The LWF Blog

Fire Safety Engineering for Design – Gaseous Fixed Fire Extinguishing Systems – Part 311

November 2, 2026 9:53 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 310, LWF began to talk about gaseous fixed fire extinguishing systems. In part 311, we continue on the subject of gaseous fixed fire extinguishing systems by discussing the types of extinguishing agents used.

Gaseous fire suppression systems rely on carefully selected extinguishing agents to control fires without necessarily using water. The three most common types are synthetic halocarbon agents, inert gases and carbon dioxide. Each works differently, and selecting the right agent depends on the type of fire, the protected environment and the concentration needed to extinguish the flames safely and effectively.

Synthetic halocarbon agents are designed to interrupt the combustion process, while inert gases reduce the oxygen concentration within an enclosure to a level that prevents combustion from continuing. Carbon dioxide suppresses fire by reducing the available oxygen and absorbing heat. Each option has specific applications, design considerations and safety implications, making the choice of agent a key part of the overall fire protection strategy.

The required concentration varies according to the fire classification. Class A fires involve ordinary combustible materials, including wood and paper. Some fires present greater challenges because they develop deep within materials such as plastics and electrical cable insulation. These are often described as higher-hazard or deep-seated Class A fires. Class B fires involve flammable liquids and combustible substances, including petrol, diesel, kerosene, paraffin, alcohols, paints and solvents.

Understanding the distinction between these fire types is essential when specifying a gaseous suppression system. An agent that extinguishes a surface fire may not achieve the same result when combustion continues within a material or concealed space. System designers must therefore establish the appropriate fire classification and use the corresponding extinguishing concentration when calculating the quantity of agent required.

Design concentrations also incorporate a safety margin above the minimum concentration needed to extinguish a fire. According to the CIBSE guidance on fire suppression, this margin is typically between 20% and 30%, depending on the design standard applied. The additional concentration provides a margin for effective suppression under the conditions anticipated by the system design.

The standards used for system design can influence the final concentration specified. CIBSE’s guidance identifies NFPA 2001 (2015b), the US standard for clean agent fire extinguishing systems, as generally recommending lower design concentrations than the relevant ISO and BS EN standards. Designers should therefore establish which standard governs a project rather than assuming that concentrations specified under different frameworks are interchangeable.

When specifying gaseous fire suppression systems in the UK, designers should follow the applicable standards and fire safety legislation for the building and its intended use. BS EN 15004-1:2024, Fixed firefighting systems – Gas extinguishing systems – Design, installation and maintenance (ISO 14520-1:2023, modified), sets out requirements for the design, installation, testing, maintenance and safety of gaseous extinguishing systems. The standard provides a framework for selecting suitable extinguishing agents and determining appropriate design concentrations for the fire risks present. Designers must also consider the relevant UK fire safety requirements and the specific characteristics of the protected enclosure.

Gaseous suppression is not the only technology available for protecting enclosed spaces. Aerosol fire suppression systems offer an alternative approach, while permanent hypoxic inerting systems reduce oxygen levels continuously to help prevent combustion from starting or developing. These technologies have different operating principles and application requirements, which need to be considered alongside conventional gaseous systems.

Choosing an appropriate fire suppression solution requires more than selecting an extinguishing agent. Accurate fire classification, suitable design concentrations and compliance with the applicable standards all contribute to system performance. Understanding these fundamentals provides a sound basis for examining alternative suppression technologies and their role in protecting buildings, equipment and critical environments.

In part 312 of LWF’s series on fire engineering we will discuss halocarbon and fluoroketone extinguishing agents. 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.