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Contents
  1. 1Fire signatures and why sensors are combined
  2. 2Types and combinations
  3. 3Carbon monoxide sensing
  4. 4Unwanted alarms and reliability
  5. 5Use in regulation and design
  6. 6Limits
  7. 7See also
  8. 8Notes
  9. 9References

A multi-sensor detector is an automatic fire detector that senses more than one fire phenomenon, or uses more than one operating principle, in a single unit.[1] In the terms of the US fire alarm code NFPA 72, as quoted in NUREG-1805, this is called combination detection.[1] Typical examples are a combined heat and smoke detector and a heat detector that combines a rate-of-rise and a fixed-temperature element.[1] Units that combine thermal, photoelectric and ionisation sensing are also available.[2]

The idea behind such detectors is that each type of sensor responds best to a different part of a fire. Ionisation detectors respond somewhat faster to open flaming fires, while photoelectric detectors respond faster to smouldering fires.[3] Modern detectors also use response algorithms instead of a simple threshold, partly to reduce false and nuisance alarms and to improve fire signature matching.[4] These algorithms differ between detectors and are generally not published.[4] For fire safety engineering this creates a problem, because the actual response of each model is affected by its signal sampling algorithm.[4] In New Zealand, C/VM2 (second edition, 2025) avoids the problem by simulating every detector with fixed heat or smoke detector criteria, regardless of the make and model installed.[5]

Fire signatures and why sensors are combined

A fire produces several signatures that a detector can sense. The SFPE Handbook lists electromagnetic radiation, invisible products of combustion smaller than 0.1 micrometre, visible smoke larger than 0.1 micrometre, a rapid change in temperature and a high temperature.[6] Each common detector type responds to only one of these.[6] Ionisation smoke detectors sense the invisible products, photoelectric detectors the visible smoke, rate-of-rise heat detectors a rapid change in temperature, and fixed-temperature heat detectors a high temperature.[6]

Grid of five fire signatures against detector types; each single detector has one mark, a combined smoke and heat unit has marks in several columns.
Fire signatures and the detector types that sense them; a combination detector covers more than one column. Based on Schifiliti et al. (2002), p. 4-4, and Iqbal and Salley (2004), pp. 11-4 to 11-6.

The smoke itself differs between fires. Early in a fire, heating of materials produces very small particles, and the particle size grows as the fire develops and particles coagulate.[7] High-energy flaming fires produce large numbers of small particles, which suit the ionisation principle.[3] Low-energy smouldering fires produce more of the larger particles, to which photoelectric detectors respond faster.[3] Every listed smoke detector must still pass the same set of test fires.[3]

Full-scale tests show the difference in practice. Purser describes a flaming furniture fire in the lounge of a test house. There, the ionisation detector in the lounge operated after 0.5 minutes and the optical detector after 1.5 minutes.[8] The response of a light-scattering detector also depends on the colour of the smoke.[9] One such detector may alarm at an optical density of 0.029 m−1 for light grey smoke from a smouldering cotton wick but only at 0.15 m−1 for black smoke from a kerosene fire.[9] A unit that combines several sensors aims to cover more of these fire types with one device.[1][3]

Types and combinations

One combination adds a heat sensor to a smoke detector. NUREG-1805 notes that some ionisation and photoelectric detectors are made with a dual mode of operation, with a fixed-temperature thermal element built into the unit.[3] A heat detector can itself combine two principles: a rate-of-rise element and a fixed-temperature element.[1] According to the SFPE Handbook, most commercial rate-of-rise heat detectors have a fixed-temperature element as a backup. The reason is that many slowly developing fires cause high ceiling temperatures without ever reaching the rate of rise needed for an alarm.[10]

A worked example in the SFPE Handbook shows how the two elements of such a combined heat detector complement each other.[10] The detector has a fixed-temperature threshold of 57 °C, a rate-of-rise threshold of 11 °C per minute and a response time index of 42 m1/2s1/2.[10] It is installed at a spacing of 10.0 m under a 2.4 m ceiling, and the fire grows as a t-squared fire with different growth times.[10] For a fast fire with a growth time of 100 s, the rate-of-rise element responds after 53 s and the fixed-temperature element after 135 s.[10]

For a slow fire with a growth time of 600 s, the order reverses: 521 s for the fixed-temperature element against 835 s for the rate-of-rise element.[10] Both respond at 454 s when the growth time is 509 s.[10] The crossover depends on the spacing: at a spacing of 25 m, the rate-of-rise element is faster only for growth times up to 228 s.[10] The handbook therefore advises considering the two elements separately when a system is designed or analysed.[10]

Two response-time curves against fire growth time crossing at 509 s; rate-of-rise is lower for fast fires, fixed-temperature for slow fires.
Response time of the two elements of a combined heat detector at 10 m spacing as a function of fire growth time; the curves cross at a growth time of 509 s. Based on Schifiliti et al. (2002), p. 4-18.

Analogue sensors go a step further. Instead of a simple alarm or no-alarm signal, they measure the conditions in the space and send this information to a computer-based fire alarm control unit.[3][2] They can report when a detector is too dirty to work properly or has become too sensitive.[2] The system designer can adjust the sensitivity to the ambient environment or use an extra-sensitive setting for a high-value area.[2] NUREG-1805 lists these sensors as photoelectric, ionisation, or combination thermal, photoelectric and ionisation units.[2]

Block diagram: smoke and heat sensors in one detector send values over a loop to a panel that gives pre-alarm, alarm or dirty warnings.
An analogue multi-sensor detector sends measured values, not only an alarm state, to the control panel. Based on Iqbal and Salley (2004), pp. 11-5 to 11-6, and Australian Building Codes Board (2020a), pp. 96-97.

The Australian verification method data sheets describe the same capability at system level. An analogue addressable detector reports its exact location and also data on its own condition.[11] This information can identify dirty detectors and give an early alert before the main alarm, as smoke or heat levels rise.[12] According to the data sheets, it may improve the efficacy of the system and reduce false alarms.[12]

Combining signals is also possible between separate detectors. In coincident detection, two or more detectors must operate before the alarm is recognised.[13] These can be two similar detectors at different locations, or two detectors of different types.[13] A multi-sensor detector combines the signals within one unit, whereas coincident detection combines the signals of several units in the fire alarm system.[1][13]

Carbon monoxide sensing

Gas sensing is a further possible input. The International Fire Engineering Guidelines list carbon monoxide concentration among the fire conditions that can be used to determine detector activation.[14] Carbon monoxide matters especially in smouldering fires. Purser notes that a smouldering fire in a dwelling can last for several hours, with a hazard that develops mainly from carbon monoxide.[15] The slowly thickening and irritant smoke may trigger detectors before the situation becomes dangerous.[15]

A Dutch study by the Netherlands Institute for Public Safety (NIPV) deals with indoor car parks for electric vehicles. It recommends a fire detection system with smoke and/or heat detectors together with a gas detection system for carbon monoxide.[16] It cites a test of several detectors in which carbon monoxide detectors were the fastest to alert.[16] The same report notes that in the Netherlands carbon monoxide detectors are in use, often in connection with LPG, but not as a form of fire detection.[16] Dutch standardisation and certification are not geared to this use either.[16]

For gas sensing detectors there is no accepted method to calculate activation times, according to the International Fire Engineering Guidelines (2005).[17] The same chapter does give calculation methods for heat, smoke and flame detectors.[13] Carbon monoxide is also one of the main asphyxiant gases in flaming furniture fires, together with hydrogen cyanide.[15]

Unwanted alarms and reliability

One reason to combine sensors and signal processing is to reduce unwanted fire alarms. The SFPE Handbook states that detector algorithms are used to reduce false and nuisance alarms and to improve fire signature matching.[4] NUREG-1805 describes analogue sensor systems as essentially free of false alarms under the conditions normally found in a building.[2] It adds that matching the detector's sensitivity to the environment is part of the design of a stable smoke detection system.[3]

Processing a signal takes time. The International Fire Engineering Guidelines list detector signal interrogation, verification and processing, and the time for coincident detector operation, among the delays between activation and notification.[18] These delays must be added to the activation time when the time to warn occupants is estimated.[18]

Complexity has its own risks. The Australian data sheets note that addressable systems depend on site-specific software and programming, which introduces the risk of software or programming errors.[11] They also note that combining different system types adds complexity, which should be considered when estimating the effectiveness of the system.[12]

Use in regulation and design

Some prescriptive codes accept combination detectors as an alternative to single-sensor detectors. The New York City building code applied to the World Trade Center required, in a Class E fire alarm system, ceiling-mounted ionisation smoke detectors or combination smoke/heat detectors at each elevator landing.[19] These detectors had to be placed directly above the call button.[19]

Performance-based design treats combination detectors more simply. In New Zealand, C/VM2 (second edition, 2025) requires detectors to be simulated with fixed values, regardless of the make and model to be installed.[5] It gives separate criteria for a heat detector and for a smoke detector.[5] The heat detector has a response time index of 30 m1/2s1/2 and an activation temperature of 57 °C, and the smoke detector alarms at an optical density of 0.097 m−1.[5] C/VM2 gives no separate criterion for multi-sensor detectors.[5] The International Fire Engineering Guidelines similarly allow a point smoke detector to be modelled as a heat detector that operates at a temperature rise of 13 °C, with a low response time index.[20]

Limits

The main limitation for engineers is that the response of a multi-sensor detector is set by its internal algorithm. The SFPE Handbook notes that even if correlations between optical density and detector response were available, the actual response of each model would be affected by its signal sampling algorithm.[4] Because the algorithms are generally not published, accurate modelling is not possible without data for the particular detector.[4] Product performance tests can give a range of possible response, because the test standard sets minimum and maximum permissible performance.[9]

For heat detectors with two elements, the SFPE Handbook advises analysing each element on its own, as in the worked example above.[10] For smoke detectors, the International Fire Engineering Guidelines note that there are few well-developed prediction methods and advise a conservative prediction.[20] The data sheets of the Australian verification method credit the extra information from analogue detectors with better efficacy and fewer false alarms.[12] Estimation methods are best used to compare how one detector's response changes with spacing or location, not to compare with egress times.[9]

See also

Notes

  1. ↑ 1.0 1.1 1.2 1.3 1.4 1.5 Iqbal and Salley (2004), p. 11-4.
  2. ↑ 2.0 2.1 2.2 2.3 2.4 2.5 Iqbal and Salley (2004), p. 11-6.
  3. ↑ 3.0 3.1 3.2 3.3 3.4 3.5 3.6 3.7 Iqbal and Salley (2004), p. 11-5.
  4. ↑ 4.0 4.1 4.2 4.3 4.4 4.5 Schifiliti et al. (2002), p. 4-22.
  5. ↑ 5.0 5.1 5.2 5.3 5.4 Ministry of Business, Innovation and Employment (2025b), p. 27.
  6. ↑ 6.0 6.1 6.2 Schifiliti et al. (2002), p. 4-4.
  7. ↑ Iqbal and Salley (2004), p. 11-3.
  8. ↑ Purser (2002), p. 2-137.
  9. ↑ 9.0 9.1 9.2 9.3 Schifiliti et al. (2002), p. 4-23.
  10. ↑ 10.00 10.01 10.02 10.03 10.04 10.05 10.06 10.07 10.08 10.09 Schifiliti et al. (2002), p. 4-18.
  11. ↑ 11.0 11.1 Australian Building Codes Board (2020a), p. 96.
  12. ↑ 12.0 12.1 12.2 12.3 Australian Building Codes Board (2020a), p. 97.
  13. ↑ 13.0 13.1 13.2 13.3 National Research Council of Canada et al. (2005), p. 2.7-2.
  14. ↑ National Research Council of Canada et al. (2005), p. 1.7-5.
  15. ↑ 15.0 15.1 15.2 Purser (2002), p. 2-140.
  16. ↑ 16.0 16.1 16.2 16.3 Van der Graaf et al. (2021/2023), p. 37.
  17. ↑ National Research Council of Canada et al. (2005), p. 2.7-6.
  18. ↑ 18.0 18.1 National Research Council of Canada et al. (2005), p. 1.7-6.
  19. ↑ 19.0 19.1 Grill and Johnson (2005b), p. 80.
  20. ↑ 20.0 20.1 National Research Council of Canada et al. (2005), p. 2.7-5.

References

  • Australian Building Codes Board. (2020a). Data sheets: Fire Safety Verification Method (Handbook annex, Print version 1.1).
  • Grill, R. A., & Johnson, D. A. (2005b). Fire protection and life safety provisions applied to the design and construction of WTC 1, 2, and 7 and post-construction provisions applied after occupancy (NIST NCSTAR 1-1D). National Institute of Standards and Technology.
  • Iqbal, N., & Salley, M. H. (2004). Fire Dynamics Tools (FDTs): Quantitative fire hazard analysis methods for the U.S. Nuclear Regulatory Commission Fire Protection Inspection Program (NUREG-1805). U.S. Nuclear Regulatory Commission, Office of Nuclear Reactor Regulation.
  • Ministry of Business, Innovation and Employment. (2025b). Verification Method C/VM2: Framework for fire safety design (2nd ed.).
  • National Research Council of Canada, International Code Council, Department of Building and Housing, & Australian Building Codes Board. (2005). International fire engineering guidelines (2005 ed.). Australian Building Codes Board.
  • Purser, D. A. (2002). Toxicity assessment of combustion products. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 2-83–2-171). National Fire Protection Association; Society of Fire Protection Engineers.
  • Schifiliti, R. P., Meacham, B. J., & Custer, R. L. P. (2002). Design of detection systems. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 4-1–4-43). National Fire Protection Association; Society of Fire Protection Engineers.
  • Van der Graaf, J., Hessels, T., & Rosmuller, N. (2023). Fire safety of indoor car parks accommodating electrically powered vehicles. Nederlands Instituut Publieke Veiligheid. (Original work published 2021)
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