The ventilator grille is first mentioned a few pages into the story as Montag returns home from work, "He stood looking up at the ventilator grille in the hall and suddenly remembered that something lay hidden behind the grille." This is the vented grille in the ceiling that allows the heat and air conditioning into the room. The grille is important to the story because it's behind the grille that Montag has hidden books he's stolen from houses where the he and the other firemen burned books. The grille is mentioned several more times before Montag actually retrieves books from there. It becomes a symbol for the forbidden books.
In Fahrenheit 451, the ventilator is an integral part of the heating system inside Montag and Mildred's house. Besides this simple purpose, however, the ventilator has particular significance because it symbolises Montag's inner rebellion. It is the place where he hides the books that he has stolen from other people's houses, for example, and the fact that he keeps it a secret (even from his wife) shows that it represents his repressed desire to rebel.
Throughout Part One, Bradbury depicts the ventilator as a visual reminder of Montag's inner desire to rebel. He is constantly reminded of its contents and it pricks at his conscience:
He stood looking up at the ventilator grille in the hall and suddenly remembered that something lay hidden behind the grille, something that seemed to peer down at him now.
Eventually, Montag can stand it no longer and he reveals the hidden collection of books to Mildred. This is a turning point in the novel because Montag can no longer ignore his inner voice and sense of conscience.
The ventilator grille is part of the house's heating/cooling system. But in the context of the story, the association is mostly with cooling. The vent becomes a literal and symbolic image of a safe haven for forbidden books. The association with books and cooling is significant as well. There is an opposition here; heat is associated with destruction, and cooling with preservation. Fire destroys the books, but they are saved in a cool space. In his conversation with Beatty, Montag underscores this association of coolness and books (as opposed to fire and destruction):
But in his mind a cool wind started up and blew out of the ventilator grille at home softly, softly chilling his face. And again he saw himself in a green park talking to an old man, a very old man, and the wind from the park was cold too.
For Montag, he even associates his encounters with Faber (the old man) with cold and life (the "green" park), again opposing the destructive force of fire. It is in this conversation with Beatty (and other firemen) that Montag asks if firemen used to put out fires rather than start them. This shows more symbolic associations of saving books by opposing fire: the cooling vent, the cold air, life (the green plants in the park), and water to put out fires.