A pre-action fire sprinkler system is similar to a dry-pipe sprinkler system. In a pre-action system, the water is held back by a pre-action valve. The valve is opened when flame, heat, or smoke is detected. Two things must happen before a pre-action sprinkler system releases water.
The first is that the detection system must detect a fire and the valve must open, releasing water into the piping. The second event is that individual sprinkler heads need to open to allow water to discharge.