Carbon Monoxide Alarms: Where to Put Them and What to Do
Where to fit a carbon monoxide alarm in a UK home, which standard to look for, how to test it, what the symptoms of exposure are and what to do if it sounds.
Carbon monoxide (CO) is a poisonous gas with no smell, colour or taste. It is produced when fuels such as gas, oil, coal and wood do not burn fully, usually because an appliance is faulty, badly installed or poorly ventilated. You cannot detect it without an alarm, which is why a working CO alarm is one of the cheapest and most important safety items in a home.
Do you need one?
Yes, if you have any fuel-burning appliance: gas boiler, gas fire, wood burner, open fire, oil boiler or solid-fuel stove. Even if your own boiler is fine, CO can enter from a neighbouring property or a blocked flue in some terraced and flat arrangements.
In rented homes in England, landlords must provide a CO alarm in any room used as living accommodation that has a fixed combustion appliance, excluding gas cookers, and check they work at the start of each tenancy. Scotland and Wales have their own rules, so check the position where you live.
Which alarm to buy
- Look for the British or European standard marking BS EN 50291 on the device and packaging, and a mark from a recognised testing body.
- Choose sealed-unit models with a 7 to 10 year battery, or mains-powered alarms with battery backup if you want to avoid battery changes.
- Avoid the cheap "black spot" stick-on indicators, which are not reliable alarms.
- A decent standalone alarm typically costs around £15 to £30.
Where to put it
Placement guidance generally points to:
| Room | Position |
|---|---|
| Room with the appliance | At head height, or on a shelf, about 1 to 3 metres from the appliance |
| Bedrooms | Near the bed, at breathing height, if an appliance is next door or the flue passes through |
| On a ceiling | Some models allow it, if the manual says so |
Do not place it inside a cupboard, behind furniture or curtains, in a damp bathroom, directly above or next to a cooker, or near a window or vent where draughts could affect it. Follow your manufacturer's guidance; it overrides general rules.
Testing and maintenance
- Press the test button once a month.
- Vacuum or wipe dust from the vents gently.
- Note the expiry date on the device, often 5 to 10 years, and replace it on time.
- Replace batteries according to the manual.
You can put reminders on the same schedule as smoke alarms. See our smoke alarm testing schedule.
Recognise the symptoms
Early signs can be mistaken for flu or tiredness, and often ease when you leave the building.
- Headache, dizziness, nausea
- Tiredness and confusion
- Shortness of breath
- Symptoms that improve when you are out, and worsen at home
- Several household members, or pets, affected at once
What to do if the alarm sounds
- Do not ignore it. Open doors and windows.
- Turn off appliances if you can do so safely, and leave the building.
- Call the National Gas Emergency Service on 0800 111 999 if you think it is gas, or call 999 if anyone is unwell.
- Do not go back in until the cause is found.
- Get medical advice if anyone has symptoms.
- Do not use the appliance until a Gas Safe registered engineer, or an appropriate registered engineer for oil or solid fuel, has inspected it.
Prevention
- Have gas appliances serviced annually; see the boiler service guide.
- Have chimneys and flues swept and checked as advised for the fuel type.
- Never block air vents or air bricks. See ventilation at home.
- Never use a barbecue or portable fuel heater indoors.
- Never leave a car or petrol generator running in an attached garage.
Support is available through gov.uk and Gas Safe Register for official advice. If you need a heating or solid-fuel engineer, you can search on Compito, compare ID-verified providers and send a request, and you should always confirm registration yourself.
A worked example: a typical terraced house
Imagine a terrace with a gas boiler in the kitchen, a gas fire in the lounge and a wood burner added to the back room. A sensible setup might be one alarm in the lounge, one in the back room, and one near the boiler or in the hall by the kitchen, plus an alarm on the landing where the bedrooms are if the boiler flue or chimney breast runs up that side. That is three or four alarms, at a typical £15 to £30 each, for a house where the risk comes from several sources.
Special situations
- Holiday lets and caravans. Fuel-burning appliances in temporary accommodation need alarms, and guests do not know the property. Put the instructions where people will see them.
- Garages attached to the house. Never run engines inside, and consider an alarm in rooms next to the garage.
- Shared walls and flats. A neighbour's faulty appliance can affect you, so an alarm is worth having even if you only use electric heating.
- Stoves and fires. Have chimneys swept as advised for the fuel and keep records.
Common mistakes
- Putting the alarm in a place with draughts or steam, causing nuisance alarms and temptation to remove it.
- Assuming a new alarm will last forever; they have an end-of-life signal.
- Taking the battery out to silence a chirp and forgetting to replace it.
- Relying on a smoke alarm to do the job of a CO alarm.
- Buying an alarm that does not carry the standard marking.
Keeping a record
Write the installation and replace-by date on the alarm or on a label inside a cupboard. It is much easier than trying to remember when it was bought. For servicing records of gas appliances, ask your engineer for a copy of the report each time and keep it with your home paperwork.
Frequently asked questions
Can a smoke alarm detect carbon monoxide?
Not unless it is a combined unit that says it does. Standard smoke alarms do not detect CO, so you need a separate CO alarm or a combined device.
How many CO alarms do I need?
At least one in every room with a fuel-burning appliance, and consider bedrooms near flues or appliances. A multi-storey home may need more.
Do electric homes need CO alarms?
If there is no fuel burning in your home, the risk is low from your own appliances, but neighbours' appliances or a garage can be a source. An alarm is cheap and may still be sensible.