Charles:
PVC is not an approved or listed material of construction for fire sprinkler piping. Chlorinated PVC (CPVC) polymers that are listed for the fabrication of pipe and fittings meeting ASTM F 442, ASTM F 437, ASTM F 438 and ASTM F 439 are approved materials when using NFPA 13, 13R and NFPA 13D.
Three things to consider because it does seem counterintuitive to allow a particular polymer as fire sprinkler piping. Please consider the following:
1) As others have stated, the use of CPVC pipe and fittings (not PVC) is limited by the UL or FM listing. In the fire protection scheme, attics are a Light hazard occupancy, which means the fire protection hazard is the structure itself since almost all apartment buildings that are not a high-rise building are constructed using wood. Interestingly, I have at least 40 high rise buildings in my jurisdiction where either apartments or hotel rooms are protected with listed CPVC pipe & fittings.
2) The fire sprinkler pipe must be protected from freezing, which is accomplished by insulation or the insulation is negated by virtue that the building in question is in a climate that the attic or other combustible concealed space temperatures don't exceed < 40F.
3) The sprinkler piping is filled with water, which has a high latent heat of vaporization. The water filled pipe, in a sprinklered attic, is adequately protected from melting by enthalpy.
I'm an approving authority for a very large metropolitan fire department for almost 30 years and have extensive experience with NFPA 13, 13R and 13D automatic sprinkler systems. CPVC can fail but the problems we're continuing to see is the chemical compatibility of CPVC with other building materials (internet search CPVD sprinkler pipe + incompatible materials for guidance). Examples of failures are CPVC vs. insulation, fire-stop assemblies, and lubricants used to cut steel fire sprinkler piping.
