The LWF Blog

Fire Safety Engineering for Design – Water Mist Systems – Part 294

July 6, 2026 8:40 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 293, LWF discussed the scope of the standards in relation to water mist systems. In part 294, we consider the properties of water mist systems.

Water mist fire suppression systems are increasingly being specified within modern buildings where reduced water consumption, enhanced life safety performance, and protection of sensitive environments are key design considerations. Unlike conventional sprinkler systems, water mist systems utilise very fine water droplets to control, suppress, or extinguish fires through a combination of cooling, oxygen displacement, and radiant heat attenuation.

Under British and European standards, water mist is generally defined as a water spray in which 90% of the droplet volume has a diameter of less than 1 mm when measured 1 m below the discharge nozzle. NFPA 750 adopts a more restrictive definition, requiring 99% of the discharged water volume to consist of droplets below 1 mm diameter.

Historically, residential and domestic water mist systems within the UK were designed in accordance with BS 8458:2015. However, this standard was formally withdrawn in July 2025 and replaced by BS EN 14972-17:2025, a part of the wider BS EN 14972 suite covering water mist systems for specific hazards and occupancies. This change reflects the wider transition from prescriptive design rules toward performance-based fire engineering supported by full-scale fire testing.

BS EN 14972-1 now provides the overarching requirements for design, installation, inspection, and maintenance, while BS EN 14972-17 establishes the fire test protocols applicable to residential occupancies using automatic nozzle systems. Rather than relying solely on standardised nozzle spacing or discharge densities, systems must now demonstrate validated fire performance for the intended application.

The effectiveness of water mist systems is fundamentally linked to droplet size. Fine droplets provide a significantly greater total surface area than larger sprinkler droplets, enabling rapid heat absorption and evaporation. Thermal cooling occurs through two mechanisms:

• sensible heat absorption as droplets are heated to 100 °C; and
• latent heat absorption during conversion from liquid water to steam.

Approximately 87% of the cooling effect is derived from latent heat of vaporisation, making evaporation the dominant suppression mechanism.

As steam is generated, oxygen concentration within the flame zone is locally reduced, inhibiting combustion. Simultaneously, the mist cloud absorbs and scatters radiant heat, helping to reduce fire spread and flashover potential.

Unlike traditional sprinkler systems, water mist performance is highly dependent upon enclosure geometry, ventilation, nozzle type, operating pressure, and fuel characteristics. Consequently, systems should only be specified where supported by third-party certification, validated fire testing, and a project-specific fire engineering assessment demonstrating suitability for the proposed risk.

In part 295 of LWF’s series on fire engineering we will continue to look at water mist systems and their performance criteria. 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.