Carbon Monoxide Alarms for UK Trades 2026 — Compliance, Siting and the Law

Why Carbon Monoxide Is So Dangerous

Carbon monoxide (CO) is produced by the incomplete combustion of any carbon-based fuel — natural gas, LPG, oil, coal, wood, pellets, charcoal and petrol all produce it when they don't burn cleanly. A correctly burning appliance produces almost none. A blocked flue, a cracked heat exchanger, poor ventilation or a badly serviced burner can push CO levels up dangerously fast.

What makes CO uniquely hazardous is that the human body cannot detect it. It is colourless, odourless and tasteless, and it does not irritate the eyes, nose or throat. Early symptoms — headache, dizziness, nausea, tiredness — are easily mistaken for flu or simple fatigue, and victims often fall asleep before they realise anything is wrong. CO binds to the haemoglobin in blood several hundred times more readily than oxygen, starving the brain and heart of oxygen. At high concentrations it can cause collapse and death within minutes. A working, correctly sited alarm is frequently the only warning anyone gets.

The Law: When a CO Alarm Is Required

There are two separate legal routes that can trigger a CO alarm requirement, and as a trade you need to understand both because they apply in different situations.

Building Regulations — Approved Document J

Approved Document J of the Building Regulations covers combustion appliances and fuel storage. It requires a carbon monoxide alarm to be fitted whenever a new or replacement fixed combustion appliance is installed in a dwelling. This applies across fuel types — gas boilers, oil boilers, wood-burning stoves, solid-fuel appliances and biomass systems all bring the requirement into play when newly fitted or replaced.

The practical point for trades is that the trigger is the installation work, not the property. If you install or replace a qualifying appliance, Part J expects a CO alarm to be provided and correctly sited as part of that job. Historically the rules were tightest on solid-fuel appliances, but the scope has broadened, and the safe working assumption now is that any new or replacement combustion appliance should come with a compliant CO alarm fitted. Gas cookers have in some cases been treated differently, but you should not assume an exemption — confirm the current requirement for the specific appliance and fuel before signing off the job.

The Smoke and Carbon Monoxide Alarm Regulations

Separately, the Smoke and Carbon Monoxide Alarm (England) Regulations place a duty on landlords. The rules require a carbon monoxide alarm to be fitted in any room used as living accommodation that contains a fixed combustion appliance. The scope was extended so that this now includes gas appliances such as gas boilers and gas fires — not only solid-fuel appliances as in the earlier version of the rules. Gas cookers are generally excluded, but a gas boiler in a kitchen or a gas fire in a living room brings the requirement in.

Landlords must make sure the alarm is working at the start of each new tenancy. Scotland and Wales operate their own broadly similar regimes — Scotland's rules require CO detection where there is a carbon-fuelled appliance or a flue, and Wales applies its own standards for rented housing. If you do contractor work for landlords or letting agents, you are often the person on site who is expected to fit the alarm and confirm it works, so it pays to know which appliances pull the duty in.

Where CO Alarms Are Required — At a Glance

Treat the table as a starting point, not a substitute for checking the appliance and the current regulations. Where a job sits in a grey area, the cautious and defensible choice is to fit a compliant alarm and record why.

The Standards: BS EN 50291 and Siting Guidance

Any CO alarm you fit on a UK job should be marked to BS EN 50291 — the British and European standard for electrical apparatus for the detection of carbon monoxide in domestic premises. A device that does not carry this marking should not be installed for compliance work, regardless of how it is sold. Look for the standard number and a UKCA or CE mark on the unit before fitting.

Siting is governed by the alarm manufacturer's instructions and by the guidance in BS 50292, which sets out where alarms should and should not be placed. The manufacturer's instructions always take priority — if they conflict with general guidance, follow the instructions for that specific product. The principles below are the common thread across most manufacturers.

Where to Place a CO Alarm

CO is roughly the same density as air, so unlike heat or smoke it does not reliably rise or sink — it mixes and moves with room air currents. That changes the siting logic compared with a smoke alarm.

• In the same room as the appliance: An alarm should be fitted in the room containing the fixed combustion appliance.
• Horizontal distance from the appliance: Typically sited between 1m and 3m from the appliance, measured horizontally. Too close and false alarms from start-up emissions become likely; too far and a leak may not be detected promptly.
• Height: If wall-mounted, the alarm should be higher than any door or window but around 150mm below the ceiling. If ceiling-mounted, it should be at least 300mm from any wall. Always follow the product instructions for exact figures.
• Rooms the flue passes through: Where a flue runs through a room that does not itself contain the appliance — for example a bedroom the flue pipe travels through — an alarm should also be fitted in that room.
• Sleeping areas: Consider an additional alarm in or near bedrooms so occupants are woken if CO migrates while they sleep.

Where NOT to Place a CO Alarm

Bad siting is as much of a failure as no alarm at all, because it produces nuisance alarms (which get the unit unplugged) or a false sense of safety. Avoid these locations:

• Directly above or immediately next to the appliance, where normal start-up emissions can trigger it.
• Inside enclosed or sealed spaces such as a boiler cupboard with the door closed, where air does not circulate.
• In a dead-air spot — the apex of a pitched ceiling, or behind curtains, furniture or doors.
• Where it will be obstructed, or where temperature or humidity falls outside the product's rated range — directly above a sink, near extractor fans, or close to windows and air vents that pull the CO away before it reaches the sensor.
• Anywhere the manufacturer's instructions specifically exclude.

Sensor Lifespan, Batteries and Replacement

CO alarms do not last forever — the electrochemical sensor inside degrades over time whether or not it has ever detected CO. Most domestic units have a working life of 5 to 7 years, marked on the casing as an expiry or replace-by date. After that date the alarm should be replaced entirely, not just have its battery changed.

There are two broad battery types. Sealed-for-life units contain a long-life battery that lasts the full rated lifespan and cannot be removed or replaced — when the alarm reaches end of life, the whole unit is binned. These are the better choice for rented properties because a tenant cannot remove the battery for another use. Replaceable-battery units are cheaper up front but rely on the occupier swapping the cell, which is a common point of failure. Whichever type you fit, note the expiry date in your records so a replacement can be scheduled before the sensor times out.

Testing and What the Alarm Stages Mean

Every CO alarm has a test button that checks the electronics and sounder — but pressing it does not test the CO sensor itself, only that the unit powers up and the horn works. Advise occupants to press the test button weekly. The standard alarm signals are:

• Steady or repeating loud alarm (four beeps, pause, repeat): CO is present. Occupants should get fresh air, open windows, turn off the appliance if safe, leave the property and call the relevant emergency line — and not re-enter until it is safe.
• Single chirp every 30–60 seconds: Low battery or fault — replace the battery (or the whole sealed unit) promptly.
• Repeated chirp pattern with an end-of-life indicator: The sensor has reached its expiry — replace the entire alarm.

Make sure the people living in the property understand the difference between a real CO alarm and a low-battery chirp, because confusion between the two is exactly how warnings get ignored.

CO Alarm vs Smoke Alarm — Not Interchangeable

A CO alarm and a smoke alarm are completely different devices and one cannot do the other's job. A smoke alarm detects the particles produced by a fire; a CO alarm detects the carbon monoxide gas produced by incomplete combustion, which is invisible and produces no smoke. A smouldering gas or oil fault can fill a home with lethal CO without ever setting off a smoke detector.

Because CO does not behave like smoke, the siting rules differ too — a smoke alarm goes on the ceiling to catch rising smoke, whereas a CO alarm is sited relative to the appliance as described above. There are combined smoke-and-CO units available, but where you fit one, confirm it is certified to both relevant standards and sited so that it satisfies the requirements for each function.

The Trade's Responsibility — Advise, Fit and Record

On any job involving a combustion appliance, the responsibility increasingly runs through the trade. If you are installing or replacing a qualifying appliance, fitting a compliant CO alarm is part of doing the job to Building Regulations. Even where it is not strictly mandatory, advising the customer to fit one — and recording that advice — is good practice and protects you if something goes wrong later.

Documentation matters as much as the physical work. For every relevant job, record the alarm make and model, the standard it meets, where you sited it and why, the test you carried out, and the sensor expiry date. Hand the customer the manufacturer's instructions and explain weekly testing and what the different signals mean. For landlord work, keep evidence that the alarm was working at hand-over. This record is your defence if a claim is ever made, and it is the kind of detail that separates a professional trade from a casual one.

One rule overrides everything else on gas work: any work on a gas appliance — installing, replacing, servicing or disconnecting — must be carried out by a Gas Safe registered engineer. Fitting a battery CO alarm is a simple task, but the appliance work that triggers the requirement is not, and only a Gas Safe registered engineer is legally permitted to do it. If you are not Gas Safe registered, do not touch the gas appliance.

Pulling It Together

CO alarms sit at the intersection of safety and compliance, and the rules have only tightened. Know the two legal routes — Building Regs Part J for installation work and the Smoke and Carbon Monoxide Alarm Regulations for landlord duties. Fit only alarms marked to BS EN 50291, site them by the manufacturer's instructions and BS 50292, track the 5–7 year sensor expiry, and record everything you do. Get those basics right on every relevant job and you protect your customers, your business and your name — and you do it without adding much more than a few minutes and a clear note to each visit.