BS 5839-6 fire alarm grades — A, D and F compared

Why grade matters

The "category" of fire alarm (L1, L2, L3 etc.) tells you what areas are covered. The "grade" tells you how robust the system itself is — power supply, interlinking, control equipment. Two systems can be the same category but very different grades, and that difference is usually what separates a compliant HMO from a non-compliant one.

Grade A

A panel-based system with manual call points, automatic detection, sounders and a dedicated 24V supply. Designed to BS 5839-1 standards. Required in larger HMOs (typically over 2 storeys or with sleeping risk in non-purpose-built accommodation), bedsit blocks and some converted flats.

Grade D1 and D2

Mains-powered detectors with an integral battery backup, interlinked between units. Grade D1 has a tamper-proof battery rated for the life of the detector; D2 has a user-replaceable battery. Suitable for most standard shared houses up to 2 storeys.

Grade F1 and F2

Battery-only interlinked detectors. F1 has long-life sealed batteries (10-year). F2 uses standard user-replaceable batteries. Only acceptable in limited circumstances — single-private-dwelling new-builds where Grade D rewiring isn't practical, and very rarely in HMOs.

Picking the right grade — typical HMO scenarios

• 2-bed shared flat above a shop — Grade D2 LD2 (mains-interlinked smoke alarms in escape route + heat in kitchen).
• 4-bed shared house (2 storeys) — Grade D2 LD2.
• 5-bed licensable HMO (2 storeys) — Grade D2 LD2, with consideration of LD1.
• 6-bed HMO over 3 storeys — Grade A LD2 typically required.
• Bedsit-style HMO with individual rooms — Grade A LD1.

Common mistakes

Mixing brands (interlink won't work), using battery-only detectors in HMOs (they don't meet the standard), and forgetting the heat detector in the kitchen are the three issues we record most often. A short LD2 retrofit on a typical shared house is usually under £400 inclusive.