Smoke rises and carbon monoxide mixes with the air, so the best place for a smoke alarm and the best place for a carbon monoxide alarm are not the same place. That single fact is why combined units are a compromise, and why the legal duties for the two are written separately even in the same set of regulations.
Two hazards, two behaviours
Smoke is buoyant and collects at the ceiling, which is why smoke alarms are ceiling mounted. Carbon monoxide has a density close to air and disperses through the room rather than forming a layer, so a CO alarm works at a range of heights, typically placed at breathing height or higher and at a set distance from the appliance. A unit fixed to a ceiling is fine for smoke and merely acceptable for CO.
Where CO alarms are required
In rented homes in England the regulations require a carbon monoxide alarm in any room used as living accommodation that contains a fixed combustion appliance, other than a gas cooker. Scotland and Wales have their own positions. A CO alarm is required because of an appliance rather than because of a room's use, so the boiler cupboard, the wood burner and the gas fire are what drive it.
When a fire alarm and carbon monoxide detector in one unit makes sense
Where the appliance is in a hall or landing that also needs a smoke alarm, one combined unit in the right place is tidy and effective. Where the appliance is in a living room and the smoke alarm belongs in the hall, a combined unit forces one of the two into the wrong place. Decide the positions for each hazard first, then see whether a single device happens to serve both.
Different lives, different tests
Carbon monoxide sensors have their own service life, often different from the smoke sensor beside them, and a combined unit is replaced as a whole at whichever comes first. The test button proves the sounder, not the CO cell, exactly as it does not prove the smoke chamber. Both are replacement items on a date, not maintenance items on a test.
A fire alarm with carbon monoxide detection, and what it cannot do
Combined units exist and they are convenient, one enclosure, one fixing, one set of batteries. The compromise is siting, because the two hazards behave differently. Smoke rises, so a smoke sensor belongs on the ceiling. Carbon monoxide mixes with air and the guidance for a carbon monoxide alarm is to site it in the room with the appliance, at the height the manufacturer specifies, which is often between head height and the ceiling and at a stated distance from the appliance itself. A combined unit on a hallway ceiling is well placed for smoke and poorly placed for the carbon monoxide risk, which is in the room with the boiler or the stove. So the honest answer is that a combined alarm is a good second layer and not a substitute: where there is a carbon fuelled appliance, put a dedicated carbon monoxide alarm in that room, sited as its instructions say, and let the smoke alarms do their own job on the ceiling.
Questions people ask about carbon monoxide fire alarm
Is a combined smoke and carbon monoxide alarm as good as two separate ones?
Only where the best position for both happens to be the same place. Smoke alarms belong on the ceiling; CO alarms are sited relative to the appliance. Decide positions first, then choose devices.
Where does a carbon monoxide alarm go?
In the room with the fixed combustion appliance, at a distance from it recommended by the manufacturer, and not in a cupboard or behind furniture. Follow the instructions supplied with the specific alarm.
Do I need a CO alarm if I only have a gas cooker?
The England rented sector duty excludes gas cookers. That is a limit on the statutory requirement rather than advice that a cooker cannot produce carbon monoxide, and many people fit one anyway.
Is a combined carbon monoxide and smoke detector a good idea?
It saves a fixing and costs you siting, which is why opinions differ. Smoke rises and wants a ceiling in the circulation space; carbon monoxide mixes with air and is usually best at breathing height on a wall a few metres from the appliance, and never directly above it or in a cupboard. One device cannot be in both places. Where a combined unit works is a hallway with an appliance close by and no better option; where it does not is a boiler in a utility room or a stove in a lounge, both of which want their own carbon monoxide alarm in the room with the appliance.