Suppression systems
— an overview
Gaseous and water mist suppression systems — what they are, how they work, the main types used in the UK, and how they interact with fire alarm systems.
Suppression systems are used in environments where the consequences of fire would be catastrophic — data centres, server rooms, archives, specialist manufacturing — or where conventional sprinklers are not appropriate. Understanding how they work and what they require from a fire alarm perspective is important for anyone responsible for such premises.
How Suppression Systems Operate
Regardless of the suppression agent used, the operating sequence for an automatic suppression system is broadly consistent. The cause and effect programming that governs the sequence is set up during commissioning and documented in the cause and effect matrix.
Coincidence detection
Two independent detectors in the protected area must both activate before the suppression sequence begins. This two-detector coincidence requirement significantly reduces the risk of accidental discharge from a single faulty or spuriously activated detector.
Pre-discharge warning
On coincidence, the suppression control panel immediately activates pre-discharge warning devices within the protected area — a specific sounder tone and flashing beacon that warn any occupants to evacuate at once.
Building alarm
Simultaneously, a signal is sent to the main fire alarm panel to trigger the general building evacuation alarm if it has not already activated.
Environmental preparation
During the pre-discharge delay, the panel triggers air handling unit shutdown, closes fire dampers, and releases door holders — sealing the protected space to maximise agent retention after discharge.
Time delay and abort
A pre-discharge delay of typically 30–60 seconds elapses to allow evacuation. A manual abort button allows the sequence to be halted if the activation is known to be a false alarm. After the delay — if abort has not been activated — the agent discharges.
Gaseous suppression
Gaseous Suppression Systems
Gaseous suppression systems extinguish or suppress fires by flooding the protected space with a suppression agent that reduces oxygen concentration below the level needed to sustain combustion, or interrupts the chemical chain reaction of the fire, or both. Unlike water, gas leaves no residue — making it the preferred solution for protecting sensitive electronic equipment, irreplaceable documents, and high-value assets.
The suppression agent is stored in pressurised cylinders adjacent to the protected area and is released through a system of pipes and nozzles designed to achieve a uniform concentration throughout the space within a defined discharge time — typically 10 seconds. Aspirating detection is often specified for the protected zone — see our detection devices guide for how aspirating systems work.
Inert Gas Systems — IG-541, IG-55, IG-100
Use naturally occurring gases — nitrogen, argon, or a mixture — to reduce oxygen concentration from approximately 21% to around 12–15%. At this level combustion cannot be sustained, but humans can survive briefly, allowing escape. No ozone depletion, no global warming potential, no residue.
Typical applications: Data centres, telecoms rooms, archives, museums. Requires significant cylinder storage volume.
FM-200 (HFC-227ea)
Extinguishes fire primarily by absorbing heat from the flame. Stored as a liquefied gas, vaporises rapidly on discharge, achieving extinguishing concentration quickly with compact cylinder storage compared to inert gas.
Note: Subject to phase-down under F-gas regulations — availability and cost are being affected by restrictions on high global warming potential fluorinated gases.
Novec 1230 (FK-5-1-12)
A fluorinated ketone that extinguishes by heat absorption. Very low global warming potential and zero ozone depletion — environmentally preferable to FM-200. At higher equipment cost but increasingly preferred where environmental credentials matter.
Note: Also subject to regulatory scrutiny under evolving PFAS restrictions — the regulatory picture is changing.
CO2 Systems
Extinguishes by displacing oxygen. Highly effective with no residue, but lethal to humans at extinguishing concentrations. Must never be used in normally occupied spaces — requires strict safety controls, lockout procedures, and clearly marked danger zones.
Typical applications: Unoccupied plant rooms, electrical switchgear rooms, marine applications, industrial hazards.
Water mist
Water Mist Suppression Systems
Water mist systems suppress fire using extremely fine water droplets — typically less than 200 microns in diameter, compared to conventional sprinkler droplets which are typically 1,000–2,000 microns. The very small droplet size dramatically increases the surface area of water exposed to the fire, giving water mist systems several suppression mechanisms simultaneously.
How water mist suppresses fire
Unlike a conventional sprinkler that primarily cools a fire by wetting the burning material, water mist works by cooling the flame gases, displacing oxygen as the droplets vaporise into steam, and wetting and cooling surfaces around the fire. The combination of these effects allows water mist to suppress fires using a fraction of the water that a conventional sprinkler system would consume — typically 50–90% less water.
| Characteristic | Detail |
|---|---|
| System pressure | Low pressure (below 12.1 bar), intermediate (12.1–34.5 bar), and high pressure (above 34.5 bar). Higher pressure systems produce finer droplets and generally more effective suppression. |
| Typical applications | Server rooms, data centres, heritage buildings, museums, listed buildings, machinery spaces, commercial kitchens, passenger vessels |
| Advantages over gaseous systems | No hazard to occupants from the suppression agent. No sealed room requirement. Lower environmental impact. Effective on a wider range of fire types. |
| Advantages over conventional sprinklers | Dramatically reduced water usage. Smaller pipe diameters. More suitable for environments where water damage would be unacceptable. |
| Standard | BS 8458 (residential and domestic), BS 8489 (industrial and commercial), NFPA 750 |
Fire alarm integration
How Suppression Systems Interact with Fire Alarms
A suppression system does not operate independently — it is closely integrated with the fire alarm system. The suppression control panel receives signals from the fire detection system, manages the pre-discharge sequence, and sends signals back to the main fire alarm panel. The integration is defined in the cause and effect matrix.
| Interface | Detail |
|---|---|
| Detection | Dedicated detectors in the suppressed area. Two-detector coincidence before suppression sequence initiates. |
| Pre-discharge warning | Dedicated sounders and beacons in the protected area activate on coincidence — distinct from the general fire alarm signal |
| General alarm | Signal to main fire alarm panel triggers building evacuation alarm |
| Environmental controls | Air handling shutdown, damper closure, door release — to seal the space and maximise suppression effectiveness |
| ARC signal | Suppression activation signal typically sent to the Alarm Receiving Centre if the system is monitored |
Maintenance — a separate discipline
Suppression systems require regular maintenance in addition to the standard fire alarm servicing programme. This should be carried out by a specialist suppression contractor. It is a separate discipline from fire alarm servicing — always check whether your maintenance contractor is competent to cover both before assuming they do.