The LWF Blog

Fire Safety Engineering for Design – Water Mist System Configuration – Part 303

September 7, 2026 8:56 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 302, LWF discussed twin fluid water mist systems. In part 303, we begin to look at water mist system configuration.

Water mist fire suppression systems are generally configured in two ways: deluge systems, which use open nozzles, and automatic systems, which use closed nozzles activated by heat. The distinction matters because each arrangement is designed to respond differently to a developing fire.

In an automatic water mist system, individual nozzles are fitted with a heat-sensitive element, such as a liquid-filled glass bulb. When the temperature around a nozzle reaches its operating point, the element releases the nozzle and water mist is discharged locally. Automatic nozzles operate independently, with only those exposed to sufficient heat opening.

Deluge systems take a different approach. Their nozzles remain open, while the pipework is dry and unpressurised. A separate fire detection system identifies the developing fire and activates the deluge valve, allowing water into the pipework. Water mist is then discharged simultaneously through all the open nozzles serving the protected zone. The British Automatic Fire Sprinkler Association describes this arrangement as the standard deluge approach for Class B and Class F applications.

Class B fires involve flammable liquids, while Class F fires involve cooking oils and fats. Both present challenges that make rapid application and effective cooling particularly important. Deluge water mist can be designed either as a total-flood system, where the protected enclosure is filled with mist, or as a local-application system, where nozzles concentrate discharge around a particular hazard or item of equipment.

For Class F fires, extinguishing the visible flame is only part of the task. Cooking oils can remain at temperatures capable of causing re-ignition after the flame has disappeared. Continued water mist discharge therefore provides cooling as well as suppression. Research published in the Fire Safety Journal, including full-scale experiments on hot cooking-oil fires, found that water mist could rapidly cool the oil and prevent re-ignition under the tested conditions.

System duration is consequently a critical design consideration. The water supply must provide sufficient capacity for the complete discharge period specified by the relevant approval and system design. Where a gas propellant is part of the system, the gas supply must also be sufficient for the required operating period.

Deluge systems can also be used for Class A hazards such as cable tunnels. In these applications, designers may divide the installation into zones with overlapping protection. Zoning helps prevent a fire from travelling beyond the area where it originated while allowing the system to target the affected section.

Selecting between automatic and deluge water mist is therefore not a matter of choosing one system as universally better. The appropriate configuration depends on the fire hazard, enclosure, detection strategy, required discharge pattern and validated fire-test performance. Water mist systems are application-specific, and the system should be designed and approved for the particular risk it is intended to protect.

In part 304 of LWF’s series on fire engineering we will continue discussing water mist system configurations by talking in more detail about automatic water 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.