What's the difference between a wet and dry fire sprinkler system?

The difference is what the pipes normally contain while standing by for a fire.

  • A wet sprinkler system has pressurized water inside the system 24/7 for extinguishing a fire when any sprinkler head gets hot enough to open.
  • A dry sprinkler system has pressurized air or nitrogen inside the piping. This keeps a dry-valve closed, and water out of the system. When a sprinkler fuses (opens), it allows the air to escape; then the dry valve opens and allows water to enter the system. Wherever there is an opened sprinkler, water will flow through it and put out the fire.

Dry systems are used in areas where water filled piping could freeze, such as unheated warehouses, freezers, unheated buildings, etc.

Additional precautions can also be taken to avoid accidental flooding when a sprinkler head is damaged. For example, a dry-valve controller may wait for a signal from a fire alarm panel before turning on the water.