These are the most common type of installation and are used in buildings where there is no risk of freezing. They are fast to react because water is always in the pipes feeding the sprinkler heads.
Wet Pipe installations are required for multi-storey or high rise buildings and for life safety purposes.

As the name suggests 'Alternate Installations' can have the pipes full of water in the summer months and be drained down and filled with compressed air for the winter months. This is important for buildings that are not heated and consequently prevents freezing of the pipes.
NB: Under the new BSEN 12845 Rules Technical Bulletin 232.4, Alternate Alarm Valves are no longer considered appropriate for sprinkler service, due to the potential for accelerating pipework corrosion. Existing Alternate Alarm Valves before the introduction of this rule may continue to operate and maintain in accordance with the standard.

The pipes are filled with compressed air and are under pressure at all times and the water is held back by a different air valve. When a sprinkler head opens the drop in air pressure opens the valve and water flows into the pipe work and on to the fire. Dry pipe systems are used where wet or alternate systems cannot be used.

Like dry pipe systems, the pipes are filled with air but water is only let into the pipes when either a smoke or a heat detector operates. Pre-action systems are used in Comms Rooms where it is not acceptable to have the pipes full of water unless there is a fire.

These are not strictly sprinkler systems and are only used in special cases for individual risks.