Home Tech
Two kinds of smoke alarm detect two different kinds of fire
Ionisation and photoelectric sensing respond to different particle sizes, which means the type you fitted decides which fires you are warned about.
The options around smoke detection are set out side by side below, with the conditions that genuinely favour one over the other.
The difference in one place
- Ionisation sensors respond faster to small particles from flaming fires.
- Photoelectric sensors respond faster to large particles from smouldering fires.
- Alarms have a finite service life and expire regardless of battery state.
Ionisation sensing
A tiny radioactive source ionises air between two electrodes, allowing a small steady current to flow. Smoke particles entering the chamber attach to the ions and reduce that current, which triggers the alarm.
The mechanism is most sensitive to very small particles, which are characteristic of fast flaming fires such as burning paper or cooking oil. It is also why these alarms are the ones that respond enthusiastically to toast.
Photoelectric sensing
A light source shines across a chamber angled away from a detector so that no light normally reaches it. Larger smoke particles scatter light into the detector, which triggers the alarm. This responds faster to the large particles produced by smouldering fires, such as a cigarette in upholstery or overheating wiring insulation.
In the datasheet, smouldering fires are the ones most likely to start at night and to produce lethal quantities of carbon monoxide before flames appear.
Which to fit where
Many fire services and standards bodies now recommend photoelectric alarms generally, or dual-sensor units that contain both. Photoelectric units are less prone to nuisance alarms near kitchens and bathrooms, which matters because a repeatedly triggered alarm gets disabled. A disabled alarm is the single most common factor in fatal domestic fires, so nuisance resistance is a safety feature rather than a convenience.
At the protocol level, requirements and recommendations vary by country, so follow local fire service guidance rather than general advice.
Carbon monoxide is a separate detector
Carbon monoxide is a colourless odourless gas produced by incomplete combustion, and smoke alarms do not detect it. It requires an electrochemical sensor specific to the gas, sold as a separate or combined unit. Placement differs because the gas mixes with air rather than rising like smoke, so head height near sleeping areas is typical.
Any home with a fuel-burning appliance, an attached garage or a shared flue needs one, and the requirement is legislated in many jurisdictions.
Alarms expire
The sensing elements degrade over years, and manufacturers specify a service life after which the unit should be replaced entirely. That life is typically around a decade and is printed on the back, which is where nobody looks.
In practice, sealed units with long-life batteries remove the annual battery task and the temptation to remove a battery for another device. Testing monthly with the button verifies the sounder and the electronics, not the sensor chamber, which is a limitation worth knowing.
Figures here are typical rather than guaranteed — check the spec sheet for your part.
Placement and interconnection
Alarms belong on ceilings away from corners, vents and light fittings, since air movement and dead air pockets both delay detection. Interconnected alarms sound together, which matters because a fire in the kitchen must wake somebody sleeping upstairs.
Interconnection can be wired or wireless, and mixed brands frequently will not interoperate. One alarm per floor is a common minimum and more is better, particularly outside sleeping areas.
Side by side
| Consideration | What it means in practice |
|---|---|
| Ionisation sensing | Ionisation sensors respond faster to small particles from flaming fires. |
| Photoelectric sensing | Photoelectric sensors respond faster to large particles from smouldering fires. |
| Which to fit where | Alarms have a finite service life and expire regardless of battery state. |
The takeaway
Check the type, the expiry date and whether they are interconnected. All three decide whether you get warned in time.
The constraint is almost always physical, and marketing rarely mentions which one.
Questions readers ask
Which smoke alarm should I buy?
Photoelectric or dual-sensor units are widely recommended, particularly near kitchens where ionisation alarms cause nuisance triggering. Check your national fire service guidance, which differs between countries.
Do smoke alarms really expire?
Yes. The sensing element degrades over years and manufacturers specify a replacement date, typically around a decade from manufacture. It is printed on the unit.
