Most UK homeowners associate carbon monoxide with boilers and faulty gas appliances. That association is correct — but it’s also incomplete. Chimneys are one of the most common domestic sources of CO exposure in Britain, and they have one quality that makes them more dangerous than a boiler: nobody thinks to test them.

You service the boiler annually because British Gas, your landlord, or your common sense told you to. The chimney sits there for years, soot building up, draw weakening, until something tips the balance — and then the only thing standing between your family and a hospital visit is a £25 alarm that may or may not be in the right place.

Carbon monoxide is the only common household poison you cannot smell.

What CO actually does

Carbon monoxide binds to haemoglobin in your blood roughly 200 times more readily than oxygen does. Even small amounts displace the oxygen your cells need to function. Your brain notices first because it’s the most oxygen-hungry organ in your body, which is why early symptoms look like fatigue or a hangover.

The insidious thing about CO is that the symptoms feel like a hundred ordinary things. A headache that won’t shift. Drowsiness in the afternoon. Mild nausea. A vague flu-like feeling that improves when you leave the house and returns when you get home. By the time anyone connects those dots to the chimney, exposure has often been ongoing for weeks or months.

The NHS estimates around 60 accidental CO deaths per year in the UK, with thousands more hospitalisations — and many cases that are misdiagnosed as flu, food poisoning, or chronic fatigue. CO is sometimes called the silent killer for the obvious reason: most victims have no idea they were exposed until a routine blood test catches it, or until they don’t wake up.

How a chimney ends up venting CO into your home

Every time you burn fuel — wood, coal, smokeless briquettes, gas — you produce carbon monoxide. That’s normal. The job of the chimney is to carry that CO out of your house, up the flue, and into the sky. As long as the flue is drawing properly, the system works.

What stops a flue drawing properly:

• Soot and creosote build-up — narrows the flue, restricts the upward draught, and starts pushing combustion gases back into the room.
• Bird and vermin nests — jackdaws are the worst offenders in the North East. A twiggy nest at the top of the pot can shut a flue almost completely, and the warmer the chimney, the more attractive the nesting site.
• Downdraughts — caused by nearby buildings, taller trees, wind eddies, or improperly sized cowls. CO from a fire can be pushed downward and back into the living space.
• Cracked or deteriorated flue liners — allow CO to leak directly through the chimney wall into bedrooms or roof voids above, where there is often no alarm.
• Negative pressure in the home — bathroom extractors, kitchen hoods, or modern airtight insulation can create suction that pulls flue gases backward.
• Re-lit chimneys after long periods of disuse — a previously unused fireplace is far more likely to have nests, debris, or moisture damage than one in regular use.

Any one of these is enough to turn a working appliance into a source of CO. Two of them at once is common.

Symptoms, ranked by severity

The NHS lists the warning signs of CO poisoning in roughly the order you’ll experience them as exposure increases:

• Dull, persistent headache
• Dizziness, especially on standing
• Nausea or vomiting
• Unusual tiredness or confusion
• Shortness of breath
• Difficulty thinking clearly or concentrating
• Loss of consciousness, collapse

The single most useful diagnostic clue: symptoms improve when you leave the house and return when you come back. If multiple people in the home are unwell at the same time — including pets — that pattern matters even more.

Why a CO alarm alone isn’t enough

A working, properly placed CO alarm is non-negotiable. The Smoke and Carbon Monoxide Alarm (England) Regulations 2015, updated in 2022, require landlords to install CO alarms in any room containing a fixed combustion appliance. Most homeowners now have at least one. Good.

But an alarm is a last line of defence, not a first. It tells you CO has already arrived in the room. By the time it sounds, the family pet may already be ill and the slowest sleeper may already have a serious headache. There are also four common failure modes for CO alarms:

• Wrong placement. CO is roughly the same density as air — it doesn’t rise like smoke. Alarms placed high on the wall (where smoke alarms go) may fail to register low-level chronic exposure at breathing height.
• Battery degradation. Sealed-cell alarms last 7–10 years; replaceable-battery alarms much less. A lot of UK homes are running alarms older than they should be.
• Sensor degradation. The electrochemical sensor inside a CO alarm wears out over time even if the battery is fresh. After about a decade, the alarm is essentially decorative.
• Low-level exposure. Many alarms only sound at concentrations of 50 ppm or higher over extended periods. Chronic exposure at 20–30 ppm — still enough to cause headaches, brain fog and long-term harm — may never trigger the alarm.

An alarm tells you the gas is already in the room.
A clean flue tells you it never arrived.

What a NACS-certified sweep catches that an alarm can’t

An annual professional sweep doesn’t just remove soot. A properly performed visit is, in effect, an annual safety inspection of the entire combustion-and-venting system. Here’s what gets checked:

1. The flue itself

Modern rotary power sweeping removes soot and creosote from the full length of the flue. While the sweep is working, the brushes meet resistance from any narrowing, deposit or partial blockage. A NACS-certified sweep will know whether what they’re feeling is normal residue or something more serious.

2. The smoke evacuation test

After the sweep, a smoke pellet is lit at the appliance and the flue’s ability to clear it is observed. This is the part that matters for CO. A flue that fails to draw smoke is a flue that will fail to draw carbon monoxide. The test catches downdraughts, partial blockages, and pressure imbalances that brushwork alone wouldn’t expose.

3. Visible structural condition

The sweep can see the appliance, the hearth, the flue entrance, the visible part of the chimney breast. Cracked flue plates, displaced bricks, deteriorating mortar lines, broken fire bricks — all of these are noted on the certificate and flagged for follow-up.

4. CCTV inspection where indicated

If the smoke test, the brushwork, or the visible inspection raises a concern, a camera inspection takes the diagnosis up the flue. CCTV catches what nothing else does: cracked liners, hairline structural damage, hidden blockages, and the early-stage masonry failure that signals a chimney needs more than a sweep.

The high-risk profiles — who should be most cautious

Some properties carry a higher CO risk than others. If any of the following describes your home, the case for an annual sweep is stronger:

• Older terraced or semi-detached homes — common across Newcastle, Sunderland, Gateshead and the wider North East — often have original brick flues that share walls with neighbouring properties. Cracks in shared masonry are not unusual.
• Period properties with original linings — an unlined Victorian flue that’s done a hundred years of service may be more porous than the homeowner realises.
• Modern wood-burner installations with twin-wall flue systems — the liner is usually fine, but the connection points and registry plate are where issues develop.
• Homes returning a fireplace to use after years of dormancy. Bird nests, debris, and degraded liners are almost guaranteed.
• Properties with cowls fitted incorrectly — the wrong cowl can create a downdraught that pushes CO back into the room every time the wind picks up.
• Highly airtight modern homes with mechanical ventilation. Combustion appliances need replacement air; airtight homes can starve a fire and force flue gases backward.

If you suspect carbon monoxide in your home right now

The NHS guidance is unambiguous. If anyone in your household has symptoms that improve when they leave the property, or if your CO alarm is sounding:

1. Stop using the appliance. Don’t relight the fire, don’t reset the stove.
2. Ventilate the property. Open windows and doors.
3. Leave the home. Get everyone out into fresh air.
4. Call for help. NHS 111 for advice if symptoms are mild. 999 if anyone is unconscious, severely confused, or struggling to breathe. If you suspect a gas appliance, the National Gas Emergency Service is 0800 111 999.
5. Don’t use the fire again until a NACS-certified sweep has cleared the flue and confirmed the system is safe.

The remedy — in order of importance

If you take one thing from this article, take this. Carbon monoxide risk from a chimney is preventable. The remedy is well-established, low-cost, and reliable:

1. An annual professional sweep by a NACS-certified sweep. Twice a year if you burn wood as your primary fuel. The sweep removes the build-up that causes both blockages and downdraughts, performs a smoke evacuation test, and produces a certificate documenting the work.
2. A working CO alarm in every room containing a combustion appliance, placed at breathing height, replaced every 7–10 years.
3. CCTV inspection after any incident, before commissioning a new appliance, or whenever the smoke test raises a concern.
4. Awareness. Know the symptoms. Take improvement-when-out-of-the-house seriously. Trust the pattern over the diagnosis.

For Newcastle, Sunderland, Gateshead and the wider North East, Black Diamond is a NACS-certified chimney sweep. Every sweep includes a smoke evacuation test and a same-day certificate. Booking takes about 30 seconds.