Carbon monoxide isn’t visible and can’t be smelled, but can make you very ill.

As millions of British  homes don’t have an alarm in place, we found out what carbon monoxide poisoning is, why you should have an alarm and what you can do to keep your boiler safe.

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1. What is carbon monoxide poisoning?

Potentially deadly carbon monoxide is created when a fuel isn't burning correctly and is usually - although not exclusively - emitted from faulty gas boilers, fires and cookers.

The symptoms of carbon monoxide poisoning include headaches, dizziness, weakness and chest pain, so it can easily be confused with other illnesses, especially the flu.

Carbon monoxide has no colour, taste or smell, which makes spotting faulty appliances hard - unless, of course, you fit carbon monoxide alarms.

Wired-in alarms should be installed by an electrician and are always on (unless the power's off), but most of us have battery-powered alarms.

These have either replaceable batteries, or sealed-for-life batteries that can't be replaced and usually last between seven and 10 years.

2. What sort of alarm should you have?

Make sure your alarm complies with EN 50291 (this should be on the box) and has a quality certification, such as the British Standards Kitemark.

The cheapest and simplest detectors only give a visual indicator - they change colour when detecting carbon monoxide, for example - but an audible alarm is much safer.

Hearing an alarm could save your life, especially at night when you're asleep and wouldn't see a visual change.

Having alarms that are linked (this can be done wirelessly - look for the Wi-Safe 2 logo) could also save your life because if one alarm goes off, they all will.

3. How do you fit a carbon monoxide alarm?

When you get a new carbon monoxide alarm, fit it according to the installation guide - it should be 1m-3m from the fuel-burning appliance, for example.

Test the alarm straightaway so you know that it works and what it sounds like, so you don't confuse it with your home's smoke alarms, and then remember to test it once a week.

4. Where should it be fitted?

A carbon monoxide alarm should be fitted on every floor of your home (ideally in all the bedrooms) and in every room with a fuel-burning appliance or a flue, even if it's concealed.

For more information on alarms and carbon monoxide in general, click here.

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5. How can I make sure my appliances aren’t emitting the gas?

Many fuel-burning appliances, such as boilers, should be serviced annually to ensure they're safe and working efficiently, something we often forget to do.

A boiler service, which includes safety checks, among other things, must be carried out by a Gas Safe Register engineer, or OFTEC registered for oil-fired boilers.