For return air grilles, fixed grilles are optimal as the direction of the air does not matter. For supply grilles, however, you'll want an adjustable grille so that you can spread the air evenly throughout the room.
HVAC Premium Hack: If you have your adjustable grille facing the upper side of the room in the summer, the cold air will fall down naturally. While in winter, you can face the grille down so the heat can rise naturally. This is more cost-effective as the HVAC system is able to circulate the air more evenly, with less effort.
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