In fire fighting, what is the wet sprinkler and dry systems?

A wet sprinkler is a system that is charged with water. A dry sprinkler is one where the pipes don’t contain water. When a sprinkler head goes off on a dry system, the head lets the air out and water follows it shortly after from a valve that opens to let the water in. Dry sprinklers are primarily used in places where freezing is possible to prevent freeze/burst lines.

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Wet and dry describes the static state of the sprinkler head feed plumbing (all the piping above and behind the heads). The system most people see is "wet", meaning the feed lines are flooded and will immediately dump when a head is triggered. A "dry" system means the feed lines are drained of water and are only flooded when the system detects a possible fire.


Dry systems are used in areas that if a leak occurred it could severely damage equipment (think data center and archival storage). Dry systems require more regulation... permitting, testing, and inspection ($$ maintenance) thus limiting their use.


The next logical question is "Then why would anyone put even a dry sprinkler based fire suppression system in a data center?" The answer is; Property insurance cost. The cost of insurance is significantly less expensive when a water sprinkler suppression system is resident.

There were other systems as well like Halon, which is no longer used and CO2 systems, which I am not even sure if they still use either since they now have a water fog system that I am told works very well. Maybe I should investigate and see what they have come up with since I retired.