Understanding Multi-Sensors


With advances in technology there is a growing selection of multi-sensors available on the market. However as with everything, when there is choice, it’s important to understand the options and impacts they have on your design.

Good fire detection is a result of the ideal combination of quick notification, with minimal false alarms. There is always going to be a compromise in finding the fastest signs of fire mixed with reliability. This is often where multi-sensors can come into their own, providing early warning while providing the reassurance of a confirmation.

Comprehensive research carried out in the UK by BRE and the FIA found that the use of advanced multi-sensors can outperform more basic models in resisting false alarms. It did also highlight the vast difference in available multi-sensors currently on the market.

What is a multi-sensor?

A multi-sensor is a detector that uses a combination of sensors to make a final decision, usually a combination of smoke, heat and/or carbon monoxide. Depending on the manufacturer these can be as simple as two sensors both needing confirmed alarm condition to intelligent models using involved algorithms making complex decisions.

Why do false alarms matter?

In addition to false alarms causing a nuisance and inconvenience, they damage the reliability of the whole fire detection system. Repeated false activation result in equipment being ignored or disabled, leaving the whole building vulnerable.

SANS10139: 2021 lists the primary false alarm risks as being:

  • Steam
  • Toaster
  • Oven usage
  • Cooking
  • Dust
  • Smoking
  • Aerosol spray
  • Hot works

Selecting the correct multi-sensor is an excellent way of maintaining early detection while reducing the negative effects of the potential false alarms listed above.

How to select the right type of multi-sensor?

Good fire detection designs takes relevant risk factors and environmental conditions into consideration before selecting the best detection for a specific area. Primary concerns when selecting which detector technologies are:

  1. Type of area to be protected? – room, open space, corridor, escape route?
  2. Predominant use of the area? – who is in the area; are they awake or asleep, people familiar with the building or members of the public?
  3. What type of fire risk is most likely? – smouldering fire, clean burning, quick spreading?
  4. Category of systems? – Life safety or property protection?

Bullet points from SANS10139:2021

Smoke detectors should not be used in:

  • Areas where principal fire hazard is flammable liquids or gases
  • where there is risk of high false alarms
  • areas of early detection of a relatively fast and clean burning flame fire is likely

Heat detectors should not be used:

  • escape routes
  • mobility impaired rooms
  • areas of Category P in which a small fire has potential to cause unacceptable damage

Carbon monoxide detectors may be used in:

  • rooms opening onto escape routes
  • any area suitable for use with heat detector, other than flammable liquids
  • escape routes within Category L3 or L4 (but not L2 or L1) in conjunction with smoke detectors
  • other areas where there is test evidence that CO detectors would offer adequate fire protection

Carbon monoxide is a heavy gas and CO detectors designed for detecting excess CO in a room are mounted around 1m above the floor and not on the ceiling. This is not their intended function in the fire detection multi-sensor, which is using the CO to confirm the smoke detected is from a fire and not from steam, dust or other misleading sources.

Watch out for!

  1. Multi-sensors with part heat detector are subject to the coverage area (5.3m) and height limitation (9m for Class A1 or 7.5m for others) of heat detectors.
  2. Some smoke and multi-sensor detectors can be programmed such that they would not then meet the fire sensitivity tests of EN54-7, which would not be compliant with SANS10139
  3. Multi-sensors with CO that only have either EN54-5 or EN54-7 are not compliant as standalone CO detectors.


The increase in multi-sensors has lead to updates in EN54 which now include specific categories. Fully compliant multi-sensors are compliant to:

  • EN 54 part 29 Multi-sensor fire detectors – Point detectors using a combination of smoke and heat sensors
  • EN 54 part 30 Multi-sensor fire detectors – Point detectors using a combination of carbon monoxide and heat sensors
  • EN 54 part 31 Multi-sensor fire detectors – Point detectors using a combination of smoke, carbon monoxide and optionally heat sensors

Many multi-sensors are approved under the older categories and have certified the individual components:

  • EN 54 part 5 Heat detectors – Point heat detectors
  • EN 54 part 7 Smoke detectors – Point smoke detectors using scattered light, transmitted light or ionization

Note: There is no EN54 for Carbon Monoxide detectors as they are not a fire detector in their own right.


SANS10139:2021 has additional comments regarding the testing of multi-sensors, which in itself is testament to their increased popularity. It specifies that the multi-sensor should be physically tested by a method that confirms that products of combustion in the vicinity of the detector can reach the sensors and that the appropriate response is confirmed at the CIE.

Where a multi-sensor is dependent on one or more detection technologies to confirm an alarm, the individual detectors should be physically tested.

Specific mention is made to remember that some sensor functions may be time-related so this must be considered when testing. It also has a specific reminder to ensure on completion of tests the system should be returned to its normal configuration.

This has lead to the growth in test equipment which is now capable of testing multiple sensors simultaneously. Check with your supplier for details of new test equipment now available.

If you remember nothing else!

  • Not all multi-sensors are made equal.
  • The rules for the individual sensors still apply.
  • Can be a valuable tool in the fight against unwanted false alarms – when used correctly.
  • Need to be added to the maintenance and commissioning testing procedures.

Written by: Nichola Allan

Email: nichola@g2fire.co.za