Signs worth taking seriously
- A yellow or orange flame instead of a crisp blue one — a properly burning gas flame should be blue.
- Soot or dark staining around the boiler casing.
- Excess condensation on nearby windows that's new or worsening.
- A pilot light that keeps going out.
- A working carbon monoxide alarm going off. Take this seriously every time — never assume it's a false alarm without checking.
Any of these can point to incomplete combustion, which is how carbon monoxide is produced. CO is colourless and odourless, which is exactly why a working CO alarm near your boiler matters — don't rely on being able to smell or see a problem.
Symptoms of carbon monoxide exposure
Headaches, dizziness, nausea, breathlessness and tiredness that improve when you're away from the property and return when you're back are classic warning signs. If you or anyone in the household experiences this, get fresh air immediately and seek medical advice — don't wait to see if it passes.
What to do
Turn the boiler off if you can do so safely, ventilate the area, and get it looked at before using it again. Don't try to diagnose or fix a suspected combustion fault yourself. If you ever smell gas specifically, see our separate guide on what to do if you smell gas — the steps are slightly different and time-sensitive.