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These are the most common systems and are used in buildings where there is no risk of freezing. They are quick to react because water is always in the pipes above the sprinkler heads.
Wet systems are required for multi-storey or high-rise buildings and for life safety.
As the name suggest Alternate systems can have the pipes full of water for the summer and be drained down and filled with air (under pressure) for the winter. This is important for buildings that are not heated.
The pipes are filled with air under pressure at all times and the water is held back by the control valve. When a sprinkler head opens, the drop in air pressure opens the valve and water flows into the pipework and onto 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 the detector operates (e.g. smoke detectors). Pre-action systems are used 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 industrial risks.