Pre-action systems are similar to dry systems in that the pipes are not filled with water prior to activation of the system. The pre-action valve is keeps the water from flowing through the pipes until it is activated by it's heat or smoke detection elements. Routine maintenance of these systems is essential.
Pre-action systems are:
Less likely to falsely discharge due to the dual action required to activate the system.
Among the more costly systems to install and maintain, due to their complexity.