and the water smells a bit ‘eggy’.

This could indicate a bacterial infection in your system. Left untreated, this could spell further problems with your boiler and needs addressing as soon as you notice it.

Bacteria thrive in warm, damp conditions. Your hot water system is a perfect place for bacteria to live and take over your system. This tends to happen if your boiler is not hot enough. 

Another common cause is the lack of use of your boiler, such as going away on holiday or an empty property where the system has been laid dormant. Bacteria can grow between 20-45ºC, so if your boiler settings work within these temperatures, you could be tempting a colony of bacteria to settle. 

There is also an increased risk of your system becoming infected with Legionella bacteria. The Health and Safety Standards recommend running your boiler or hot water cylinder at temperatures above 60ºC, as the bacteria cannot survive at these temperatures. 

Remember to also check the cold water

If you run your cold water for around a minute and detect an unusual smell, it could indicate an issue with municipal water supplies. This could be the best-case scenario, as your boiler is working fine, and the problems lie with the water supply to your street.

If this is the case, it's best to consult your neighbours about their water and inform your local water board.

If, however, there is no unusual smell coming from your cold water, this could spell an issue with your drainage, boiler or system.

How to treat the hot water bacteria problem

This is a job for a professional. A qualified engineer can treat the issue of a bacteria infection with biocide flushing, which contains Chlorine Dioxide to kill off infections. 

To ensure the infection has been treated successfully, your engineer will conduct tests to determine if the treatment has been successful.

How can I reduce the risk of gas leaks and bacterial infections in my boiler and system?

To keep on top of your boiler’s health, it’s essential to book annual boiler servicing that helps to spot minor faults before they develop into something more serious. 

It’s also important to ensure that your engineer is Gas Safe registered. Rest assured, all BOXT engineers are qualified and experienced in the field. 

Our boiler repair services and comprehensive Home Cover plans protect your boiler and ensure you’re in expert hands.

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Final thoughts

In short, a boiler that smells like rotten eggs is not pleasant, and could be dangerous. If you’re worried about your boiler and believe there’s something wrong, we always recommend contacting our boiler engineers.