Pre-action and deluge systems are variations of the dry pipe system. Pre-action systems must be triggered twice before water is dispensed from the sprinkler head. The first trigger -- usually a smoke detector or a heat detector that is separate from the sprinkler head -- allows water into the pipes. Once this trigger occurs, the system acts like a wet pipe system. The second trigger, the individual sprinkler heads, releases the water, pushing it through the sprinkler head to extinguish the fire. This type of system provides added protection against false sprinkler head activation. If a sprinkler head is falsely triggered, an alarm will sound, but no water will be released, since the pipes remain dry until the second trigger has been activated. Although sprinkler heads are not likely to accidentally activate -- sprinkler heads have a one in 16 million false activation rate -- pre-action systems are very popular when dealing with sensitive property like in a museum or library [source: USFA].
In deluge systems, like the pre-action system, water enters the pipes when triggered by a heat or smoke detector, separate from the sprinkler heads. There may also be a manual function, where pushing a button or pulling a cord activates the system. The sprinkler heads in a deluge system are open, meaning they don't have a trigger function and are activated simultaneously. Deluge systems are usually installed in chemical plants or other areas where the spread of fire would be exceptionally hazardous.
The widespread use of fire sprinkler systems and the resulting innovations are no doubt a result of laws, regulations and building codes that require their installation. We'll explore existing codes and the possible need for home regulations on the next few pages.