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Is my boiler leaking carbon monoxide?

alarm on a ceiling with a finger pushing the button

If you think a gas appliance is leaking carbon monoxide, call the free National Gas Helpline immediately

Call: 0800 111 999

The service is open 24 hours a day, 7 days a week.

What is carbon monoxide?

Carbon monoxide is a gas produced in a boiler when fuel is not burned completely due to a lack of oxygen. When carbon monoxide is produced, boilers release the gas outside the home via a flue. However, this gas can sometimes escape from the boiler system into the home and cause carbon monoxide poisoning.

What are the signs of carbon monoxide poisoning?

Carbon monoxide poisoning can cause significant health issues if left undetected. The most common symptoms of carbon monoxide poisoning include:

  • Headaches
  • Dizziness
  • Nausea
  • Confusion
  • Shortness of breath
  • Loss of consciousness

Exposure to carbon monoxide can be harmful, even at low levels, because it causes the levels of oxygen in the brain to drop. If you experience these symptoms over an extended period of time you should seek help from a medical professional immediately and turn off the gas appliances in your home.

How do I know if my boiler is leaking carbon monoxide?

Carbon monoxide leaks are notoriously difficult to detect because the gas is colourless, odourless and tasteless. There are common signs, however.

For instance, if your boiler struggles to stay alight this could be a sign of a carbon monoxide leak. This is because the release of carbon monoxide in the system depletes the levels of oxygen needed to keep the flame alight.

Other signs include seeing greater amounts of condensation on windows than usual, as well as unusual dark staining around your gas appliances. These two things point to carbon monoxide being present inside your home, which often go unnoticed unless coupled with the physical symptoms of carbon monoxide poisoning.

If you find these signs in your home, turn off all gas appliances at the mains and leave the premises immediately. You should then call a Gas Safe Registered (GSR)engineer to inspect your appliances. In general, the most common cause for carbon monoxide leaks are faulty gas appliances, so it is important to have them serviced regularly by a GSR engineer.

Do I need a carbon monoxide detector for a gas boiler?

It is recommended to install a carbon monoxide detector in every room where there is a gas appliance. Currently, UK law dictates that in England and Wales a carbon monoxide detector must be installed in any room which is used at least partly as ‘living accommodation and contains a solid fuel burning appliance.’ Living accommodation includes bathrooms.

Solid fuels are defined as wood and coal, which means gas and oil appliances are exempt. While private homeowners and landlords are not obligated by law to install a carbon monoxide alarm in rooms without solid fuel appliances, we highly recommend that they do so.

Faulty gas boilers are one of the most common reasons for carbon monoxide leaks in the UK, and installing a carbon monoxide detector (which costs less than £30) helps detect these leaks before they cause harm.

What appliances leak carbon monoxide?

Any fuel-burning appliance could theoretically create a carbon monoxide leak. Aside from a gas or oil burner, stove burners, water heaters, gas tumble dryers and charcoal grills could all leak carbon monoxide.

Will opening windows reduce carbon monoxide?

Opening windows can help dilute the carbon monoxide by bringing in fresh air. However, it will not do much to help reduce the level of carbon monoxide in the rest of the home. The best thing to do if you suspect your appliances are leaking carbon monoxide is to turn all your appliances off at the mains, exit the premises and call a GSR engineer to run the necessary checks.

Can a boiler release carbon monoxide when off?

No. If your boiler is switched off at the mains, your boiler will not be burning fuel and therefore no waste carbon monoxide gas will be produced.


  • Carbon monoxide is a gas produced in a boiler when fuel is not burned completely due to a lack of oxygen.
  • Common symptoms of carbon monoxide poisoning include dizziness, headaches and shortness of breath.
  • A stuttering pilot light or dark staining around gas appliances is a sign of carbon monoxide leaks.
  • Boilers should be serviced yearly by a GSR engineer to reduce the risk of carbon monoxide leaks.
  • Homeowners should install carbon monoxide alarms to detect leaks as soon as they occur.

For more information on carbon monoxide, visit the NHS website