Air velocity limits by duct role

A duct can be big enough for the CFM yet still run the air too fast. Branches and returns stay under ~700 fpm, trunks under ~900; past that, ducts get noisy.

A velocity scale from 0 to 1,200 feet per minute. A green band up to 700 fpm is the quiet range for branches and returns, an amber band from 700 to 900 fpm is for main trunks only, and a red band above 900 fpm is where ducts whistle and rumble. Dashed markers sit at 700 (branch/return) and 900 (trunk). A worked example marks 1,200 CFM in a 16-inch trunk at about 860 fpm, inside the trunk band. — Source: ACCA Manual D velocity guidance

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A velocity scale from 0 to 1,200 feet per minute. A green band up to 700 fpm is the quiet range for branches and returns, an amber band from 700 to 900 fpm is for main trunks only, and a red band above 900 fpm is where ducts whistle and rumble. Dashed markers sit at 700 (branch/return) and 900 (trunk). A worked example marks 1,200 CFM in a 16-inch trunk at about 860 fpm, inside the trunk band.

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