The LWF Blog

Fire Safety Engineering for Design – Sprinkler Systems – Part 239

June 16, 2025 10:12 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 238, LWF talked about dry installations, tail-end alternate and tail-end dry systems. In part 239, we will discuss pre-action installations.

Pre-action installations

A pre-action installation is primarily a dry installation which pre-arms should a fire be detected by a fire detection system. There are two distinct types of pre-action sprinkler installations – Type A and Type B systems.

Type A

Water is fed into the pipework through a pre-action valve and is only released into the system pipework when the fire detection system is activated by the congruent operation of two fire detectors.

Prior to becoming operational and at all other times, the system is in normal operating mode, which means the pipework is charged with low-pressure compressed air, which will escape in the event of  damage to a sprinkler head or to the system pipework. This occurrence would raise the alarm but not result in water entering the system pipework.

Concurrent operation of the fire detection system and a sprinkler head is required before water is able to discharge from the sprinkler system.

Such systems are most commonly seen in environments where the inadvertent operation of the sprinkler head or a damaged pipe would have very costly or disruptive consequences, should water be delivered unexpectedly and inappropriately.

As a pure fire suppression system, the added complication in addition to reliance on a fire detection system means a less reliable system overall than a straightforward wet charged sprinkler system. Such systems should therefore only be considered where there is no viable alternative and are not appropriate for use in high hazard environments.

Type B

A Type B system is an alternate wet and dry or dry installation in which the fire detection system charges the sprinkler system pipework with water at an early stage of fire development, prior to operation of the sprinkler heads.

A Type B is appropriate on large systems with a high volume and where high hazards are involved and rapidly growing fires may occur. Should the fire detection system fail, the Type B pre-action sprinkler will revert to operate as a conventional alternate or dry installation.

In part 240 of LWF’s series on fire engineering we will talk about recycling installations and deluge installations (which must be the type seen most commonly in Hollywood films). 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.