Carbon Monoxide From a Boiler: Warning Signs

Why a boiler makes carbon monoxide at all

Carbon monoxide (CO) comes from gas that burns without enough air. A boiler that is set up, flued and serviced correctly burns cleanly and sends what little CO it makes straight out through the flue. Three things upset that: a burner that is dirty or badly adjusted, a flue that is blocked, cracked or disconnected, and a boiler casing whose seal has failed so combustion gases escape into the room.

CO has no colour, taste or smell. The rotten-egg smell of gas is an added odorant, so you can have a CO leak with no smell at all. That is why the signs below matter, and why an alarm matters more.

Warning signs on the boiler and in the room

What you can see depends on the type of boiler. Most boilers fitted in the last twenty years are room-sealed: the burner is behind a sealed case and you cannot see the flame. Older open-flued boilers, back boilers behind gas fires and many gas fires themselves have a visible flame or pilot, and on those a pilot light that keeps going out is worth taking seriously.

• A flame that is yellow, orange or floppy rather than crisp and blue, on any appliance where you can see it
• Soot, black marks or yellow-brown staining on the boiler casing, the wall above it or around the flue
• A pilot light that keeps blowing out
• Much more condensation than usual on the windows of the room the boiler is in
• A flue pipe that has come apart at a joint, is visibly damaged, or has been boxed in with no way to inspect it
• The boiler running with its front panel off or loose

Never run a room-sealed boiler with its casing removed or its case screws missing. The case is part of the seal that keeps combustion gases out of the room.

Symptoms in people: how carbon monoxide poisoning shows up

The NHS lists headache, dizziness, feeling or being sick, weakness, tiredness and confusion, chest and muscle pain and shortness of breath. Early on these look like flu or a hangover, which is why CO is so often missed.

The patterns that point to CO rather than a bug are more useful than any single symptom: the symptoms ease when you go outside or leave the house for the day and come back when you return; several people in the home, or pets, are unwell at the same time; and they are worse when the heating has been on for a while or in the room with the boiler.

What to do right now if you suspect CO

Act on the suspicion. You do not need to be sure.

• Open doors and windows
• Switch the boiler off at the controls and turn the gas off at the meter if you can do it quickly
• Get everyone, including pets, out of the house into fresh air
• Call the National Gas Emergency Service on 0800 111 999. It is free and open 24 hours
• Get medical advice: NHS 111, or 999 if anyone is confused, drowsy, has chest pain or has collapsed
• Do not use the boiler, or any gas appliance you suspect, until a Gas Safe registered engineer has checked it

If a CO alarm is sounding, treat it as real. Do not press the silence button and carry on. The test button only checks the battery and sounder, so an alarm that tests normally afterwards has not been proved wrong.

What the engineer checks

The gas emergency engineer will make the appliance safe, which usually means switching it off, fixing a "Danger, Do Not Use" label and recording it as "Immediately Dangerous" or "At Risk" on a warning notice. The fault itself is then found and fixed by a Gas Safe registered engineer.

On a modern condensing boiler that means a combustion analysis: a flue gas analyser probe goes into the test point on the flue and reads the CO level and the ratio of CO to CO2. The reading is compared with the maker’s limits. A high ratio points to the burner, the gas valve setting, a failing fan or a blocked heat exchanger. The engineer then checks the flue along its whole length, every joint, the terminal outside and the case seal. On an open-flued appliance they also do a spillage test to prove the flue is drawing combustion gases away.

Flues hidden in ceiling voids or boxing are a known risk in newer flats. Gas Safe Register guidance has expected inspection hatches along concealed flue runs since the end of 2012. Where an engineer cannot see the flue, they classify the boiler "At Risk" and advise that it stays off until hatches are fitted.

Our emergency boiler repair service covers this kind of fault once the emergency service has made it safe. The £60 call-out gets an engineer to you, the repair price is agreed on site before work starts, and gas work is carried out by Gas Safe registered engineers. Check any engineer first using how to check the Gas Safe Register.

CO alarms: which, where, and what the law says

Buy an alarm marked to the European standard EN 50291 with a sealed battery. They typically cost £15 to £35 in DIY shops and last seven to ten years; the replace-by date is printed on the back. Fit one in the room with the boiler and, in a bigger home, one in each bedroom and any room with a gas fire.

Follow the maker’s instructions for placement. As a rule it goes high up, one to three metres horizontally from the boiler, not directly above it, not in a cupboard, and not right next to a window or an extractor fan.

For rented homes in England, the Smoke and Carbon Monoxide Alarm (Amendment) Regulations 2022 have required landlords, private and social, since 1 October 2022 to fit a CO alarm in any room used as living accommodation that has a fixed combustion appliance, gas cookers excepted, and to repair or replace an alarm once told it is faulty. Landlords must also have gas appliances checked every year under the Gas Safety (Installation and Use) Regulations 1998 and give tenants a copy of the record within 28 days. If you rent, see landlord responsibilities in a plumbing emergency.