Where to put a carbon monoxide alarm, room by room

Which rooms need a carbon monoxide alarm

Carbon monoxide comes from fuel that does not burn completely. Anything that burns gas, oil, coal or wood can produce it if it is faulty, badly installed or starved of air, or if its flue is blocked or leaking. The alarm needs to be where the gas would first build up, which means the room the appliance is in.

Heating season is when most of these appliances run for hours at a time, so early autumn is the right moment to check that every alarm is in the right place and still in date. If a gas boiler sits in a kitchen cupboard, the alarm goes in the kitchen, outside the cupboard. If you have a gas fire in the living room, that room needs its own alarm too, even if there is one near the boiler.

• Any room with a gas, oil or solid fuel boiler
• Any room with a gas fire, open fire, wood burner or multi-fuel stove
• Bedrooms and living rooms that a flue or chimney runs through, for example a boiler in the loft with its flue passing through a bedroom below
• Bedrooms generally, if you want to be woken by an alarm at night, placed near head height when lying down
• A room next to an appliance that has no alarm of its own, such as a garage with a boiler in it

Exact position: distance and height

Carbon monoxide mixes with the air in a room rather than sinking or rising sharply, so the position rules are about getting the alarm into the air the appliance affects, while keeping it out of draughts and dead spots. The distances below follow the placement guidance used by gas engineers and alarm manufacturers. Where your alarm's own instructions differ, follow them.

Where not to put a carbon monoxide alarm

Most false alarms and most alarms that fail to go off when they should come from a bad position rather than a faulty unit. A detector sitting in the wrong place can miss a real leak, or chirp at cooking steam so often that someone takes the batteries out.

• Inside a cupboard or boiler housing, or behind curtains or furniture
• Directly above a sink, hob or cooker, where steam and grease reach it
• Next to an extractor fan, air vent, open window or door, where fresh air dilutes the gas
• In a bathroom or shower room, where condensation can damage the sensor
• In a very cold, hot or dusty space, unless the alarm is rated for it
• Closer than 1 metre to the appliance, where brief puffs at start-up can set it off

A carbon monoxide alarm is a backup, not a substitute for a safe appliance. It warns you once the gas is already in the room. An annual service and safety check on every gas appliance is what stops it being produced in the first place.

How many carbon monoxide alarms do you need?

Count the rooms that contain a fuel-burning appliance, then add any bedrooms or living rooms that a flue passes through. That is the minimum. A typical London flat with one combi boiler in the kitchen and no other gas appliances needs one alarm, in the kitchen. A house with a boiler in the utility room, a gas fire in the lounge and a flue running up through a bedroom needs three.

Choose an alarm marked as meeting BS EN 50291, which is the standard for domestic carbon monoxide alarms. Models with a sealed battery that lasts the life of the alarm save you worrying about flat batteries, and some mains alarms can be interlinked so that one sounding sets off the others. Whatever you choose, press the test button once a month and replace the unit by the end-of-life date printed on it.

The rules for landlords and new appliances

In England, private and social landlords must fit a carbon monoxide alarm in any room used as living accommodation that contains a fixed combustion appliance, apart from a gas cooker. This has applied since 1 October 2022, and the landlord must repair or replace an alarm once told it is faulty. Tenants who find a missing or broken alarm should report it in writing.

Separately, the Building Regulations in England require a carbon monoxide alarm in the room whenever a new or replacement fixed combustion appliance, such as a gas boiler, is installed. Scotland and Wales have their own rules for rented homes, so check the current guidance if your property is there.

A landlord's annual gas safety check covers the appliances and flues, not the alarms themselves, so do not assume the gas engineer has checked the detectors. Our guide to what certificates a landlord needs sets out the other annual paperwork.

If your carbon monoxide alarm goes off

Treat it as real. Open doors and windows, turn off gas appliances if you can do so safely, get everyone out of the property and call the National Gas Emergency Service on 0800 111 999, free and open 24 hours a day. Seek medical help if anyone has a headache, dizziness, nausea or breathlessness. Do not use the appliance again until it has been checked by a Gas Safe registered engineer.

A short chirp every minute or so is usually a low battery or an alarm at the end of its life rather than a detection, and our guide to a carbon monoxide alarm beeping explains how to tell the two sounds apart. For the symptoms and warning signs on the appliances themselves, see carbon monoxide signs at home.

Getting your boiler and gas appliances checked

If you are fitting alarms because the heating is about to go on for winter, it is a good time to have the boiler looked at too. Sooty marks around the case, a lazy yellow flame on an older appliance, a boiler that keeps locking out or an alarm that has sounded are all reasons to book a check rather than wait for the annual service.

Loyal Plumbers carries out boiler servicing, repairs and gas safety certificates across London and everywhere inside the M25, with gas work done by Gas Safe registered engineers. We run 10 vans, 24 hours a day, and our call-out charge is £60. Call or send an enquiry with the boiler make, where it is and what you have noticed, and we will arrange a visit.