Carbon Monoxide Safety: Symptoms, Alarms, Action
Carbon monoxide is colourless, odourless and produced when fuels burn incompletely — which is why a faulty or poorly ventilated boiler, fire or heater can produce it without anyone noticing until people feel ill. The defences are simple and non-negotiable: an audible CO alarm to the current British Standard, annual servicing of every fuel-burning appliance by a registered engineer, and knowing the symptoms and the emergency response before you ever need them.
Recognising the symptoms
CO poisoning symptoms resemble flu without the fever: headache, dizziness, nausea, breathlessness, collapse and loss of consciousness in severe exposure. The pattern that should alarm you is contextual — symptoms that improve when you leave the property and return when you come back, several people (or pets) in the same building feeling unwell together, or symptoms appearing when a particular appliance is running.
Take that pattern seriously even if the CO alarm hasn't sounded: alarms fail, batteries die, and an alarm in the wrong room may not see what you're breathing. Anyone with suspected CO poisoning needs fresh air immediately and urgent medical advice — tell the medical service you suspect carbon monoxide, because it changes what they test for and how they treat.
Warning signs on appliances
Visible indicators of incomplete combustion: a lazy yellow or orange flame where it should be crisp and blue (on appliances where the flame is visible), sooty or brown/black staining around an appliance, excessive condensation in the room, a pilot light that frequently blows out, and increased soot or scorching near the flue. Any of these on a gas appliance means stop using it and get a Gas Safe registered engineer to inspect.
Structural causes matter too: blocked or damaged flues and chimneys, blocked air vents (frequently sealed up during draught-proofing or insulation work), and appliances installed in unventilated spaces. A serviced, correctly ventilated appliance venting through a sound flue is the state you're maintaining — and it's exactly what the annual service verifies with instruments rather than assumptions.
What to do if you suspect CO
Immediately: get everyone out into fresh air, open doors and windows if you can do so on the way, turn off the appliance if it's safe to do so without delay, and do not go back in. If you also smell gas, call the National Gas Emergency line free on 0800 111 999. Seek urgent medical attention for anyone with symptoms, and tell them you suspect carbon monoxide.
Then: do not use the appliance again until a Gas Safe registered engineer (or the appropriate registered engineer for oil or solid fuel) has inspected and cleared it. An engineer finding an appliance immediately dangerous will disconnect or turn it off with your permission and label it — that outcome is the system working, not an inconvenience to negotiate.
Alarms: the practical detail
Fit audible alarms to the current British Standard (look for BS EN 50291 marking and a kitemark), not cheap unmarked units or the colour-changing patches that can't wake anyone. Place them in every room containing a fuel-burning appliance, plus sleeping areas — typically at roughly breathing height or as the manufacturer directs, a sensible distance from the appliance itself (not directly above it), and away from windows and extractors that would ventilate the sample away.
Test monthly with the test button, replace batteries as the unit requires, and note that sensors themselves expire — most units carry a 7–10 year replacement date printed on the case. Landlords have specific obligations around CO alarms which vary by nation and have been extended in recent years; check current requirements for your jurisdiction and treat them as a floor rather than a ceiling.
| Item | Detail |
|---|---|
| Symptoms | Headache, dizziness, nausea, breathlessness, collapse |
| Key pattern | Better away from the property, worse on return |
| Appliance signs | Yellow/orange flames, sooty staining, condensation, pilot blowing out |
| Emergency action | Everyone out, ventilate, don't return, seek medical help |
| Gas smell too? | Call 0800 111 999 (National Gas Emergency) |
| Alarms | BS EN 50291 audible units; test monthly; replace at expiry date |
Where advice stops and engineers start
This site's safe-check boundary is the boiler casing, and it doesn't move: filling loop, condensate pipe, thermostat, bleed valves and one reset are yours; everything behind the case is Gas Safe engineer work by law and by sense. Any source telling you otherwise about gas appliances is worth closing. The upside of a firm boundary is that everything on your side of it is genuinely safe to attempt.
Frequently asked questions
Can I smell carbon monoxide?
No — it's completely odourless, colourless and tasteless, which is exactly what makes it dangerous. An audible alarm to the current British Standard is the only reliable detection, alongside annual servicing of every fuel-burning appliance.
Where should CO alarms be placed?
In every room with a fuel-burning appliance and in sleeping areas, positioned per the manufacturer's instructions — not directly above the appliance, and away from windows and extract fans that would ventilate the sample away.
What are the first signs of CO poisoning?
Headache, dizziness and nausea most commonly — flu-like but without fever. The revealing pattern is symptoms that ease when you leave the building and return when you do, or several occupants unwell at once.
Does an annual service prevent carbon monoxide?
It's the main preventive measure: the service verifies combustion with a flue-gas analyser and checks the flue and ventilation. Combine it with a working alarm — the service prevents, the alarm catches what prevention misses.
Fixing causes, not symptoms
The theme running through our guides holds here too. A repair that addresses why the fault happened — the pressure that keeps dropping, the sludge that keeps returning, the condensate pipe that freezes every January — is the one you don't repeat next winter. When you book through our network, say you want the cause addressed rather than the symptom silenced; it occasionally costs a little more on the day and reliably costs less across the years you'll own the system.
Carbon monoxide: what to watch for and how to respond — the warning signs on appliances, and why annual servicing is the main defence.