I agree with almost every piece of advice given to you BUT - you knew that was coming - cpvc is very capable to withstand the location and the method used to install the hot water heater. The copper pipe also is capable to withstand the attempts to bend it. The cpvc broke because it was subjected to extreme torsion while it was being bent in an attempt to bend the copper pipe to get it in place after the cpvc was installed.
I am not arguing that it should have been done that way, only that the materials used are perfectly capable of withstanding the usage except for the abuse they were afforded trying to make them do something that they cannot - which is bend. Copper can be bent, but it is not designed to be - it is rigid. And unless something is terribly wrong with the gas or fuel oil water heater, the temperature of the flue should never get anywhere near that which the cpvc will fail at under pressures up to around 300 psi! But still - No, do not install the water heater using any kind of plastic or rigid copper unless you wish to deal with fun like that.
All of that said, the solution is flexible copper pipes - about 18" long with two female connectors already installed. One 3/4" male on the water heater, one on the supply line, one on the hot water output and one on the hot water line to the house; all 3/4" male. The two flexible pipes solve the problem. no more worrying about vibration, leaking, BS, some wiggle-room for replacement unit - the only way to go IMHO.