The LWF Blog

Fire Safety Engineering for Design – Water Mist System Configuration – Part 304

September 14, 2026 9:10 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 303, LWF began to look at water mist system configurations. In part 304, we continue discussing water mist system configurations by talking in more detail about automatic water systems.

Automatic water mist systems are most commonly used to protect Class A fire risks, providing an alternative approach to conventional sprinkler protection where the application and system approval make water mist appropriate. The systems can be configured as wet or pre-action installations, following principles familiar from sprinkler systems. When sufficient heat from a fire reaches an automatic nozzle, the frangible bulb breaks and water is discharged locally around the risk.

Water mist systems differ from conventional sprinklers because they discharge water as very small droplets. These droplets absorb heat from the fire and surrounding environment and can evaporate rapidly, helping to control the fire and limit heat transfer. The effectiveness of the system depends on the characteristics of the water mist equipment, the hazard being protected and the fire tests or approvals supporting its use.

BS 8489 provides guidance for industrial and commercial water mist systems, including considerations for system design and installation. The standard is particularly relevant when establishing the design area and water supply requirements for an installation. BS 8489-1:2016 is the code of practice for the design and installation of industrial and commercial water mist systems.

The design area for a water mist system should not be selected using a generic assumption. It needs to be supported by the relevant third-party approval, fire test evidence and applicable standard for the system being installed. For light and ordinary hazard risks where an appropriate water mist approval exists, the design area is typically between 72 m² and 140 m². The actual requirement will depend on the approved system and the specific risk, so the figures should be treated as design parameters rather than universal values.

Hydraulic calculations are another critical part of the design process. The water mist system must have sufficient flow and water storage to satisfy both the most hydraulically favourable and most hydraulically unfavourable demands. Pressure losses through pipework, changes in elevation, nozzle characteristics and the configuration of the system can all affect the water available at individual discharge points. Accurate calculations are therefore essential to ensure that the system can deliver the required performance throughout the protected area.

Water storage capacity must also correspond with the required discharge duration. For the systems described in the source guidance, design discharge times should be no less than 30 minutes and are more commonly 60 minutes. The precise duration should be established from the applicable approval, standard and project requirements rather than assumed during the early stages of design.

For building owners, consultants and contractors, the key consideration is the competence of the company designing and supplying the system. Water mist protection is not simply a matter of selecting nozzles and providing a water tank; the design approval, hydraulic calculations, pipework, water storage and discharge characteristics all need to work together. Using a company with the appropriate water mist design knowledge helps ensure that these parameters are calculated correctly and that the completed installation is capable of delivering the protection for which it was designed.

In part 305 of LWF’s series on fire engineering we will discuss water mist components. 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.