Carbon monoxide is produced whenever a carbon fuel burns. In a correctly working system all of it goes up the flue and outside. The chimney is not a convenience — it is the safety device.
What makes CO dangerous is that none of your senses detect it. It has no color, no smell and no taste, and the early symptoms — headache, nausea, dizziness, fatigue — are exactly the symptoms of a winter virus. People go to bed with what they think is flu.
A tell worth knowing
If symptoms appear when the appliance is running and ease when you leave the house or when it has been off for a while, that pattern is worth taking seriously. Genuine flu does not improve because you went to the supermarket.
How a chimney produces it
Blockage
A bird's nest, collapsed liner tile, or heavy creosote build-up. Exhaust that cannot go up goes into the room. Nesting is seasonal and can appear over a single spring in a flue that was clear the previous fall.
A cracked liner
Combustion gases escape into the masonry and can find their way into the building — often through an adjacent unused flue in the same stack, or through the framing where the chimney passes through the structure.
Negative pressure in the house
This one surprises people. Modern homes are tightly sealed, and powerful extraction — a range hood, a bathroom fan, a clothes dryer — can pull the house into negative pressure. The flue then becomes the easiest route for air to enter, so it reverses and pulls exhaust down into the room. Nothing is broken; the pressure balance is simply wrong.
An oversized or cold flue
A flue that never reaches temperature draws weakly. Add a cold day and a downdraught and a weak draw becomes no draw.
Why a draft test is a different question
Confirming a flue is clear tells you nothing is blocking it. A draft test measures whether the flue is actually pulling combustion gases out of the building under the conditions the house runs in — doors shut, extraction on, in the weather you actually have.
Those are two different questions, and it is entirely possible for a chimney to pass the first and fail the second. A CO test at the appliance completes the picture by measuring what is present in the room while it runs.
Alarms are the last line, not the first
Fit them — on every level, near sleeping areas, and to the manufacturer's placement guidance. Test them monthly and replace the unit at the end of its stated life; sensors degrade whether or not the alarm has ever sounded.
But an alarm tells you that CO has already reached the room. The chimney is what stops it getting there. The alarm is your backup, not your system.
If an alarm sounds
- Get everyone outside into fresh air immediately.
- Call emergency services from outside — do not go back in to investigate.
- Do not use the appliance again until the cause has been identified and fixed.
- Seek medical attention if anyone has symptoms, and say you suspect CO exposure.
An alarm that has sounded means the appliances and their venting get looked at, not that the alarm gets reset.
Where to start
ABC Chimney Solutions looks at the chimney, the venting and the conditions the house runs in together, which is what the draft and safety hub covers. Where the flue is not carrying gases out properly, the fix is usually a draft diagnosis or a correctly sized furnace or boiler liner rather than anything else.
Frequently asked
Can a gas fireplace produce carbon monoxide?
Yes. Any combustion appliance can, and gas systems have a particular risk profile because they produce no soot and no smoke — so a developing venting problem gives none of the visible warnings a wood fire would. ABC Chimney Solutions would say annual inspection matters more on gas, not less. Learn more about this service.
Do I need a CO alarm if I only have a wood stove?
Yes. Solid fuel produces carbon monoxide like any other. A blocked flue, a downdraft or a slow smoldering burn with the air shut down can all put it into the room, and ABC Chimney Solutions recommends an alarm on every level of the home regardless of fuel. Learn more about this service.
What does a draft test involve?
ABC Chimney Solutions looks at whether the chimney and venting are actually carrying combustion gases out of the building under the conditions the house runs in — doors closed, extraction operating — because a flue that is clear is not the same as a flue that draws. Where that points to a venting fault, the fix is a liner, a damper or a draft correction rather than a test result. Learn more about this service.