Recently I came across a leak on the hot water line coming from a hot water heater. The house was piped using CPVC, which is different than PVC in that it goes through an extra chlorination process that makes it capable of handling higher temperatures. PVC, the white plastic pipe most commonly known from irrigation systems, can only handle temperatures up to 140 degrees. CPVC, which looks the same as PVC, but slightly yellow in color, can handle temperatures up to 200 degrees. In this case, a repair had been made to the hot water CPVC line using a PVC coupler. The pipe was leaking from both sides of the coupler.
The proper fix was to have a plumber replace the leaking coupler using a CPVC coupler using primer and cement that is specifically for use with CPVC.
One other item to note would be that plastic piping of any type should not be connected directly to the water heater, whether or not it is capable of higher temperatures. Instead an 18 inch water heater connector made of copper should be used.