CO Alarm Regulations UK Explained
A carbon monoxide alarm is a last line of defence, not a first — the rules tell you where one is required by law, but siting and servicing decide whether it ever does its job.
Carbon monoxide is colourless, odourless and lethal at low concentrations, which is why UK law now mandates alarms in specific situations and why building control expects them alongside new combustion appliances. For a gas engineer the alarm sits at the end of a chain that starts with a sound appliance, correct ventilation and a clear flue — get those wrong and no alarm makes the installation safe.
This guide sets out where a CO alarm is legally required, how to site and test one to BS EN 50291 and the manufacturer’s instructions, and why fitting an alarm never replaces an annual service. It is written for the person on the job who has to advise a customer, not quote marketing copy at them.
Key takeaways
- In rented homes in England, a CO alarm is required in any room used as living accommodation that contains a fixed combustion appliance, excluding gas cookers.
- Building Regulations Approved Document J requires a CO alarm when a new or replacement combustion appliance (other than a cooker) is installed.
- Site alarms to BS EN 50291 and the manufacturer’s instructions — high on a wall or on the ceiling, roughly 1 to 3 m horizontally from the appliance, out of dead-air corners.
- Press-to-test only proves the electronics and sounder, not the CO sensor — replace the unit at its stated end-of-life date.
- A CO alarm is a backup, never a substitute for servicing the appliance and checking the flue and ventilation.
Where a CO alarm is legally required
The Smoke and Carbon Monoxide Alarm (Amendment) Regulations 2022 tightened the rules for rented homes in England. Since October 2022 a landlord must provide a carbon monoxide alarm in any room used as living accommodation that contains a fixed combustion appliance — for example a gas boiler, a gas fire or a solid-fuel stove. Gas cookers are specifically excluded from that requirement. The alarm must be repaired or replaced once the landlord is told it is faulty.
Separately, Building Regulations Approved Document J requires a CO alarm to be fitted whenever a new or replacement fixed combustion appliance is installed, other than an appliance used solely for cooking. That obligation bites at the point of installation regardless of tenure, so it applies to owner-occupied homes too when you swap a boiler or fit a new fire. Scotland and Wales have their own parallel requirements, so confirm the position for the nation you are working in rather than assuming the England rules apply everywhere.
Cooker exception, not a free pass
Gas cookers are excluded from the mandatory-alarm rules, but that reflects how they vent, not that they are risk-free. Where a room combines a cooker with another combustion appliance, the appliance drives the requirement.
Siting the alarm to BS EN 50291
A CO alarm only works if the gas can reach the sensor, so placement matters as much as the model. Follow BS EN 50291 and the manufacturer’s instructions together. As a rule the alarm goes at head height or above — high on a wall or on the ceiling — and roughly 1 to 3 m horizontally from the appliance. Too close and steam, dust or servicing can nuisance-trip it; too far and a leak may disperse before it registers.
Keep the unit out of dead-air pockets: not tucked in a corner where wall meets ceiling, not behind curtains or furniture, and not immediately above a sink or cooker where moisture collects. If it is wall-mounted, keep it clear of the ceiling by the distance the maker specifies. In a room with a sloped ceiling, treat the high side as the reference point. Where one appliance serves a space that opens into another, a second alarm may be sensible even if only one is strictly required.
Testing, maintenance and end of life
The press-to-test button checks the battery, electronics and sounder — it does not test the carbon monoxide sensor itself. That is an important distinction to give the customer: a chirping test does not mean the sensor can still detect gas. Advise them to test weekly, keep it dust-free, and never paint over or cover it.
Every CO alarm has a fixed service life, commonly around seven to ten years, printed on the unit with a replace-by date. The sensor degrades whether or not it has ever seen CO, so an old alarm can pass its button test and still fail to alarm on a real leak. When you service an appliance, glance at the alarm’s date and flag any that are past or near end of life.
Log it with the service
TradePlanr lets you record the CO alarm’s presence, siting and replace-by date on the gas service record and CP12 you issue on site, so the evidence sits with the appliance history rather than in someone’s memory.
An alarm is not a substitute for servicing
The single most important message to leave with any customer is that a CO alarm is a backup. It tells you something has already gone wrong; it does nothing to stop it going wrong. Incomplete combustion, a blocked or leaking flue, or inadequate ventilation are the causes, and only inspection and servicing address them.
Frame it plainly: the alarm is there for the day the appliance, flue or ventilation fails between services. The annual service — checking combustion, the flue integrity and the air supply — is what keeps that day from arriving. A working alarm and a serviced appliance are two different controls, and one never covers for the other.
Frequently asked questions
Do I legally have to fit a CO alarm with a new boiler?
Yes. Approved Document J requires a carbon monoxide alarm when you install a new or replacement fixed combustion appliance other than one used solely for cooking, so a new or swapped boiler triggers it regardless of whether the property is rented or owner-occupied.
Does a gas cooker need a CO alarm?
A gas cooker on its own is excluded from the mandatory-alarm requirements in the rented-homes regulations and from the Approved Document J trigger. If the same room also holds another combustion appliance, such as a boiler or fire, that appliance creates the requirement.
How close to the boiler should the CO alarm go?
Follow BS EN 50291 and the manufacturer’s instructions: generally high on a wall or on the ceiling and roughly 1 to 3 m horizontally from the appliance, clear of dead-air corners, moisture and anything that would block airflow to the sensor.
From guidance to action
Related guides
CP12: The Landlord Gas Safety Record
What a CP12 checks, the landlord’s legal duties, and how to complete one correctly.
Flue Gas Analysis and Combustion Ratios
Reading a flue gas analyser — CO/CO₂ ratio, acceptable combustion figures, and what fails.
Ventilation Requirements for Gas Appliances
When purpose-provided ventilation is needed and how to size it to BS 5440-2.
