What is the difference between wet-pipe and dry-pipe systems?

• A. Wet-pipe systems hold pressurized air or nitrogen
• B. Dry-pipe systems have water-filled piping
• C. Wet-pipe piping is water-filled; dry-pipe holds pressurized air or nitrogen in the pipes; water enters when a head is activated in a dry-pipe
• D. Dry-pipe systems are not used for fire protection

Answer: (C)

The main idea is what fills the pipes and how water reaches the sprinklers. In a wet-pipe system, the piping is always filled with water, so when a sprinkler head is heated and releases, water immediately flows through the lines to the activated head. In a dry-pipe system, the pipes are kept under pressure with air or nitrogen, and water is held back in a separate vessel. Water only enters the piping after a dry-pipe valve is released by a sprinkler head, so there’s a brief delay as the pipes fill and water moves to the head.

This setup makes wet-pipe simpler and faster to discharge, while dry-pipe is better for unheated or cold spaces where water-filled pipes could freeze, at the cost of more complexity and a little delay in water delivery. The statement that dry-pipe systems are not used for fire protection is incorrect, since both types are designed for fire suppression.