Carbon Monoxide Alarm Going Off

What's happening?

Carbon monoxide is produced when a carbon fuel — gas, oil, coal, wood — burns without enough air, or when the products of combustion cannot escape through the flue. It is colourless, tasteless and has no smell, so an alarm is the only warning you will get before symptoms start. HSE records around seven deaths a year in Great Britain from CO poisoning caused by gas appliances and flues that were badly installed, poorly maintained or short of ventilation, and many more non-fatal exposures. A sounding alarm is not a nuisance to silence; it is a reading. Treat it as a genuine detection until a Gas Safe registered engineer has tested the appliances and proved otherwise.

What usually causes it?

• A blocked or partially blocked flue or chimney — nests, debris, or a collapsed liner
• A boiler, fire or water heater burning rich because the burner or heat exchanger is fouled with soot
• An air vent or airbrick blocked up, covered by furniture, or filled in during draught-proofing
• A flue passing through a sealed void with no inspection hatch, so a joint has separated unseen
• A flueless appliance such as a gas cooker used for long periods to heat a room
• A neighbouring flue terminating close to your window or air vent, pushing its products back in
• A solid fuel stove or open fire sharing a chimney with a gas appliance

When should you call a gas engineer?

After the emergency service has attended, a Gas Safe registered engineer needs to find the cause: a flue gas analyser reading on each appliance, a flue integrity check, and a ventilation calculation for the room. That inspection is the only way to know which appliance was at fault and whether the rest are safe. Book it before you use any gas appliance again, and ask for the combustion readings in writing.

What will the gas engineer actually do?

• Test each appliance with a calibrated flue gas analyser and record the CO and CO/CO2 ratio readings
• Inspect the flue or chimney along its full route, including any hatch into a concealed void, for blockage or separated joints
• Measure the purpose-provided ventilation to the room and compare it with what the appliance needs
• Strip, clean and re-set the burner, injectors and heat exchanger where combustion is poor
• Classify anything unsafe under the industry unsafe-situations procedure and cap or disconnect it with your permission
• Recommission the appliances, re-test, and advise on alarm placement to current standards

If the alarm sounds

1. Turn off every gas appliance, including the hob.
2. Open the doors and windows to ventilate the rooms.
3. Get everyone out into fresh air.
4. Ring the National Gas Emergency Service on 0800 111 999.
5. If anyone has symptoms, get urgent medical advice and say you suspect carbon monoxide.
6. Do not use any gas appliance again until an engineer has tested it.