The LWF Blog

Fire Safety Engineering for Design – Water Mist Systems – Part 297

July 27, 2026 9:30 am

LWF’s Fire Safety Engineering blog series is written for Architects, building designers and others in the construction industry to highlight and promote discussion on all topics around fire engineering. In part 296, LWF looked at the performance of water mist systems. In part 297, we will discuss fire test protocols for water mist systems.

A well-designed water mist system is only as effective as the evidence supporting its performance. Unlike conventional sprinkler systems, water mist technology cannot be specified solely on the basis of nozzle characteristics or hydraulic calculations. Its effectiveness depends on how the complete system performs against a defined fire hazard, making full-scale fire testing a fundamental requirement of every recognised water mist standard.

International standards and certification schemes consistently require water mist systems to demonstrate their performance through independent fire testing using recognised third-party protocols that are directly relevant to the intended hazard. This approach recognises that water mist extinguishes fires through a combination of cooling, oxygen displacement by steam generation, and radiant heat attenuation. The relative importance of these mechanisms varies significantly depending on the fuel, enclosure geometry, ventilation conditions, and fire development. As a result, successful performance in one fire scenario cannot be assumed to translate to another.

Several internationally recognised fire test protocols provide the benchmark for demonstrating system performance. These include FM 5560: Water Mist Systems, UL 2167: Standard for Water Mist Nozzles for Fire-Protection Service, the VdS Approval Programme, and BS 8489, specifically Parts 4 to 7, which define fire performance testing for a range of applications. Together, these documents establish the testing criteria, fire scenarios and acceptance requirements needed to verify that a system is suitable for a particular hazard.

The protocols are hazard-specific rather than universally applicable. For Class A combustibles, such as those typically encountered in light hazard occupancies that broadly correspond to Ordinary Hazard Group 1 under BS EN 12845, the applicable test methods include FM 5560 Appendix G and BS 8489-7:2016. These evaluate the system’s ability to control or extinguish fires involving solid combustible materials under representative building conditions.

For Class B flammable liquid hazards, testing is considerably more varied due to the range of possible fire scenarios. Machinery spaces, for example, are covered by FM 5560 Appendices A to F, with additional requirements for local application systems contained in Appendix I. Equivalent testing is also provided within BS 8489-5:2016, which addresses machinery space protection for land-based applications.

Specialised protocols also exist for high-risk industrial processes. Class F hazards, including industrial cooking equipment using vegetable or animal oils, are addressed by FM 5560 Appendix J, reflecting the unique fire behaviour associated with high-temperature cooking oils.

A critical consideration when selecting a water mist system is the distinction between marine and land-based approvals. Although marine-certified systems are frequently considered for terrestrial applications, the underlying fire test protocols differ substantially. Marine testing is generally limited to exposed spray fires within machinery spaces, whereas land-based protocols require successful performance against a broader range of hazards, including concealed spray fires as well as both exposed and concealed pool fires. Consequently, marine approval alone does not demonstrate suitability for land-based installations.

In part 298 of LWF’s series on fire engineering we will continue discussing fire test protocols. In the meantime, if you have any questions about this blog, or wish to discuss your own project with one of our fire engineers, please contact us.

Lawrence Webster Forrest has been working with their clients since 1986 to produce innovative and exciting building projects. If you would like further information on how LWF and fire strategies could assist you, please contact the LWF office on 0800 410 1130.

While care has been taken to ensure that information contained in LWF’s publications is true and correct at the time of publication, changes in circumstances after the time of publication may impact on the accuracy of this information.