What Is the Difference Between a Carbon Monoxide Alarm and a Smoke Alarm?

A smoke alarm senses smoke particles in the air. When the level rises, it sounds a loud alarm. This gives you time to wake up, leave the building, and call 999. Smoke rises, so you fit smoke alarms on the ceiling.

Types of Smoke Alarm

• Optical alarms: These detect smouldering fires, such as a burning sofa. They suit hallways and landings because cooking sets them off less often.
• Ionisation alarms: These react fast to flaming fires, such as burning paper. They set off more false alarms near kitchens.
• Multi-sensor alarms: These combine sensors for wider coverage.
• Heat alarms: These react to a rise in temperature instead of smoke. They suit kitchens, where steam and toast set off standard smoke alarms.

A CO alarm senses carbon monoxide gas. The gas forms when fuel such as gas, oil, coal, or wood burns without enough air. You are unable to see, smell or taste it. The alarm is your only warning. When the gas reaches a harmful level, the alarm sounds.

Where Carbon Monoxide Comes From:

• Gas, oil and solid fuel boilers
• Gas fires and gas cookers
• Wood burners, open fires and coal fires
• Portable heaters and generators
• Barbecues used indoors or in a garage
• Car exhaust in an attached garage

Faults raise the risk. A blocked flue, poor ventilation or a boiler without regular servicing lets gas escape into the room.

Signs of Carbon Monoxide Poisoning:

The symptoms look like flu without a fever. Watch for:

• Headache
• Dizziness
• Nausea
• Tiredness and confusion
• Breathlessness
• Stomach pain

Symptoms often improve when you leave the property and return when you come back. If several people in the home feel unwell at the same time, suspect carbon monoxide.

What to Do When a CO Alarm Sounds:

Step 1. Open all doors and windows.

Step 2. Turn off the appliance if you know which one it is.

Step 3. Leave the property and stay outside.

Step 4. Call the National Gas Emergency Service on 0800 111 999.

Step 5. Seek medical help. Call 999 if someone has collapsed or feels seriously unwell.

Step 6. Do not use the appliance again until a registered engineer checks it. Use a Gas Safe registered engineer for gas, an OFTEC registered engineer for oil, and a HETAS registered engineer for solid fuel.

They Detect Different Dangers

Fire creates smoke. A faulty appliance creates carbon monoxide. A gas leak from a boiler produces no smoke, so a smoke alarm gives you no warning. A CO alarm is not built to detect fire, so it gives you no early fire warning.

They Sit in Different Places

Smoke rises, so smoke alarms go on ceilings. Carbon monoxide mixes with the air in a room, so CO alarms go at breathing height near the appliance.

They Last for Different Periods

Most smoke alarms last about 10 years. Most CO alarms last 5 to 10 years. Look for the replace-by date on the label.

They Follow Different British Standards

Look for BS EN 14604 on a smoke alarm. Look for BS EN 50291 on a CO alarm. A combined unit shows both numbers.

Do You Need Both a Smoke Alarm and a Carbon Monoxide Alarm?

Yes. Fire and carbon monoxide leaks are separate dangers. A smoke alarm will not warn you about a leaking boiler. A CO alarm will not warn you about a kitchen fire. Combined smoke and CO units exist. Check the box for both British Standards. Two separate alarms let you place each one in its best position.

Smoke Alarm Placement:

• Fit at least one on every floor.
• Place alarms in hallways and on landings to cover your escape route.
• Mount the alarm in the centre of the ceiling, at least 30 cm from walls and light fittings.
• Fit a heat alarm in the kitchen.
• Keep alarms away from bathrooms and cookers, where steam triggers them.

CO Alarm Placement:

• Fit one in every room with a fuel-burning appliance.
• Place it 1 to 3 metres from the appliance. Follow the maker’s instructions.
• Mount it at head height on a wall or shelf, or on the ceiling if the manual says so.
• Fit one in the bedroom if a boiler or flue sits nearby.
• Keep it clear of cupboards, curtains, furniture, fans and damp rooms.

The Smoke and Carbon Monoxide Alarm (England) Regulations 2015 set the rules for rented homes. The 2022 amendment widened them from 1 October 2022. If you rent out a home in England, you must:

• Fit at least one smoke alarm on every storey used as living space.
• Fit a CO alarm in any room with a fixed combustion appliance. Gas cookers are excluded.
• Test each alarm on the day a new tenancy starts.
• Repair or replace a faulty alarm once a tenant reports it.

A fixed combustion appliance is any appliance fixed in place that burns fuel, such as a gas boiler, oil boiler or wood burner. The rules apply to private landlords and social housing providers. Licensed HMOs follow similar duties through their licence conditions. Councils enforce the rules. If you ignore a remedial notice, you face a fine of up to £5,000. Scotland and Wales set their own rules. Both require interlinked smoke alarms. Check with the local council before you let a property there. If your rental has a gas boiler, you also need a Gas Safety Certificate every 12 months.

• Press the test button on every alarm once a month.
• Replace batteries as the manual says. Choose sealed 10-year battery alarms for fewer changes.
• Vacuum the alarm cover to remove dust.
• Replace each alarm before the date on its label.
• Never remove batteries or cover an alarm.
• Landlords: keep a dated record of every test at the start of each tenancy.

Does a smoke alarm detect carbon monoxide?

No. A smoke alarm senses smoke or heat. It does not sense carbon monoxide gas. You need a separate CO alarm or a combined unit marked BS EN 50291.

Is a combined smoke and CO alarm enough?

A combined alarm that meets both British Standards gives you both warnings. Two separate alarms give you better placement. Smoke alarms belong on ceilings. CO alarms belong near the appliance at breathing height.

Do I need a CO alarm if I only have an electric boiler?

In England, the law for rented homes links CO alarms to fixed combustion appliances. If you have none, the law does not require one. Fit one anyway if you burn any fuel at home, such as wood or coal.

Who must supply alarms in a rented home?

The landlord supplies the alarms and tests them at the start of the tenancy. The tenant tests them during the tenancy and reports faults. The landlord then repairs or replaces any faulty alarm.

How long does a CO alarm last?

Most last 5 to 10 years. Check the replacement date on the label.