Different Types of
Fire Detector Heads (Sensors)

Last Updated: June 29, 2022

Fire alarms have different heads, or sensors, to detect different fire types. There are four primary types of detectors that detect the presence of a fire. Understanding the difference between heat and smoke detectors will help you to determine which detector is ideal for any application.

The four types of fire detectors are heat, optical (ionisation), photoelectric, and ionisation/photoelectric. The differences in each of these are how they detect fires, heat being from temperature, and the other three being from smoke. The best detector is the combination ionisation/photoelectric.

In existing commercial buildings, premises are subject to The Regulatory Reform (Fire Safety) Order 2005. The Order states that the ‘Responsible Person’ within the organisation has a legal duty to undertake a suitable and sufficient fire risk assessment at their premises. This assessment is a detailed report of the fire risk in your building, the adequacy of existing fire precautions, and the need for any additional fire precautions. The fire risk assessment will determine the level of fire alarm system required for your business premises.

The guidance usually indicates appropriate British Standards. The main standard for fire alarm systems is BS5839 Pt1:2017.

smoke detectors

The three most common smoke detectors are ionisation, photoelectric, and combination ionisation/photoelectric. All smoke detectors sound an alarm, when they identify smoke, to notify a building’s occupants. What differentiates these detectors from one another is the way that they detect smoke.

Ionisation Smoke Alarms

These detectors are excellent at detecting fast-burning fires. These are the cheapest and cost very little to purchase. They are very sensitive to small particles of smoke produced by fast flaming fires, such as paper and wood, and will detect this type of fire before the smoke gets too thick.

Ionisation Smoke Alarm

Photoelectric (Optical) Alarms

These detectors are effective at detecting larger particles of smoke from slow-burning, ‘smouldering’ fires. They’re extremely reliable and produce few false alarms. These are more expensive but more effective at detecting larger particles of smoke produced by slow-burning fires than an ionisation smoke detector.

Photoelectric (Optical) Alarms

Ionisation/Photoelectric (Multi-sensor) Alarms

They are designed to be sensitive to a wide range of fires and provide a faster reaction to both fast-flaming and slow smouldering fires. As a result, BS 5839-6:2019+A1:2020 recommends the use of multi-sensor technology in all rooms except the kitchen. As the unit monitors for two different by-products of fires (smoke and temperature), its response to all types of fire is vastly improved over traditional single-sensor alarms. They are also less prone to false alarms associated with ionisation and optical type alarms.

Multi-sensor detector

heat detectors

These can detect an increase in temperature and are insensitive to smoke. Heat detectors detect the presence of heat and an increase in air temperature. These detectors have few false alarms but, they take longer to identify a fire than a smoke detector. They are ideal in situations where smoke detectors may cause false alarms such as steamy, humid, or dusty environments and therefore best suited for kitchens, garages, and lofts.

Heat sensitive point detectors

Point detectors can again be subdivided into a further two types;

     1. Fixed temperature – which will operate when it is exposed to a pre-determined temperature. Normally fixed temperature detectors employ a fusible alloy element that must be replaced after the detector has operated. Different temperature-rated elements are available to take account of varying ambient air temperatures.

     2. The second type operates on the rate of temperature rise. The rate of rise temperature detector may also include a fusible element for fixed temperature operation.

Heat Detector

best practice for positioning cO and fire detectors

Smoke and Heat Detectors

BS 5839-6:2019+A1:2020 recommends that smoke and heat alarms be installed:

     1. On the ceiling, as central as possible in the room

     2. Sited 300mm from walls and light fittings – this ensures the alarm is out of any ‘dead air’ spaces where the airflow may be blocked

     3. Placed within 3m of every escape door and bedroom door to ensure audibility

     4. Positioned between high-risk rooms and bedrooms

     5. For peaked and sloped ceilings – make sure there is a maximum of 600mm vertically down from the apex for smoke alarms, and 150mm vertically down for heat alarms

Carbon Monoxide Detectors

The BS EN 50292:2013 standard recommends that a CO alarm should be installed:

     1. Between 1m–3m from all potential sources of carbon monoxide (fuel-burning appliances)

     2. Sited 300mm from walls and light fittings – this is to ensure that they are outside of any ‘dead air’ spaces that occur in corners and spaces where the airflow may be blocked

     3. If the fuel-burning appliance is in a confined space (e.g. a boiler room) then the alarm should be sited on the ceiling just outside the room

     4. If there is no fuel-burning appliance, then place the alarm at breathing height e.g. bed’s head height in the bedroom

Fire Risk Consultancy Services have the knowledge and experience to assist your business to comply with all legal requirements surrounding Fire Safety including the legislation regarding Fire Alarms. Be sure to read our accompanying guides: British Standards Relating to Fire Alarms,  A Guide to Fire Alarm System Types, Categories of Fire Alarm Systems, Fire Alarms in the Home, AND Fire Extinguishers, Emergency Lighting, Fire Door Regulations, Fire Safety Signs, also published on this site. 

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