Carbon Monoxide Detectors: What London Homeowners and Landlords Must Know

What Is Carbon Monoxide and Why Is It So Dangerous?

Carbon monoxide (CO) is a gas produced by incomplete combustion — when a fuel-burning appliance (gas boiler, gas cooker, gas fire, oil boiler, wood stove) does not have sufficient oxygen to burn completely. It is completely colourless and completely odourless. Unlike natural gas, which has a distinctive sulphur smell added deliberately by suppliers, CO gives no sensory warning whatsoever. This is why it is called the silent killer.

CO kills by binding to haemoglobin in the blood approximately 200 times more readily than oxygen. The result is that red blood cells can no longer carry oxygen to the body's organs. At low concentrations, CO causes headaches, dizziness, nausea, and confusion — symptoms that closely mimic flu or food poisoning. At higher concentrations, it causes unconsciousness and death. In the UK, CO poisoning causes approximately 40–50 deaths per year, with a further 200–300 non-fatal poisonings serious enough to require hospital treatment.

The insidious danger is that victims frequently do not realise what is happening. A person waking with a headache in a bedroom with a faulty flue may put it down to illness and go back to sleep — at which point rising CO concentrations can become lethal. A CO alarm is the only reliable protection.

Legal Requirements Since October 2022

The Smoke and Carbon Monoxide Alarm (Amendment) Regulations 2022, which came into force on 1 October 2022, significantly extended the requirement for CO alarms in private rented properties in England. The key provisions for landlords are:

• A CO alarm must be installed in any room used as living accommodation that contains a fixed combustion appliance. This includes rooms with a gas boiler, a gas fire, an oil boiler, or a solid fuel appliance. Gas cookers are specifically excluded from this requirement — but see below for why fitting one near a gas cooker is still recommended best practice.
• CO alarms must be in working order at the start of each new tenancy. Landlords must test them on the day the tenancy begins.
• If a tenant reports that a CO alarm is not working, the landlord must repair or replace it as soon as reasonably practicable.

For homeowners — not landlords — fitting CO alarms is not a legal requirement but is very strongly recommended by the Gas Safe Register, the Health and Safety Executive, and the British Standards Institution.

Where to Fit Carbon Monoxide Alarms

Position matters for effective CO detection:

• Near the gas boiler: Within 1–3 metres of the boiler, on the wall at head height (roughly the same height as your head when standing — approximately 1.5–1.8 metres from the floor). Do not fit directly above the boiler where it may be triggered by normal steam or condensate vapour.
• Near a gas fire: In the same room as any gas fire or gas wall heater, again at head height within 1–3 metres of the appliance.
• Near a gas cooker: Although legally excluded from the Regulations, a CO alarm near a gas cooker is best practice — particularly for ranges and older cookers that may have incomplete combustion. Position it to one side of the cooker at head height, not directly above where cooking steam could trigger it unnecessarily.
• On each floor: Where there are gas appliances on multiple floors, fit a CO alarm on each floor — and consider the bedroom floor as a priority since occupants are most vulnerable when asleep and unable to respond to early symptoms.

CO is slightly lighter than air and distributes fairly evenly in a room, so the head height recommendation is practical rather than critical — unlike smoke alarms, which must be on the ceiling because smoke rises. The key is proximity to the appliance and an unobstructed location.

Which Alarms to Buy: Standards and Specifications

Buy a CO alarm that meets BS EN 50291-1 (for household use) or BS EN 50291-2 (for caravans and boats — not relevant here). Look for the Kitemark logo on the packaging — this is the British Standards Institution's conformity mark, indicating the product has been independently tested and certified. Do not buy alarms that carry only CE marking without a Kitemark — CE marking indicates manufacturer self-declaration of compliance, not independent testing.

Alarms are available in two main types:

• Battery-powered alarms: The most common type for domestic use. Easy to install (no wiring required), portable if repositioning is needed. Most use either standard AA/AAA batteries or a sealed lithium battery. Suitable for the majority of rental properties.
• Mains-powered alarms with battery backup: Preferred for HMOs and higher-specification installations. More reliable than battery-only (the battery backup activates if the mains supply fails). Required by some London borough HMO licensing conditions.

Lifespan: Why the Cheap Option Is a False Economy

CO alarms have a finite sensor lifespan. The electrochemical sensor that detects CO degrades over time, regardless of whether it has detected any CO. When the sensor expires, the alarm stops functioning even though it may still appear to be working. Most alarms signal end-of-sensor-life with an end-of-life alert (a different beep pattern from the normal CO alarm).

Alarm lifespans vary significantly by price point:

• Budget alarms (£10–£15): Typically 3-year sensor lifespan. Some go even shorter. Acceptable as a temporary measure but require replacement every 3 years — the cumulative cost of frequent replacement makes them poor value.
• Mid-range alarms (£20–£35): Typically 5–7 year sensor lifespan. The sweet spot for most properties.
• Premium alarms (£35–£60): Sensor lifespans of 7–10 years. Brands such as Kidde, Fireangel, and Aico offer 10-year sealed battery units where the battery and sensor are designed to last the same period — the whole unit is replaced after 10 years. For a landlord with multiple properties, these minimise ongoing management overhead.

For rental properties, invest in the 7–10 year models. The reduction in replacement frequency, the reduced risk of an expired sensor going unnoticed, and the compliance documentation simplicity are all worth the modest additional cost.

Carbon Monoxide Alarm vs Smoke Alarm: Not the Same

A smoke alarm does not detect carbon monoxide. A CO alarm does not detect smoke. They are entirely different sensors responding to entirely different phenomena. Some combination alarms exist (smoke and CO in one unit) — these are legitimate products that meet both BS 5446-2 (smoke) and BS EN 50291 (CO) standards simultaneously. Combination units are convenient but should not be confused with either a standalone smoke alarm or a standalone CO alarm.

Symptoms of CO Poisoning vs Flu

The classic differentiator: flu symptoms get worse regardless of where you are; CO poisoning symptoms improve when you leave the property and return when you come back inside. Additional indicators of CO rather than flu:

• Multiple people in the household or building showing similar symptoms at the same time
• Pets appearing unwell — dogs and cats are also susceptible to CO poisoning
• Symptoms worst in the morning after sleeping in a poorly ventilated room
• No fever (CO poisoning does not cause fever; flu usually does)

What to Do if a CO Alarm Sounds

1. Do not attempt to find the source or investigate the appliances.
2. Get everyone (including pets) out of the property immediately.
3. Leave the door open as you leave to ventilate the property.
4. Do not re-enter.
5. Call 999 if anyone is showing symptoms. Call the National Gas Emergency Service on 0800 111 999 to report the alarm.
6. Do not re-enter until the National Gas Emergency Service or a Gas Safe engineer has attended, identified the source, and confirmed the property is safe.

Landlord Fines for Non-Compliance

Local housing authorities in London can issue landlords with a remedial notice requiring CO alarms to be fitted within 28 days. If the landlord fails to comply, the council can fit the alarms and charge the landlord up to £5,000 per breach. Enforcement has increased since the October 2022 regulations extended the requirements — London borough councils have a clear mandate to act on complaints from tenants.