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Understanding Pre-Action Sprinkler Systems: Safeguarding High-Value Spaces with Precision Fire Protection

Pre-action fire protection systems are vital for high-value, temperature-dependent, or disruption-critical areas. In this blog, we’ll discuss what pre-action sprinkler systems are, how they work, their types, benefits, disadvantages, typical applications, and installation and maintenance.

What are pre-action sprinkler systems:

Pre-action sprinkler systems are a type of sprinkler system that protects spaces that additional safeguarding from accidental discharge (e.g. high-value electronics). This system employs automatic sprinklers that are attached to a pressurized piping system containing air or nitrogen. The system holds water from the distribution piping via a pre-action valve; the activation of that valve is via a smoke detector and/or a sprinkler activation, depending on the type of pre-action system.

Pre-Action Sprinkler System

Types of pre-action systems:

  1. Single Interlock System: This system admits water to sprinkler piping upon operation of a detection device.

  2. Non-interlock System: This system admits water to sprinkler piping upon operation of a detection device or automatic sprinklers.

  3. Double Interlock System: This system admits water to sprinkler piping upon operation of detection devices and automatic sprinklers.

Benefits and disadvantages:

To discuss the benefits and disadvantages to this system we must first discuss alternative approaches to protect spaces and compare them to the pre-action system relatively. Below is a list of common solutions of protection albeit not a complete list.

  • Wet sprinkler system: Typical solution to protecting a space in compliance of NFPA 13, lower cost as long as there is access to a water supply of sufficient flow and pressure. No additional safety measures are present to protect the space from accidental discharge outside of protecting the sprinklers from physical and temperature damage. No protection from temperature related damage

  • Antifreeze sprinkler system: Similar to wet system above with the addition of an antifreeze solution. This provides freeze protection in cold climates but at the cost of additional components to facilitate the antifreeze solution into the system. In addition to that the solution must be listed for fire protection systems which restricts the amount of approved products. This system provides no additional safety measures outside of temperature related damage.

  • Deluge sprinkler system: Deluge sprinkler systems are typically used in high-challenge fire areas where water is required for the entire area. Deluge systems are used to extinguish or control a fire, and they are also installed to provide exposure protection to areas adjacent to a fire. Deluge piping contains no water and is exposed to the atmosphere. The pipe is subject to the effects of corrosion resulting from condensation or improper draining of the system. Similar to Single interlock preaction system water is released to the piping via a detection device or a wet or dry detection sprinkler but all the sprinklers are nozzles resulting in complete instant coverage of the space. These systems are for large storage areas or hangers not in office spaces as Hollywood would have you believe. These systems are overall less complex then a standard wet system but offer less protection against accidently discharge.

  • Dry sprinkler system: A dry system is essentially a non-interlock pre-action system without the detection device activation. This system releases similar to a wet system where once a sprinkler is activated the air pressure holding the dry valve closed separating the dry piping form the wet service piping separate. Once the sprinkler is activated the air pressure quickly releases through the sprinkler followed by water flow shortly after. Because of this operating the system needs an additional 30% design area increase since during a fire the system essentially is blasting the fire with a jet stream of air for around 60 seconds prior to putting the fire out. This increases the system size in comparison to a standard wet system. Benefits include temperature related protection over a wet system but offers no additional release protection.

  • Water mist system: This system offers a wide variety of options within the category of “water mist” but as an umbrella definition this system mixes water and high pressure air to provide a water mist. Benefits include a smaller footprint of stored water on site or incoming water service if flow and pressure on site is low. Cost is higher than a typical wet sprinkler system but offers less water damage in the event of a discharge unlike all the other options listed here. The activation of the system can be similar to the predation system where a detector and/ or a nozzle activating via temperature is required to activate the system since the configuration is up to the designer.

  • Pre-action system: Cost effective in comparison to the options listed above that offers additional protection against unintended discharges.

Pre-Action Sprinkler System


  • Health care (LINAC, MRI, etc)

  • High-value storage

  • Museums

  • Libraries

  • Data Centers

  • Freezer warehouses

Installation and maintenance:

Utilizing NFPA 25 section 14.3, the following is a condensed maintenance and testing on a pre-action system hitting the main requirements.

  • Inspect pre-action valves monthly for damage, valves in appropriate position, leaks, pressure gauges upstream and downstream reading with the appropriate delta (NFPA 25,

  • Conduct a trip test annually, inspect the valve condition internally. Note that valves that can be reset without removal of the faceplate may be inspected every 5 years (NFPA 25 and

  • If nitrogen is used to reduce C value in calculations, the concentration of nitrogen must be biannually tested for a 98% or greater content (NFPA 25,

  • Every 3 years the pre-action valve shall be trip tested with the control valve fully open (NFPA 25,

  • Every 3 years the pre-action system shall be tested for air leakage (NFPA 25,


Pre-action fire protection systems are a valuable tool for a fire protection engineer. Their unique design, combining detection and activation, ensures precision, cost effective and minimizes water damage due to unintended discharge of the system. With most manufacturers offering packaged units that contain a nitrogen generator and compressor inside the unit this type of system is an excellent option for protecting a space that needs additional protection against unintended discharges.


National Fire Protection Association. (n.d.-a). NFPA 13 Standard Development.

National Fire Protection Association. (n.d.-b). NFPA 25 Standard Development.

Ryan McCarthy

Written By:

Ryan McCarthy, PE

Fire Protection Technical Specialist


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