Carbon monoxide alarms: where to fit them and what to do if one sounds

A carbon monoxide alarm needs the right position for its particular model, not simply a convenient shelf. Follow its manufacturer's instructions. If an alarm is sounding now, stop reading fitting advice and get everyone outside: carbon monoxide can be present without any smell or visible smoke.

If your CO alarm sounds, get outside first

1. Leave the property promptly, taking other occupants with you. Do not stay inside to find the source or silence the alarm.
2. Open doors and windows only if safe on your way out. Turn off the suspected appliance only if you can do so safely without delaying escape. Do not return to do either.
3. From outside, call the National Gas Emergency Service on 0800 111 999 where gas appliances are involved. Explain that a carbon monoxide alarm has sounded.
4. Seek medical help if exposure is suspected. Call NHS 111 for possible symptoms; call 999 for severe symptoms, including difficulty breathing, collapse, confusion or chest pain. Tell the call handler that carbon monoxide may be involved.
5. Stay out until advised that it is safe to return. Do not use the appliance again until it has been checked and declared safe by a Gas Safe registered engineer for gas, or an appropriately registered engineer for other fuels, such as OFTEC for oil.

For solid-fuel or oil appliances, explain the fuel involved when seeking emergency advice and arrange the appropriate specialist. If anyone is seriously unwell or there is immediate danger, call 999. A stopped alarm or an open window does not prove the building is safe.

Recognise symptoms without waiting for a detector

Headache, dizziness, nausea, breathlessness and unusual tiredness can indicate carbon monoxide poisoning. Symptoms may improve outdoors and return indoors; several occupants feeling unwell is another warning. Do not wait for everyone to develop symptoms, or assume a silent alarm rules out exposure.

Carbon monoxide is colourless and odourless. Gas, oil, wood and coal appliances can produce it when combustion or the removal of fumes goes wrong. If you smell gas as well, avoid flames and electrical switches, leave, and make the emergency call from outside. Our gas leak and suspected gas escape service information explains the professional help involved; emergency reporting comes first.

Choose a position using the alarm's own instructions

Provide alarms in rooms containing fuel-burning appliances. A position 1–3 metres horizontally from the appliance is commonly recommended, but check the instructions for your model and room layout. Do not put a detector directly over a boiler or cooker merely because it seems closest to the source.

Head-height positioning is commonly recommended for suitable wall-mounted alarms; ceiling-mounted alarms are commonly positioned at least 300 mm from walls. These are not interchangeable rules for every product. Follow the maker's specified heights, clearances and mounting method, and seek advice if a sloping ceiling, partition or unusual layout makes them unclear.

Consider additional protection in bedrooms and regularly occupied living areas, following the manufacturer's guidance for rooms without an appliance. Make sure the alarm can be heard where people sleep. A hallway alarm alone should not be assumed to protect every appliance room.

• Avoid cupboards, curtains and furniture that obstruct airflow around the detector.
• Keep clear of vents, extractor fans and openable windows where instructions require it.
• Check restrictions for damp, steamy or unusually hot locations before fitting.
• Choose a suitable mounting surface without drilling near concealed pipes or cables.

Select, test and retire the alarm deliberately

Choose a domestic CO alarm marked BS EN 50291 and suitable for the intended location. Read the instructions before installation; a smoke alarm is not a substitute unless it is a combined unit specifically designed to detect carbon monoxide too.

Test regularly using the test button at the interval specified by the manufacturer. Keep the sound openings clear and follow the approved cleaning method. Record the replacement date and replace the alarm at the end of its stated life, even if its button still produces a sound. Replace batteries where the design allows; do not dismantle sealed units.

Rental alarms and appliance care do different jobs

Since 1 October 2022, the rules in England require relevant landlords to provide a CO alarm in any room used as living accommodation containing a fixed combustion appliance, excluding gas cookers. They must check required alarms work at the start of a new tenancy and repair or replace faulty alarms once informed and the fault is confirmed. This is a summary; particular tenancy arrangements may need separate advice.

Alarms do not replace annual servicing or required landlord gas safety checks. Keep ventilation unobstructed and arrange boiler servicing; our guide to what a boiler service includes explains its separate purpose. Never open the boiler casing or attempt combustion adjustments yourself. If switching heating off leaves the home cold, read how to judge a heating emergency without restarting a suspect appliance.

Once emergency services have addressed immediate danger, call 01732 444555 to discuss an appliance inspection, or use the quote form for planned safety work. Do not use the enquiry form instead of the emergency number when an alarm is sounding.