In a wet sprinkler system, a network of pipes is filled with water. When the system goes off, water is released from several sprinklers in an effort to douse any nearby flames. In a dry sprinkler system, the pipes are filled with air instead of water. When the system goes off, water quickly pours first into the pipes, and then out of the sprinklers.
Each sprinkler head contains either a heat-sensitive glass bulb or a link held together by a heat-sensitive material. When the head is exposed to heat, the bulb or link releases, and air flows from the head, changing the pressure inside the pipe. The change of pressure allows water to enter the system and flow out through the head.
The single advantage of a dry sprinkler system is that it can provide fire protection to areas where the temperatures fall below the freezing point. If a wet sprinkler system were installed in these areas, the pipes would freeze, leaving the area with no protection against fires.
Some believe that a dry sprinkler system is also helpful in areas with art collections or other expensive water-sensitive objects. In truth, a dry sprinkler system would be virtually no more advantageous than a wet sprinkler system in this case. Water would flow through the dry sprinkler system less than a minute after it would flow through a wet sprinkler system, which would do next to nothing to protect water-sensitive objects.
Dry sprinkler systems are more complex than wet sprinkler systems, so they require more maintenance and are more costly. The delayed response time, although negligible, can also increase the risk of damage in the case of a fire. In addition, after a dry sprinkler system is set off, it must be completely drained and dried; wet sprinkler systems do not require this step. Failure to do this in a dry sprinkler system can lead to pipe erosion and sprinkler failure.