A carbon monoxide alarm sounding beside your lit fire is not a suggestion to check the batteries later. It is the only warning you will get that an invisible, odourless gas is accumulating in the room where you are sitting. The correct response is not debate; it is movement.
What to Do in the First Minutes
If the alarm activates while the fire or stove is running, act in this order. Turn the appliance off or extinguish the fire only if you can do so quickly and without breathing the air near the hearth for longer than necessary. Open every door and window on your way out. Get everyone, including pets, into fresh air outside. Do not stop to gather belongings. Do not assume one open window is sufficient. From outside, call the National Gas Emergency Service on 0800 111 999 if the appliance is gas-fired, or dial 999 if anyone has a headache, dizziness, or nausea. Do not re-enter until a professional says it is safe. Do not relight the fire that evening, that weekend, or that week. The appliance stays off until the cause is found and fixed.
Why Chimneys Fail to Contain Carbon Monoxide
The chimney’s single job is to carry combustion gases upward and out. When it cannot, those gases sink back into the room. The alarm sounds because something has broken that upward path.
A blocked flue is the most obvious culprit. Bird nests, particularly from rooks or jackdaws, can choke a chimney at the pot or halfway down. Soot and tar buildup from infrequent sweeping does the same more gradually. Fallen masonry from a deteriorating stack can create a partial obstruction that still allows some draw while forcing gases past the blockage.
Leaks are harder to spot. A cracked flue liner, degraded mortar between courses of brick, or damage to the chimney structure itself lets carbon monoxide escape into wall cavities, loft spaces, or neighbouring rooms. The gas may never reach the pot. A smoke pellet test, performed by a sweep, reveals these breaches by tracing where smoke emerges where it should not.
Poor draught has subtler causes. The chimney may be too short for the appliance fitted beneath it. Nearby trees or taller buildings can create downdraught. A cold flue, common in external walls or unused chimneys, struggles to establish the thermal lift that pulls gases upward. Inadequate ventilation in modern airtight homes compounds the problem: the fire starves for combustion air, and the chimney starves for the pressure difference that makes it work. Building Regulations Part J sets minimum ventilation requirements for this reason.
When the Chimney Is Not the Problem
The alarm demands investigation of the whole system, not just the flue. The appliance itself may be at fault. A gas fire with a cracked heat exchanger or incorrect pressure burns incompletely and produces carbon monoxide directly. A solid fuel stove with damaged door seals, a warped baffle plate, or a stuck air control cannot regulate its burn. The result is spillage into the room.
Other household equipment can trigger the alarm without any fault in the fire or chimney. A powerful kitchen extractor fan, a bathroom extractor running simultaneously, or a tumble dryer can create negative pressure strong enough to overcome the chimney’s natural draught and pull combustion products back down. Burning unseasoned wood, with moisture content above twenty percent, or treated timber, or household waste, produces far more carbon monoxide than the flue was designed to handle and accelerates dangerous creosote buildup.
Even the alarm itself can fail, though this is the least likely explanation and must never be assumed. Treat every activation as genuine.
What a Professional Investigation Involves
A chimney sweep will start with the flue: visual inspection of the stack, pot, cowl, and liner; a smoke pellet test for leaks; a draught test with a meter or smoke match to measure the strength and direction of pull. They will check permanent ventilation openings for correct sizing and obstruction. They will look for signs of soot leakage, damaged liner, or escaping gases at joints and connections.
For solid fuel stoves, the sweep will inspect door seals, firebricks, baffle plates, and air controls. For gas appliances, a Gas Safe engineer must perform combustion analysis, check gas pressures, and examine the heat exchanger for cracks. A HETAS technician may be needed for specific solid fuel installations. The investigation is collaborative: the sweep understands the flue, the engineer understands the appliance, and neither alone may find the answer.
A calibrated carbon monoxide detector will measure ambient levels and test specifically for spillage from the appliance or flue connections, confirming both the presence and the source.
The Danger of Retesting Yourself
Some homeowners want to relight the fire to see if the alarm sounds again. This is the most dangerous impulse in the entire process. Carbon monoxide is undetectable without equipment. You cannot smell it, taste it, or see it. The alarm may have sounded at a level that was already harmful. Relighting the appliance risks immediate re-exposure to potentially lethal concentrations while the underlying fault, whatever it is, remains unchanged and invisible. The only safe test is the professional’s test, performed with the right equipment, after the right checks. Your living room is not a laboratory, and your senses are not instruments.
The alarm is a full stop, not a comma. Ventilate, evacuate, call for help, and keep the appliance cold until someone qualified tells you otherwise.