Sizing a return or filter grille by face velocity

Grilles are sized by face velocity, not the duct friction method: pick the fpm limit for the type (400 for a filter grille), then solve for an opening that keeps the air that slow. 800 CFM → a 20×20 grille.

A three-step flow. On the left, the inputs: 800 CFM through a filter grille at 400 fpm and 75 percent free area. In the middle, the sizing formula gross opening equals CFM divided by face velocity times 0.75 times 144, giving about 384 square inches. On the right, a 20 by 20 grille running about 384 fpm, at or under the limit. Below, a table of face-velocity limits: filter grille 400, return grille about 500, supply register about 600 fpm. — Source: ACCA Manual D grille face-velocity limits

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A three-step flow. On the left, the inputs: 800 CFM through a filter grille at 400 fpm and 75 percent free area. In the middle, the sizing formula gross opening equals CFM divided by face velocity times 0.75 times 144, giving about 384 square inches. On the right, a 20 by 20 grille running about 384 fpm, at or under the limit. Below, a table of face-velocity limits: filter grille 400, return grille about 500, supply register about 600 fpm.

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    <img src="https://remodelcalculators.com/api/figure-embed/return-grille-face-velocity" alt="A three-step flow. On the left, the inputs: 800 CFM through a filter grille at 400 fpm and 75 percent free area. In the middle, the sizing formula gross opening equals CFM divided by face velocity times 0.75 times 144, giving about 384 square inches. On the right, a 20 by 20 grille running about 384 fpm, at or under the limit. Below, a table of face-velocity limits: filter grille 400, return grille about 500, supply register about 600 fpm." width="800" height="440" style="width:100%;height:auto;border:0" loading="lazy">
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