Smoke Alarms and Heat Alarms — Grade D1, Grade F2, Optical vs Ionisation, and BS 5839-6 for UK Electricians

Smoke alarms and heat alarms are mandatory across UK residential properties under the Smoke and Carbon Monoxide Alarm (England) Regulations 2015 (amended 2022) and BS 5839-6:2019. Choosing the wrong alarm type — fitting a smoke alarm in a kitchen, or using a Grade F2 battery alarm where a Grade D1 mains-wired system is required — can result in false alarms, enforcement notices, or failure of building control sign-off. This guide covers the key selection decisions every UK electrician and landlord faces: alarm grades, detection technology, room placement, and interlinked systems. Browse the full range at our Smoke Alarms collection.

Alarm Grades — Grade F2, Grade D, and Grade D1 Explained

BS 5839-6:2019 defines six alarm grades (A through F) that describe the power source and level of protection. In domestic installations, three grades are most relevant:

Grade F2 — Battery-Powered, No Central Control

Grade F2 alarms run entirely on a replaceable battery (typically 9V alkaline or sealed 10-year lithium). There is no mains connection and no interconnection to a central panel. Grade F2 is the minimum requirement for existing rented properties under the Smoke and Carbon Monoxide Alarm Regulations 2015: landlords must fit a working alarm on each storey of a dwelling before a tenancy begins. Grade F2 alarms are simple to install — no rewiring required — and are appropriate for low-risk retrofit situations where running mains cable is impractical.

Grade D — Mains-Powered with Integral Backup Battery

Grade D alarms are powered from the 230V mains supply with a built-in battery backup. When mains power fails, the battery maintains operation for at least 72 hours. Grade D is the minimum standard recommended for new dwellings under Approved Document B (Fire Safety) of the Building Regulations. Because Grade D alarms are wired into the mains, installation is notifiable work under Part P unless carried out by a competent person registered with a self-certification scheme (NICEIC, NAPIT, Elecsa).

Grade D1 — Mains-Powered with Sealed Tamper-Proof Battery Backup

Grade D1 is a subset of Grade D where the backup battery is sealed into the alarm body and cannot be removed by the occupant. This prevents accidental or deliberate battery removal — a common cause of alarm failure in rental properties and HMOs. Aico's Ei1 Series alarms, including the Ei146e and Ei144 range, are Grade D1. BS 5839-6:2019 strongly recommends Grade D1 for new builds, major refurbishments, and all HMO installations.

Optical vs Ionisation vs Heat Detection

Optical (Photoelectric) Smoke Detection

Optical alarms contain a light source and a photosensor. In normal conditions, the light beam does not reach the sensor. When smoke particles enter the chamber, they scatter the light, triggering the alarm. Optical sensors respond fastest to slow-smouldering fires — the kind that produce dense smoke before a visible flame develops. This includes smouldering upholstery, electrical faults behind wall plaster, and overheating cable insulation. Optical alarms are less sensitive to steam and cooking fumes, making them safer to fit in hallways adjacent to kitchens than ionisation types.

Ionisation Detection

Ionisation alarms use a small radioactive source (americium-241) to ionise the air inside a detection chamber. When smoke particles enter, they reduce the ionisation current and trigger the alarm. Ionisation alarms respond fastest to fast-flaming fires with small combustion particles. They are more prone to false alarms from cooking steam and toast, which is why BS 5839-6 does not recommend them in areas adjacent to kitchens or bathrooms.

Heat Detection

Heat alarms do not detect smoke at all — they respond to rising temperature. Two technologies are used:

• Fixed temperature (thermistor): Triggers when the room temperature reaches a set threshold — typically 58°C. The Aico Ei144E uses this technology.
• Rate-of-rise: Triggers when temperature rises faster than a defined rate per minute, even if the absolute temperature is below the fixed threshold. The Aico Ei144RC combines rate-of-rise detection with a fixed temperature upper limit.

Heat alarms are the correct solution for kitchens and garages where cooking, toasting, or vehicle exhaust fumes would cause nuisance alarms from any smoke detector. Heat alarms are slower to respond than smoke alarms in a fully developed fire, so they must always be used alongside smoke alarms — never as a standalone system in living spaces.

Where to Fit Each Alarm Type

Under BS 5839-6 Category LD2 — the recommended minimum for new builds — alarms are required in every room forming part of the escape route, plus rooms where fire is most likely to start (kitchen, living room). Category LD1 adds protection in all rooms, including bedrooms. Your building control surveyor will specify the required category.

BS 5839-6 and Building Regulations

Approved Document B — New Builds and Extensions

For new dwellings and major refurbishments requiring building control approval, Approved Document B (2019 edition) requires at minimum Grade D, Category LD2 smoke alarm coverage. In practice this means:

• Mains-wired smoke alarms on every storey, covering hallways and principal rooms
• Interconnection so that all alarms sound when any single alarm activates
• Heat alarm in the kitchen (not a smoke alarm)

Smoke and Carbon Monoxide Alarm Regulations (England) 2015 / 2022

For private rented properties in England, landlords must:

• Fit at least one smoke alarm on every storey where there is a room used as living accommodation
• Fit a carbon monoxide alarm in every room with a fixed combustion appliance (boiler, fire, gas cooker)
• Test alarms at the start of each tenancy
• Repair or replace alarms when notified they are faulty

The 2022 amendment extended CO alarm requirements to all rooms with combustion appliances, including gas boilers — not just solid fuel appliances as under the original 2015 regulations.

HMO Licensing — Additional Requirements

Houses in Multiple Occupation (HMOs) requiring a licence face stricter requirements under the Housing Act 2004 and local HMO licensing conditions. These typically require Grade D (mains-wired) interconnected smoke alarms in all common areas, plus heat alarms in shared kitchens. Some councils require Grade D2 (Grade D with sealed rechargeable battery) or Grade C (alarms connected to a central panel). Always check the specific conditions of the HMO licence before specifying alarms.

Interlinked Alarms and RadioLINK Systems

Interconnection means that when one alarm detects smoke or heat, all alarms in the system sound simultaneously. This provides the maximum warning time for occupants — critical in a two-storey property where a kitchen fire might not be audible from an upstairs bedroom with the door closed.

Hard-Wired Interconnection

Traditional interconnection runs a dedicated third wire between all alarms in addition to the live and neutral mains supply. All alarms must be from the same manufacturer (or be confirmed as compatible) to ensure the interconnect signal voltage is correctly matched. Aico's Ei1 Series alarms interconnect using this method.

Wireless Interlink — Aico RadioLINK

Aico's RadioLINK technology interconnects alarms via a dedicated 868MHz radio frequency without requiring a third wire. RadioLINK is ideal for retrofit projects where running additional wiring is impractical — for example, interconnecting alarms across a solid-floor extension or in a listed building. Up to 12 Aico RadioLINK alarms can be linked per installation. The Aico Ei144RC heat alarm is RadioLINK-compatible.

Products from APM Electricals

9V Battery Operated Photoelectric Smoke Alarm

• Photoelectric (optical) smoke detection technology
• 9V battery powered — no mains wiring required
• Grade F2: meets minimum requirement for existing rented properties under Smoke Alarm Regulations 2015
• 85dB alarm output; test/hush button; LED fault indicator
• Ceiling mounting bracket included

The Grade F2 battery smoke alarm is the quickest compliance solution for landlords fitting alarms in existing tenanted properties. Optical technology reduces nuisance alarms compared to ionisation types. Not suitable for new builds or major refurbishments where Approved Document B requires Grade D mains-wired alarms.

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Aico Ei146e Optical Smoke Alarm, 85dB, 230V

• Grade D1 — 230V AC mains-powered with sealed tamper-proof backup battery
• Optical smoke detection: ideal for living rooms, hallways, and bedrooms
• 85dB alarm volume; wired interconnect with all Aico Ei1 Series alarms
• Hush function silences nuisance alarms for up to 10 minutes
• BS 5839-6 Grade D1 Category LD2 compliant; approved for HMOs and new builds

The Aico Ei146e is the standard choice for new build and major refurbishment projects requiring Approved Document B compliance. Its sealed Grade D1 battery prevents tenant tampering. Wire in alongside the Ei144E or Ei144RC heat alarm in the kitchen to create a fully interlinked BS 5839-6 compliant system. Part P notifiable work.

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Aico Ei144E Heat Alarm — Fixed Temperature Thermistor, 230V

• Grade D1 — 230V AC mains-powered with sealed backup battery
• Fixed temperature thermistor sensor: triggers at 58°C
• Correct alarm type for kitchens and garages
• Wired interconnect with all Aico Ei1 Series alarms; 85dB output
• BS 5839-6 Grade D1 compliant

Under BS 5839-6 and Approved Document B, a smoke alarm must not be fitted in a kitchen — the correct device is a heat alarm. The Aico Ei144E triggers at 58°C, above normal cooking temperatures but well below fire temperature. Interconnects with the Ei146e so a kitchen fire sounds all alarms throughout the property.

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Aico Ei144RC Heat Alarm — Rate-of-Rise, RadioLINK Compatible

• Grade D1 — 230V AC mains-powered with sealed backup battery
• Dual detection: rate-of-rise plus fixed temperature upper limit
• RadioLINK compatible: wireless interlink without third wire
• Also supports wired interconnect with Ei1 Series alarms
• 85dB alarm volume; BS 5839-6 Grade D1 compliant

The Ei144RC is the premium heat alarm choice for retrofit projects where running a third interconnect wire is impractical. Its RadioLINK capability allows wireless interlink with Aico Ei2 or Ei3 Series smoke alarms. Rate-of-rise detection triggers on the speed of temperature increase, giving additional warning time in fast-developing kitchen fires.

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Selecting the Right System — Practical Decision Guide

New Build or Major Refurbishment (Building Control Required)

1. Confirm BS 5839-6 category with building control (typically LD2, sometimes LD1 for HMOs)
2. Install Aico Ei146e optical smoke alarms on each storey — hallway, landing, living room, bedrooms if LD1
3. Install Aico Ei144E or Ei144RC heat alarm in the kitchen (not a smoke alarm)
4. Interconnect all alarms via third wire or RadioLINK
5. Connect to mains via a dedicated lighting circuit spur or unswitched fused connection unit
6. Test interconnect function before handover and record on the Electrical Installation Certificate

Existing Rented Property (Landlord Compliance)

1. Confirm at least one smoke alarm per storey — use battery Grade F2 for simple retrofit with no rewiring
2. Add CO detector in every room with a fixed combustion appliance (gas boiler, gas fire, wood burner)
3. Test all alarms at start of each tenancy — record in tenancy check-in documentation
4. Upgrade to mains-wired Grade D1 alarms if undertaking any rewiring

HMO Installation

1. Obtain HMO licence conditions from the local authority — alarm specification varies by council
2. Grade D1 typically required in all common areas and sleeping rooms
3. Heat alarms in all shared kitchens; full interconnection across all alarms
4. Log alarm locations and test records in the HMO management file

Browse the full range of smoke alarms and heat alarms at APM Electricals. Same-day collection from the Acton W3 trade counter.

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