auxiliary boiler vs boiler

auxiliary boiler and boiler both are nouns.

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

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

auxiliary boiler: (nautical) an extra boiler (as a ship's boiler that is used while the ship is in port)
boiler: sealed vessel where water is converted to steam

Other hypernyms of auxiliary boiler include steam boiler.

auxiliary boiler (noun)	boiler (noun)
(nautical) an extra boiler (as a ship's boiler that is used while the ship is in port)	sealed vessel where water is converted to steam
	a metal pot for stewing or boiling; usually has a lid