There isn't a good response for this on the internet all in one place - so here it is. It is fine to use PVC on your cold water inlet to the hot water heater. But the hot water lines going OUT of the heater should be CPVC. This is why. PVC begins to leach vinyl chloride at temperatures nearing 140 degrees. After 140 degrees, the leaching speeds up and the pipe fails, sometimes due to unsupported pipes bending and separating in the high heat. Most hot water heaters are right around that temperature 120-140 degrees.
The vinyl chloride originally keeps the pipes soft and bendable, but also reacts with heat. This is how the pipe material is made, heated up and then formed in tubing machines.
It is a double-edged sword, as a pipe that holds up initially (does not soften too much) can still over time leach the plasticizer (vinyl chloride) into the water as it passes through the pipe at temps over 73 degrees. Once the plasticizer (the material softener) is gone from the pipe, it can become brittle and break when shocked, moved or even from increased PSI in the pipeline during a time of kick/on kick/off at the water heater.
Normal cold water temperature is 73 degrees Fahrenheit. The PSI rating on PVC pipe is the PSI the pipe could withstand at 73 degrees, and this is stamped along each length of pipe. As the temperature goes above 73 degrees the PSI the pipe withstands is 'de-factored' or reduced by a percentage per number of degrees raised. According to one of the charts I had read (from Harvel plastic company who posts their tech spec online) a pipe that could withstand 350 PSI at 73 degrees can only withstand 105 PSI at 130 degrees, and less and less as temp goes up. Hot water heaters are high PSI appliances. The 'pressure relief valve' on a hot water heater sets off between 120 and 150 PSI, depending on manufacturer. Do the math - a 350 PSI plastic PVC pipe can withstand less PSI at 130 degrees than the hot water heater's backup relief valve puts out under normal operating conditions. And over time, even a 600 PSI plastic PVC pipe will leach enough plasticizer to become brittle and break from internal pressure alone.
A broken hot water heater pipe can damage the whole house, or even cause explosive effects. It depends on so many factors. And that is hard to explain to customers who are more worried about just getting something done NOW and how much it will cost them to have hot water tonight. etc etc... Some of them don't even care they will have the same problem again within a few years.
The answer? They've had it for years now. CPVC pipe. It does not begin to leach plasticizer until 180 degrees, which water heaters are never supposed to attain much less maintain for a long period of time. It will not 'de-factor' it's PSI below 180 degrees.
I still have to tangle with customers daily who do not understand this and it is difficult to explain to them in a store environment. I had a lady last night whose father was dying of cancer and 'Daddy always used PVC on the hot water heater.' Uh huh. There are studies out there showing the leached material, over long term exposure, can cause increased chance of cancer, as well. Like all 'carcinogen' materials, that is hard to prove definitively. I did not try to inform the customer of this issue, as she was already upset and unreasonable.
One of my coworkers said it nicely to me this morning. 'You can't feel bad about it.. you can't warranty stupidity.' I know. But sometimes you just want to protect them from themselves and you know that you can't. And it gets frustrating.