I hate redundancy. Especially "exactly the same" or "the same exact thing." It just bugs me.
Someone pointed out that you don't have to say "hot water heater" - it's just "water heater." Hadn't thought about that one.
I guess that's true. Shouldn't it be called "cold water heater?" If the water is already hot, why heat it?
Depends on how you parse it.
For example, if I were to refer to grandma as the sweater knitter, I wouldn't be corrected with "no, she knits the yarn, and a sweater results".
A hot water heater is like a sweater knitter, the heater creates the hot water. It's the thing that's creating the result you have described.
A cold water heater describes the object of the action of the heating, so it's also correct. More correct than calling grandma a yarn knitter, because knitting doesn't specify the outcome, while heating does.
But both are redundant because water heater alone clearly indicates that it will heat cold water into hot water. So regardless, it's a wasted word.
Thankfully for me, I enjoy redundancy, it's great for the more poetic uses of language. Twelve conjugations of "fuck" in a sentence are far more effective at getting a point across than just one.
"What the fuck?" could be a casual, light hearted question.
But "what the fucking, fuck, fuckballs, fuckery?" really grasps the nuances of someone's utter, ragey disbelief.