In a wet pipe system, the piping is always filled with water under pressure. When a sprinkler head is triggered by heat, water is discharged immediately. This is the most common sprinkler system for general buildings.

Dry pipe: Pipes contain air or nitrogen; water enters only after valve activation.

Deluge: Pipes are empty, and all heads open simultaneously, releasing water everywhere.

Pre-action: Pipes are dry until detection and valve activation; reduces accidental discharge risk, recommended for IT rooms.

Since the definition describes immediate discharge from water-filled pipes, this is a wet pipe system.