Smoke Alarm Types: Ionisation vs Optical vs Heat Detector — Which Do You Need?

The Three Types of Fire Detector

There are three detector technologies used in domestic and light-commercial installations, each optimised for a different type of fire:

Optical Smoke Alarms — The Recommended Default

Optical alarms use an infrared LED inside a sensing chamber. When smoke particles enter and scatter the light beam, the alarm triggers. They are particularly effective at detecting smouldering fires — the kind that start slowly in furniture, wiring, or bedding before producing flames.

Smouldering fires are the most dangerous type in residential properties because they produce large quantities of carbon monoxide and toxic smoke before open flames appear. An optical alarm can detect this early. BS 5839-6:2019 recommends optical alarms as the primary detector in living rooms, bedrooms, hallways, and landings.

Optical alarms are also far less prone to nuisance alarms from cooking or shower steam than ionisation types — making them the practical choice for occupied properties.

Ionisation Alarms — Largely Obsolete for Domestic Use

Ionisation alarms use a small amount of Americium-241 to ionise the air between two plates. When smoke particles enter, they reduce the ionisation current and trigger the alarm. They respond quickly to fast-flaming fires that produce very little visible smoke — but these fires are less common in domestic settings.

The practical problem with ionisation alarms is false alarms. They trigger readily from cooking fumes, toast, and bathroom steam. As a result, many people disconnect or silence them — making the property less safe. Current guidance from BAFE, NFCC, and most fire services recommends optical over ionisation for domestic installations.

If you have old ionisation alarms in your property (particularly common in homes built or rewired before 2010), replacing them with optical units is a worthwhile upgrade.

Heat Detectors — Kitchens and Garages Only

A heat detector does not respond to smoke at all. It triggers when the air temperature in the room reaches a fixed threshold — typically 58°C for a standard fixed-temperature type, or when the temperature rises faster than 5.4°C per minute for a rate-of-rise type.

Heat detectors are the correct choice for:

• Kitchens — cooking produces smoke and steam that would constantly trigger a smoke alarm
• Integral garages — vehicle fumes and dust would cause nuisance alarms
• Loft spaces — dusty environments with temperature variation
• Utility rooms where tumble dryers produce steam

Heat detectors must be interlinked with smoke alarms throughout the property — a fire starting in the kitchen needs to trigger the alarms on the landing and bedrooms above.

Interlinked Alarms — Why They Matter

A standalone alarm only alerts people in the room it is fitted in. An interlinked system means every alarm in the property sounds when any single alarm detects a problem — giving occupants on the opposite side of the building, or asleep on a different floor, the maximum possible warning time.

Interlinked alarms communicate either by a mains wiring interconnect (a three-core cable between units) or by radio frequency (RF) for retrofit situations where running new wiring is impractical. Mains-wired interlinked systems are the most reliable — RF systems are useful when cable runs are genuinely not possible but require battery management.

Legal requirements for interlinked alarms:

• New-build homes — Building Regulations Approved Document B (2022 revision) requires mains-wired interlinked alarms in all new dwellings
• Scotland — The Housing (Scotland) Act 2006 requires all homes to have interlinked alarms under the Tolerable Standard (enforced from February 2022)
• HMOs — HMO licensing conditions universally require interlinked Grade D (mains with battery backup) alarms at minimum
• Rented properties (England & Wales) — The Smoke and Carbon Monoxide Alarm (Amendment) Regulations 2022 require at least one smoke alarm per storey and a CO alarm in every room with a combustion appliance, from 1 October 2022

BS 5839-6 Grades Explained

BS 5839-6:2019 classifies domestic alarm systems by grade (power source) and category (coverage):

Category refers to coverage: Category LD2 (protecting escape routes and high-risk areas) is the minimum recommended for most homes. Category LD1 (full coverage of all rooms) is recommended where the occupant may not be able to evacuate quickly — for example, elderly or disabled occupants.

Where to Fit Smoke Alarms: Room-by-Room

Testing and Maintenance

All smoke and heat alarms should be tested monthly by pressing the test button until the alarm sounds. The test button checks the electronics and sounder — it does not test the actual sensor response to smoke or heat. Replace the unit if it fails to sound, sounds weakly, or if the low-battery chirp persists after battery replacement.

Manufacturer guidance and BS 5839-6 recommend replacing detectors every 10 years — the sensor sensitivity degrades over time and old units become unreliable. The manufacture date is printed on the back of every unit.

For mains-wired systems, a competent electrician should check the interconnect wiring and confirm all units trigger together during any consumer unit or rewire work.

Mains-Wired Installation Costs in Somerset 2026

Frequently Asked Questions