|These systems contain an additional fire-detection device that will recognize a fire before the sprinklers are activated. The sprinklers are attached to a pipe containing air that may or may not be pressurized.
When the detection device senses a fire, it opens the main valve, allowing water to flow through the pipes before the sprinklers are set off. When the heat activates the sprinklers, water flows through immediately, as in a wet-pipe system.
Pre-action systems are usually employed in areas that are at risk for serious water damage due to damaged sprinklers and/or piping.
They operate faster than dry systems, but tend to be significantly more expensive.