A plenum box is a small, sheet-metal box connected between the duct and the diffuser or grille. It serves to distribute air evenly into the room. Instead of blowing air directly from the duct, the plenum slows it down and spreads it smoothly.
You will see plenum boxes behind ceiling diffusers, grilles, slot diffusers, and linear diffusers. Without a plenum, the air can be noisy and uneven, which reduces comfort.
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