The LWF Blog

Fire Safety Engineering for Design – Water Mist Systems – Part 295

July 13, 2026 9:20 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 294, LWF considered the properties of water mist systems. In part 295, we continue to explore water mist systems and their performance criteria.

Water mist systems are not universally interchangeable with conventional sprinkler protection. Their effectiveness depends upon the interaction between droplet characteristics, fire dynamics, enclosure conditions, and the specific fuel hazard involved. For building professionals, understanding these distinctions is essential when evaluating the suitability of water mist for any given fire strategy.

Water mist systems are commonly used to protect Class A, Class B, and Class F fire risks, although the suppression mechanisms differ significantly between each category.

Class A fires involve ordinary combustible materials such as timber, paper, textiles, and many plastics. These fires are often deep-seated and can continue smouldering even after visible flames have been suppressed. In these applications, water mist primarily acts through cooling and fuel wetting. Sufficient droplet momentum is required to penetrate the fire plume and reach the burning fuel surface. If droplets are excessively fine, they may evaporate before reaching the seat of the fire, reducing extinguishing effectiveness.

Class B fires involve flammable liquids such as petrol, diesel, paraffin, kerosene, solvents, and alcohols. In this environment water mist performs differently to the scenarios already described. Rapid evaporation of fine droplets within the flame zone absorbs significant thermal energy while simultaneously displacing oxygen locally through steam generation. Research referenced within NFPA 750 indicates that droplets below approximately 400 microns can be particularly effective for liquid fuel fires due to their increased evaporation rates and enhanced interaction with the flame front.

Class F fires involve cooking oils and fats and present unique challenges due to their very high auto-ignition temperatures and rapid fire development. An improper application of water can result in dangerous fuel splashing and fire spread. For this reason, water mist systems intended for kitchen protection must be specifically tested and approved for such hazards under recognised fire test protocols. Modern systems certified to BS EN 14972-17 are validated through full-scale testing to demonstrate safe extinguishment performance under representative fire conditions.

One of the key advantages of water mist is the significantly lower water demand compared with conventional sprinkler systems. This can reduce secondary water damage, minimise storage tank requirements, and improve suitability for heritage buildings, residential developments, data environments, and high-value assets.

Water mist systems are highly application-specific and cannot be designed solely using prescriptive density-area calculations. System performance depends upon aspects such as the nozzle design, operating pressure, droplet spectrum, enclosure ventilation, compartment geometry, and fuel type. For this reason, modern standards increasingly require validated fire test evidence and project-specific fire engineering assessments to confirm water mist system suitability for the intended risk profile.

For designers and approving authorities alike, the successful application of water mist depends upon understanding not only the technology itself, but also the limitations of the tested system configuration and the importance of maintaining compliance with the certified design basis.

In part 296 of LWF’s series on fire engineering we will continue looking at the performance of water mist systems. 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.