Fire sprinkler systems are very difficult and expensive to replace or repair because the piping is often inside walls and ceilings. Some companies sell two-part epoxy adhesives that are fast curing and designed specifically for repairing this type of leak. In fact, you can buy kits that comprise an epoxy putty stick plus an impregnated fiberglass tape.

Another product is a low-viscosity anaerobic methacrylate liquid that will seal many pinhole leaks. In order to use this, you have to empty the pipe of water and dry it thoroughly. Heat the pipe using a flame or hot air gun and then apply the anaerobic. As the pipe cools down, the liquid will penetrate into the porosity and then cure. These types of repairs do not have any sort of code approval, however, and you need to access the specific leaks.

New technology is just coming available that will coat the inside of fire sprinkler piping with an epoxy sealant/lining and avoid the necessity to replace the piping. I know of two companies that have recently had products certified.

To comply with the International Fire Code, testing included the severe Fire Test in UL 852 (coated pipe fitted with sprinkler heads exposed to an actual fire with temperatures exceeding 1,600˚F). The coatings were intact after the testing, with no observations of cracking, flaking, or other deformation of the lining material. In addition, no fragments were determined to have negatively impacted sprinkler flow, and no fragments were found inside any sprinkler used in testing. This is a very attractive repair technique because it requires only minimal access to the fire sprinkler system. 

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