The LWF Blog

Fire Safety Engineering for Design – Water Mist Systems – Part 298

August 3, 2026 8:35 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 297, LWF discussed fire test protocols for water mist systems. In part 298, we continue talking about fire test protocols.

Water mist systems have become an established option for fire protection across a wide range of buildings and industrial facilities. Their ability to control or suppress fire using significantly less water than conventional sprinklers makes them attractive in applications where water damage, limited water supplies or sensitive equipment are concerns. The effectiveness of any system relies not only on the technology available, but also whether the system has been tested and approved for the specific conditions in which it will operate.

Fire test protocols are developed to demonstrate that a water mist system can control or suppress a fire under clearly defined conditions. Organisations including FM Approvals, UL Solutions and VdS have published protocols covering a variety of occupancies and hazards. These include light hazard spaces, data processing rooms, wet benches used in semiconductor manufacturing, concealed ceiling and floor voids, cable tunnels, car parks and storage occupancies. Each protocol represents a specific fire scenario rather than a general approval for all buildings.

Fire tests should have defined boundaries. Ceiling height, room volume, ventilation rates, the size and position of openings, and the quantity and type of combustible materials all influence how a fire develops and how water mist performs. If one of these variables is changed, the behaviour of both the fire and the suppression system may also change. A water mist system approved for a light hazard occupancy with a maximum ceiling height of 5 metres should not be assumed suitable for the same occupancy at a greater height without further evidence.

These limitations become particularly relevant when a building design changes during development or when an existing building is refurbished for a different use. Alterations that appear relatively minor may place the installation outside the conditions covered by the original fire testing. Relying solely on an existing approval when circumstances have changed can be dangerous and potentially disastrous.

Where the proposed application differs only in one measurable parameter, such as ceiling height, repeating the established fire test under the revised conditions may provide the evidence needed to support the design. The testing should replicate the original configuration as closely as possible, with independent witnessing and technical review to confirm that the relevant acceptance criteria have been satisfied.

Some applications have no recognised fire test protocol. In these cases, a bespoke programme of full-scale fire testing is normally required. Annex C of NFPA 750 provides guidance on developing suitable test methods, establishing realistic fire scenarios and defining objective pass or fail criteria before testing begins. The intention is to demonstrate system performance under conditions that accurately represent the intended installation rather than relying on comparisons with unrelated approvals.

Independent verification is an essential part of the process. Fire tests and technical reports should be reviewed by recognised third parties, with input from the authority having jurisdiction where appropriate. Insurers and certification organisations such as FM Approvals, LPCB, UL Solutions or VdS may also participate in validating the testing and confirming that the evidence supports the proposed application.

A detailed fire risk assessment should underpin every water mist design. Certification demonstrates that a system has performed successfully within defined limits, not that it is suitable for every project. When an installation falls outside those limits, fire engineering judgement supported by appropriate fire testing provides the evidence needed to show that the proposed solution is fit for purpose.

In part 299 of LWF’s series on fire engineering we will look at the types 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.