Individual-head (i.e. not deluge) sprinkler systems come in three flavors:
- Wet-pipe systems have water pressure at the head at all times -- they are the quickest to respond, but can't be subject to freezing conditions and are too risky for water-sensitive spaces
- Dry-pipe systems have compressed air in the pipe between the head and the dry-pipe valve -- when the head fuses, the dry-pipe valve is opened by the pressure change and delivers water to the system. This can be used when a freeze risk exists, but is slower to respond.
- Preaction systems are used to provide external confirmation (from say an alarm system) before the sprinklers activate -- the alarm system must open the preaction valve before water can flow onto the fire. This is used when accidental discharges must be controlled at all costs.
This raises the question: under NFPA 13/13R/13D, can individual fire zones be protected by a dry-pipe or preaction system while the remainder of the building has wet-pipe sprinkler protection? For instance, the attic, garage, and unfinished basement in a home could be dry-pipe protected to avoid freeze-ups while the finished spaces are protected by a conventional wet-pipe system.