ventilation shaft vs air duct

ventilation shaft and air duct both are nouns.

Word	Noun	Adjective	Verb	Adverb
ventilation shaft	Yes	No	No	No
air duct	Yes	No	No	No

As nouns, air duct is a hypernym of ventilation shaft; that is, air duct is a word with a broader meaning than ventilation shaft:

ventilation shaft: a shaft in a building; serves as an air passage for ventilation
air duct: a duct that provides ventilation (as in mines)

Other hypernyms of ventilation shaft include air passage, airway, shaft.

ventilation shaft (noun)	air duct (noun)
a shaft in a building; serves as an air passage for ventilation	a duct that provides ventilation (as in mines)