Can a Carbon Monoxide Alarm Detect a Gas Leak?

No. A carbon monoxide alarm cannot detect a gas leak. It is built for one job only: to sense carbon monoxide, the colourless, odourless gas that faulty or badly maintained fuel-burning appliances can produce. Carbon monoxide detectors, commonly known as CO alarms, are devices specifically designed to detect the presence of this odourless and invisible gas¹. They are not gas leak detectors, and they will not react to natural gas or to the propane and butane in a cylinder.

That distinction matters because the two dangers need different responses. A gas leak is usually noticed by smell, and dealt with by the gas emergency service on 0800 111 999². Carbon monoxide gives no smell at all, which is why it needs an alarm. If you have a fuel-burning boiler, fire or stove, you should have a CO alarm in every room where fuel is burned¹. If you use LPG, you need a separate gas detector as well.

This page sets out what each alarm does, why LPG is a special case, where to fit a CO alarm, and what to do when one sounds.

What a carbon monoxide alarm actually detects

A CO alarm is a single-purpose device. It watches the air for carbon monoxide and raises an audible alarm when levels rise, for example from a blocked flue or chimney or a faulty appliance⁵. It does not sense smoke, and it does not sense the gas in your supply pipe or cylinder. CO alarms are specifically designed to detect carbon monoxide, while smoke alarms are designed to detect smoke and fire¹.

That narrowness is deliberate. Carbon monoxide is dangerous precisely because you cannot see, taste or smell it, so the alarm is the only thing in the house that will notice it while you are asleep or in another room. The purpose of an alarm is to alert you to the presence of carbon monoxide and give you time to exit the building safely⁶.

What it will not do is tell you where the gas is coming from, how much has built up, or whether your boiler is about to fail. It is a warning, not a diagnostic tool. That is why the fire and rescue services pair the alarm with maintenance: fitting an alarm should not be used instead of regular checks of appliances⁷.

LPG leaks need a separate gas detector

Liquid petroleum gas, the propane and butane sold in cylinders and bulk tanks, is a different hazard from carbon monoxide, and a CO alarm will not detect it. LPG equipment can be particularly vulnerable: blocked or partially blocked burners can lead to higher carbon monoxide levels and flame failure, and LPG can also leak from damage to hoses and connections⁴. So an LPG household faces two separate risks, and needs two separate responses.

The gas emergency freephone number, 0800 111 999, is the number to call for a gas or carbon monoxide emergency, or if a gas pipeline is struck, even if there is no obvious leak². Keep it somewhere you can find it in the dark.

Why a CO alarm is still worth fitting in a home with gas

It would be easy to read the first half of this page and conclude that a CO alarm is not much use in a gas home. The opposite is true. Every home with a solid fuel or gas appliance should also have a working carbon monoxide alarm installed nearby⁸. If you have a gas boiler or solid fuel appliance, a carbon monoxide alarm should be fitted in your home which will warn you⁹.

The reason is that a gas appliance can produce carbon monoxide without leaking gas at all. A blocked flue, a poorly serviced boiler or a burner that is not burning cleanly can send CO into the room, and nothing about the smell of gas will tell you. Alarms can provide early warning of a carbon monoxide leak, allowing for a prompt response¹⁰. That early warning is the whole point: it gives you time to get out and call for help.

Watch for the physical signs too. A floppy yellow or orange flame where it should normally be crisp and blue, extra condensation inside windows, pilot lights that frequently blow out, and soot or yellow stains around appliances are all worth acting on². If you see them, stop using the appliance and get it checked.

A CO alarm is not a substitute for servicing. CO alarms must never be regarded as a substitute for the proper installation, maintenance, servicing and cleaning of appliances that use gas, coal, oil or any other solid fuel⁴. Use the alarm as a backstop, and keep the annual service going.

Where to fit a CO alarm, and how many you need

The rule of thumb from the fire and rescue services is simple: one alarm for every room that contains something that burns fuel. Get one for each room of your home containing appliances that burn gas, oil, coal or wood⁶. Carbon monoxide alarms should be fitted in all rooms containing solid fuel appliances, any gas appliances or paraffin heaters¹¹. That includes boilers, fires including open fires, heaters, and stoves, or a room with a flue¹².

• Gas boiler or gas fire: fit a CO alarm in the same room¹².
• Open fire, log burner or solid fuel stove: fit one in the room, and have the chimney swept¹².
• Paraffin heater: fit one in the room where it is used¹¹.
• Bedroom: you may wish to keep a CO alarm in the bedroom so it would wake you when sleeping¹³.
• Rented home: your landlord must ensure a carbon monoxide alarm is equipped in any room used as living accommodation which contains a fixed combustion appliance, excluding gas cookers¹⁴.

Position matters. In rented homes, alarms should be installed in every room used as living accommodation containing a fixed combustion appliance, excluding gas cookers, positioned at head height, either on a wall or shelf, approximately 1 to 3 metres away from a potential source of carbon monoxide¹⁵. Test the alarm weekly, alongside your smoke alarms³.

What the alarm sound means and what to do next

When a CO alarm sounds, treat it as real. They rarely beep as a false alarm, so if your carbon monoxide detector is beeping you should act fast and get out of the property³. The alarm is not telling you to open a window and carry on; it is telling you to leave.

The fire and rescue service sequence is:

1. Open all doors and windows³.
2. Stop using and turn off all fuel-burning appliances³.
3. Evacuate, leaving doors and windows open³.
4. Do not re-enter the property until the alarm has stopped³.
5. Contact your gas or fuel supplier on their emergency number³. For gas appliances, call the gas emergency freephone number, 0800 111 999².

If anyone in the household has symptoms, get medical help. The NHS advises that you get a carbon monoxide alarm for each room of your home that contains appliances that burn gas, oil, coal or wood⁶, and to know the symptoms of carbon monoxide poisoning⁷. Symptoms and next steps are set out in full on our pages on carbon monoxide poisoning symptoms and what to do if your CO alarm sounds.

Alarms, detectors and the limits of each

It helps to keep the family of alarms straight, because each covers a different failure.

CO alarms and smoke alarms are not a substitute for one another, and both types of alarm should be installed in your house¹⁰. The emergency services advise the fitting of a carbon monoxide and smoke alarm in the house, thus protecting lives and property¹⁶. If you live in a house or flat that is entirely powered by electricity, you will not need a carbon monoxide detector, but if you have a fuel-burning boiler, fire or stove you should have one in every room where fuel is burned¹⁰.

There is one more case worth knowing. Even if you do not have any fuel-burning appliances in your home, carbon monoxide can seep through walls, so if you live in a semi-detached home or a terrace and your neighbour burns gas, LPG, oil or wood, the safety-first approach would be to install a carbon monoxide detector¹⁷. If you are choosing between a combined unit and separate alarms, our comparison of combined or separate alarms sets out the trade-offs, and carbon monoxide alarms: what they do and how to choose covers the wider range.

Where to get help and further advice

If you smell gas, or suspect a carbon monoxide leak, call the gas emergency freephone number, 0800 111 999². For gas appliances, that is the number the fire and rescue services direct you to¹. Keep it by the phone and in your emergency kit.

For households that need extra support, a telecare service can add a carbon monoxide detector that raises an audible alarm within the home and an instant alarm call to a monitoring centre when carbon monoxide levels are increased, for example from a blocked flue or chimney or faulty appliances⁵. This is worth asking about if someone in the home is older, disabled or living alone. Our guide to extra help in an emergency explains the wider options.

If you are renting, your landlord has duties. Landlords must supply you with a carbon monoxide detector and alarm in any room of the premises which is used wholly or partly as living accommodation and contains a solid fuel burning combustion appliance¹⁹. The rules differ in Scotland, Wales and Northern Ireland; our page on Wales and Northern Ireland: alarm rules and fire services sets out the differences.

Finally, make the alarm part of your routine. Test smoke alarms monthly and consider getting a carbon monoxide alarm for rooms with gas, oil, coal or wood-burning appliances²⁰. If you are unsure whether you need one at all, our page on whether you need a carbon monoxide alarm answers that directly, and where to put a carbon monoxide alarm goes into placement in more detail.