Why do some people insist on using the term 'hot water heater'? Hot water doesn't need heating. The correct term is 'water heater', without the 'hot' prefix. This really burns me up when I hear it.
Ozarkers use a lot of redundancies. I can not think of too many right now, but do you know any old widow women?
Emmett
A person's perspective depends on the implied relationships involved. Water heater might be paraphrased as Heater of water. Hot water heater might rather be paraphrased as Heater for hot water.
It's not an uncommon construction for a noun phrase to contain the result of the action or even the final product in mind, i.e. X to make/produce Y. Some examples:
popcorn popper
tea kettle
candle mould
cake batter
pie filling
lightbulb
cookie sheet
hot water heater is another example of a noun phrase focused on the outcome, rather than the ingredient. Perhaps the emphasis on the product came about to distinguish between the heater that produces the house's hot tap water as opposed to the heater of water that heats the house through radiators.
but do you know any old widow women?
"I have commanded a widow woman there to sustain thee" - King James Bible.
Hot water heater
Here in Mexico it's called a boiler, even though there's something wrong with yours if it boils your water. Ironically, the "proper" Spanish term for it simply means water heater.