hot-water heater vs water heater

hot-water heater and water heater both are nouns.

Word	Noun	Adjective	Verb	Adverb
hot-water heater	Yes	No	No	No
water heater	Yes	No	No	No

As nouns, hot-water heater and water heater are synonyms defined as:

hot-water heater and water heater: a heater and storage tank to supply heated water

Other synonyms of hot-water heater include hot-water tank.

hot-water heater (noun)	water heater (noun)
a heater and storage tank to supply heated water	a heater and storage tank to supply heated water