Unlike a wet system that is filled with water, a dry sprinkler system is filled with pressurized air. When heat opens a sprinkler head, the loss of air pressure in the piping will cause a special dry valve to open, or trip. Piping will then fill with water and out of any open sprinkler head. The water flowing into the system will also sound a local alarm that alerts people in the building. The alarm may also be transmitted to a central station company. This station will then notify the local Fire Department that there is a fire.
Dry sprinkler systems are most commonly used when a wet pipe system cannot be heated to prevent freezing. In regions using NFPA regulations, dry pipe systems cannot be installed unless the range of ambient temperatures reaches below 40F. They are most often used in unheated buildings, in parking garages, in outside canopies attached to heated buildings, or in refrigerated coolers.