The LWF Blog

Fire Safety Engineering for Design – Sprinkler Systems – Part 237

June 2, 2025 9:38 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 236, LWF looked at the types of sprinkler system. In part 237, we give an overview of wet installations and alternate wet and dry installations.

Wet Sprinkler Installations

By far the most common of all sprinkler installations is the wet installation. In simple terms, it means that the system pipework is charged with water and is at operational pressure at all times. If a fire occurs, a sprinkler head activates and the water is released immediately.

The main reason it is the most commonly used type of sprinkler installation is that it is suitable for the majority of risks, but would be unacceptable in conditions where the temperature may be below freezing, causing the pipes to freeze. Additionally, it should not be installed where the temperature may exceed 70 °C.

Alternate Wet and Dry Installations

An alternate wet and dry installation is one designed specifically for areas where it may be below freezing in winter, but not the rest of the year. During the warmer seasons, the installation may be operated as a wet installation, with pipes charged with water. During the winter, the system undergoes maintenance involving the thorough draining of water from the pipework and the control valves being set to ‘winter’ operation.

In the winter, the pipes are filled with air charged under moderate pressure. In the case of a fire, a sprinkler head will operate and the air will escape, reducing pressure. The control valve actuates and water is allowed to flow into the pipework to the operating sprinkler head.

Once the risk of frosts is past, the system should be returned to a wet (water-charged) system.

A disadvantage of using this kind of system is that during the cold months, when the system is in dry mode, there may be a delay between sprinkler operation and the arrival of water at the site of the fire. This leads to a system whereby the number of sprinkler heads which may be fed from this type of installation tends to be fewer than the number on a traditional wet sprinkler system.

A slight concession is made when using a system fitted with an exhauster or accelerator. These are designed to detect the drop in air pressure and operate to charge the system with water more quickly than would otherwise be the case.

An alternate wet and dry installation would not be suitable for high hazard storage risks. In addition, new installations of alternate wet and dry sprinkler systems are not permitted in the UK.

In part 238 of LWF’s series on fire engineering we will talk about dry installations, tail-end alternate and tail-end dry systems. 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.