boiler vs donkey boiler

boiler and donkey boiler both are nouns.

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

As nouns, donkey boiler is a hyponym of boiler; that is, donkey boiler is a word with a more specific, narrower meaning than boiler:

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

Other hyponyms of boiler include auxiliary boiler, fluidized bed boiler.

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