A pre-action system is a sprinkler system whose pipework is normally filled with air, where water enters the pipe only after an independent detection system operates. Its purpose is to make accidental discharge close to impossible.
Why it exists
In a wet system the pipe holds pressurised water at all times; if a head is broken, water flows. In spaces such as data centres, archives, museums and cleanrooms an accidental discharge costs as much as the fire itself. A pre-action system reduces that risk without giving up sprinklers.
Three types
| Type | For water to enter the pipe | For water to leave a sprinkler |
|---|---|---|
| Non-interlock | Detector or sprinkler operation | Sprinkler operation |
| Single interlock | Detector operation | Sprinkler operation |
| Double interlock | Detector operation and loss of pipe pressure | Sprinkler operation |
The last step is the same in all three: no water goes anywhere until a sprinkler opens. The difference is when the valve opens.
Where double interlock is needed
Where pipe damage is the biggest risk: cold stores, areas with heavy forklift traffic, production lines with a high risk of mechanical impact. In a double interlock system a detector alone does not open the valve; there must also be a pressure drop in the pipe. An accidental detector alarm discharges no water.
The price is delay: water enters the pipe later and leaves the sprinkler later. To compensate, NFPA 13 requires the design area to be increased by 30% (for dry and pre-action systems) and limits the time for water to reach the most remote sprinklers.
The cold store exception. A wet system cannot be used because of the freezing risk. A dry system is one option; a double interlock pre-action system prevents freezing and accidental discharge at the same time. That combination is the typical cold store solution.
What to watch in design
- The detection must be more sensitive than the sprinklers. If the detector operates after the sprinkler, the pre-action logic collapses and — outside the non-interlock type — the system is late.
- Pipe volume is limited. Water transit time is critical; large installations need an accelerator or sectioning.
- The air must be dried. Moist compressed air corrodes the pipe; a dryer or nitrogen is preferred.
- The testing regime is heavy. Detection, valve and air pressure all need regular testing to the NFPA 25 intervals.
- Standby power. Detection and valve control depend on electricity, and the system's behaviour on loss of supply must be defined.
A common mistake
Feeding gaseous suppression and the pre-action system from the same detection line in a data centre. Unless the logic for agent release is separated from that for opening the water valve, the pipe fills the moment the gas discharges. The correct arrangement is a separate detection zone for the gas and a second level for the pre-action system.
Frequently Asked Questions
What is the difference between single and double interlock?
In single interlock, detector operation alone opens the valve. In double interlock a loss of pipe pressure is also required, so an accidental detector alarm does not fill the pipe.
Where are pre-action systems used?
Where accidental discharge costs as much as the fire: data centres, archives, museums, cleanrooms; the double interlock type is also used in cold stores and production areas with a high impact risk.
Why is the design area increased for a pre-action system?
Because water enters the pipe late, discharge from the sprinkler is delayed. NFPA 13 requires the design area to be increased by 30% for dry and pre-action systems.
Why must the air in pre-action pipework be dried?
Moist compressed air corrodes the pipe from inside. An air dryer or nitrogen protects pipe life and system reliability.
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Download MEP Calc on the App StoreNFPA 13 (2025) · NFPA 15 · NFPA 855 · BS EN 12845:2015+A1:2019 · FM Global DS 5-33 · FM Global DS 4-1N. Definitions are for information only.