boiler vs vessel

boiler and vessel both are nouns.

Word	Noun	Adjective	Verb	Adverb
boiler	Yes	No	No	No
vessel	Yes	No	No	No

As nouns, vessel is a hypernym of boiler; that is, vessel is a word with a broader meaning than boiler:

boiler (noun)	vessel (noun)
sealed vessel where water is converted to steam	a craft designed for water transportation
a metal pot for stewing or boiling; usually has a lid	an object used as a container (especially for liquids)
	a tube in which a body fluid circulates