Fire safety glossary
Smoke detector types
Definition
Smoke detectors (and smoke alarms) use one of two core technologies — ionisation or photoelectric — or combine both. Each responds differently depending on whether a fire is producing large visible particles (smouldering) or small invisible particles (fast-flaming). Choosing the right type for each room significantly affects how much warning time you have.
Detector comparison
Four alarm types and where to use each
Ionisation
A tiny amount of radioactive material ionises air between two plates, creating a small electrical current. Smoke particles disrupt the current, triggering the alarm.
Pros
- Fast response to fast-flaming, open fires
- Generally less expensive
- Widely available
Cons
- Prone to false alarms from cooking steam and toast
- Slower to respond to smouldering fires
Photoelectric
A light beam shines across a detection chamber. Smoke particles scatter the beam onto a photosensor, triggering the alarm.
Pros
- Better at detecting slow smouldering fires
- Fewer false alarms from cooking
- Recommended for sleeping areas
Cons
- Slightly slower on fast-flaming fires
- More expensive than ionisation
Combination (dual)
Contains both ionisation and photoelectric sensors in one unit. Either sensor can trigger the alarm independently.
Pros
- Detects both fire types effectively
- NFPA-recommended for sleeping areas
- Best overall coverage
Cons
- Higher cost than single-technology units
- Slightly larger unit size
Heat alarm
Triggers when temperature rises above a set threshold (typically 58°C / 136°F) or rises rapidly. Does not detect smoke.
Pros
- No false alarms from cooking
- Suitable for dusty or steamy environments
- Ideal for kitchens and lofts
Cons
- Responds after fire is already developed
- Not a substitute for smoke detection in living areas
Why the distinction matters
Both ionisation and photoelectric alarms can detect both fire types eventually — the difference is response time. A photoelectric alarm may detect a smouldering sofa fire 15–50 minutes before an ionisation alarm does. An ionisation alarm may detect a fast-flaming kitchen fire 30–90 seconds earlier than a photoelectric alarm. For sleeping areas, where a smouldering fire can fill a room with toxic smoke before flames are visible, that earlier warning from a photoelectric alarm is potentially life-saving.
The NFPA recommends that homes have both photoelectric and ionisation technology — either as separate alarms or as a single combination unit. The UK government guidance for landlords specifies optical (photoelectric) or heat alarms as preferred types for most room positions.
Placement still matters
Even the best alarm type is ineffective if positioned incorrectly. Alarms should be mounted on the ceiling, at least 30 cm (12 in) from any wall or corner, and away from cooking steam, bathroom steam, and supply air vents. For full guidance on room-by-room placement, see our smoke alarm placement guide.
Test monthly, replace every 10 years
Press the test button on every alarm monthly. Replace all alarms every 10 years from the manufacture date (printed on the back of the unit). The test button checks the circuit — not the sensor — so an old alarm may pass a test while its sensor degrades. Check the date on the back of every alarm in your home today.
Interlinked versus standalone
Regardless of sensor type, all alarms in a home should ideally be interlinked — either by wired connection or wirelessly — so that a detection in one room activates alarms throughout the building. This is especially important in larger homes and two-storey properties where a fire at the far end of the house or on a different floor may not be heard from a bedroom. Interlinked alarms have been mandatory in all Scottish homes since February 2022 and are required in new builds across the UK.
Smoke alarm FAQ
Questions about smoke detector types and placement
The NFPA 72 standard and NFPA smoke alarm guidance recommend a dual-sensor (combination ionisation + photoelectric) alarm in bedrooms and sleeping areas. Ionisation sensors respond faster to fast-flaming fires; photoelectric sensors respond faster to slow, smouldering fires — the type that most commonly occurs at night when a cigarette or wiring fault develops slowly. A combination alarm covers both scenarios. If a single-technology alarm is used, NFPA recommends photoelectric for sleeping areas due to the smouldering fire risk.
A smoke alarm is a self-contained device: it detects smoke AND sounds an alarm from the same unit. A smoke detector is a sensing component in a larger fire alarm system — it detects smoke and sends a signal to a control panel, which then activates separate alarm devices such as sounders and strobes. In domestic settings, "smoke alarm" is the correct term. In commercial or large residential buildings, individual "smoke detectors" feed into a building fire alarm system. Both types use ionisation, photoelectric, or combined detection technology.
Ionisation alarms are more sensitive to small particles — including the aerosols and steam produced by cooking — which is why they false-alarm more frequently in kitchens. Solutions include: replacing the kitchen ionisation alarm with a heat alarm (responds to temperature rise rather than particles — appropriate for kitchens and garages), or fitting a photoelectric alarm further from the cooking area. Never remove the battery or disable an alarm to stop nuisance activations — relocate or replace the alarm with the appropriate type instead. See our smoke alarm placement guide.
Smoke alarms should be replaced every 10 years from the manufacture date (printed on the back of the unit). The sensing chamber degrades over time and the alarm becomes less reliable, even if it still responds to a test button press. The test button only checks the alarm circuit, not the sensor sensitivity. CO alarms should be replaced every 7–10 years. Always check the manufacturer's guidance for the specific unit.
Interlinked alarms communicate with each other so that when one detects smoke, all alarms in the network sound simultaneously. This is critical in larger homes or homes with multiple floors where a fire on one level might not be heard from a bedroom on another. In Scotland, the law requires interlinked alarms in all homes as of 2022. In England, Wales, and Northern Ireland, interlinked alarms are strongly recommended but not yet legally mandated for existing properties (required in new builds). NFPA recommends interconnection for all smoke alarms.