water heater vs storage tank

water heater and storage tank both are nouns.

Word	Noun	Adjective	Verb	Adverb
water heater	Yes	No	No	No
storage tank	Yes	No	No	No

As nouns, storage tank is a hypernym of water heater; that is, storage tank is a word with a broader meaning than water heater:

water heater: a heater and storage tank to supply heated water
storage tank: a large (usually metallic) vessel for holding gases or liquids

Other hypernyms of water heater include heater, warmer, tank.

water heater (noun)	storage tank (noun)
a heater and storage tank to supply heated water	a large (usually metallic) vessel for holding gases or liquids