The traditional method of installing residential sprinklers requires three separate sets of pipe in a home. The first two are for the plumbing - one for hot water and one for cold. The third set of pipes supplies the fire sprinklers. Multipurpose sprinkler/plumbing systems are now available that make sprinklers part of the cold water plumbing. This eliminates one run of pipes, which reduces cost.
Both methods of installation comply with NFPA 13D, the standard for installing residential fire sprinkler systems. The difference between the traditional method and multipurpose is in how the water is supplied to the sprinklers. Traditional systems must be isolated from the potable water. To install the sprinklers, the plumber installs a backflow prevention device to the plumbing system and the sprinkler contractor attaches the sprinkler pipe to these devices. The devices are required because the water in the sprinkler system is stagnant. The check valve or backflow prevention device keeps stagnant water from contaminating the potable water.
In a multipurpose system, there is no separate piping for the sprinkler system. Instead, the water for the sprinklers and plumbing is distributed by the same pipe. Using the same pipe for both the domestic water and fire sprinklers has several advantages. For one, it requires less pipe and fittings. Another advantage is the cost savings with the deletion of a backflow prevention device. These devices add hundreds of dollars to the cost of a residential sprinkler system. Since the multipurpose system uses potable water, there is no stagnant water to worry about. The same fresh water that supplies the plumbing fixtures is constantly available to control a fire.
NFPA 13D has allowed multipurpose systems for several years. They were not widely used before 1999, and many fire officials may not be aware of them. But revisions made to the 1999 edition of NFPA 13D changed that. The earlier editions required a minimum pipe diameter of 3/4," but the 1999 edition accepted 1/2" pipe that is commonly used for plumbing systems. This new system is creating more interest in multipurpose systems, and the prospect of lower installation costs is sparking interest in residential sprinklers in general.