You should get your boiler checked immediately if you notice any signs that may indicate carbon monoxide poisoning. Carbon monoxide is a highly poisonous gas that is responsible for killing around 50 people a year in the UK. As it can easily go undetected due to lack of colour, scent or taste, it is important to be aware of possible symptoms:

Nausea and vomiting
Stomach pain
Shortness of breath and difficulty breathing
Confusion, fast and irregular heart rate and seizures

These symptoms can be mistaken for food poisoning, viral infections or the flu, but if the symptoms disappear when you leave your house, you should have your boiler checked. As a safety precaution, it is advisable to purchase an audible carbon monoxide alarm; available from most DIY stores and costing between £20-30.
Boilers installed and regularly serviced by a qualified Gas Safe registered engineer will be safe and reliable. However, if a boiler has been fitted badly or not properly maintained, there will be warning signs if it isn’t working properly:

Lazy yellow flames on your gas appliances rather than a crisp blue one
Pilot lights that frequently blow out
Excessive condensation on windows
Unusual dark marks or staining around or on gas appliances

For further information on this topic, please take a look at our articles on carbon moxoxide poisoning and gas safety awareness in the home.