Smoke Alarm vs Heat Detector: What to Fit and Where (BS 5839-6 Guide)

Fire detection is one of those jobs where getting it right matters — and where a lot of electricians, builders and landlords are still specifying the bare minimum rather than the right solution. This guide covers the difference between smoke alarms and heat detectors, where each should go under BS 5839-6, and what to fit for different property types.

Smoke Alarm vs Heat Detector: What's the Difference?

Smoke alarms detect the products of combustion — smoke particles — before a fire fully takes hold. They give the earliest warning and are the default choice for most rooms in a property.

Heat detectors respond to a rapid rise in temperature or a fixed temperature threshold (typically 58°C for rate-of-rise, or 58°C fixed). They do not detect smoke. They're slower to react than smoke alarms but are designed for environments where a smoke alarm would false-alarm constantly.

The key distinction: use smoke alarms everywhere a fire is likely to smoulder or develop smoke first. Use heat detectors where smoke is expected in normal use — kitchens, garages, loft spaces with poor ventilation.

What Does BS 5839-6 Require?

BS 5839-6 is the British Standard for fire detection and alarm systems in domestic premises. It defines Grades (the type of system — standalone battery vs mains-interlinked) and Categories (the level of protection).

For most domestic work in England, Part B of the Building Regulations and Approved Document B point to BS 5839-6 Grade D, Category LD2 as the minimum for new builds:

• Grade D — mains-powered with battery backup (not just battery-only)
• LD2 — alarms in escape routes (hallways, landings) plus rooms with highest fire risk (living room, kitchen)

For a full LD1 system (highest protection), you add alarms in every room including bedrooms. Landlords in Scotland must comply with the interlinked alarm requirements under the Tolerable Standard — which effectively requires LD1 with interlinked mains alarms in all rooms.

Where to Fit Each Type

Optical Smoke Alarm

Best for: living rooms, hallways, landings, bedrooms. Optical sensors detect large smoke particles from slow-burning fires (smouldering foam, fabric) earlier than ionisation types. For most domestic rooms, optical is now the recommended default.

Ionisation Smoke Alarm

Best for: areas where fast-flaming fires are the primary risk. Less common now — optical has largely replaced ionisation as the standard choice for domestic installs.

Multi-Sensor (Optical + Heat)

Best for: kitchens adjacent to living areas, open-plan spaces. Combines an optical sensor with a heat sensor to reduce false alarms without losing early detection. A good spec upgrade where the client has complained about nuisance alarms.

Heat Detector

Best for: kitchens, garages, loft spaces, boiler rooms. Kitchens are the most common call — a smoke alarm in a kitchen is almost guaranteed to false-alarm during cooking. A heat detector gives fire protection without the nuisance factor. Do not substitute a heat detector for a smoke alarm in escape routes or living areas.

Mains vs Battery: Which Grade Do You Need?

For new builds and notifiable rewires, mains-powered (Grade D) is required by Part P and BS 5839-6. Battery-only alarms (Grade F) are only acceptable for replacement of existing battery alarms where no rewire is taking place — and even then, modern sealed lithium battery alarms with 10-year life are a better recommendation than standard 9V battery units.

Interlinked alarms — where triggering one sets off all of them — are required in new build and increasingly expected as best practice in refurbishments. Mains alarms can interlink via the interconnect wire (typically orange in Aico and similar). Battery alarms can use radio frequency (RF) interlink — useful in retrofit situations where you can't run additional cable.

Products We Stock

APM carries a range covering Grade D mains, battery, and RF interlinked options:

• Axiom Mains Optical Smoke Alarm with Battery Backup (RF Interlinked) — Grade D mains with sealed battery backup and radio frequency interlink, ideal for new installs and full rewires
• Fireangel WST-630T 10-Year Battery Lithium Smoke Detector — Grade F battery with 10-year sealed lithium, suitable for retrofit where mains is not practical
• Aico Ei144E Heat Alarm — mains-powered heat detector for kitchens and garages, interlinks with Aico smoke alarm range
• Google Nest Protect 2nd Gen Wired (Smoke + CO) — combined smoke and carbon monoxide detector, smart home integration, suitable for premium installs

Browse the full Fire, Heat & Smoke Alarms collection for the complete range. Same-day collection from our Acton trade counter — 24 Western Avenue, W3 7TZ. Call 020 8702 8080 to confirm stock before you travel.

Practical Positioning Tips

• Mount on the ceiling, ideally central — keep smoke alarms at least 300mm from walls and light fittings. Heat detectors: same rule.
• Minimum one alarm per floor on escape route — LD2 minimum means hallways and landings are non-negotiable.
• Don't fit smoke alarms within 3m of a kitchen — nuisance alarms cause residents to remove batteries. Fit a heat detector or multi-sensor at the kitchen boundary instead.
• Keep alarms away from air vents and drafts — airflow dilutes smoke before it reaches the sensor and can cause delayed detection.
• Test and record — on completion, test all alarms and note locations on the electrical installation certificate or a separate commissioning sheet.