The equal-friction duct sizing method

Equal-friction sizing holds one friction rate (0.08 in./100 ft is the residential default) across the whole system and solves for the duct that carries the flow: 1,200 CFM → 16″ round at ~860 fpm.

A three-step flow. On the left, the inputs: 1,200 CFM of airflow and a 0.08 in.w.c. per 100 ft friction rate. In the middle, the ASHRAE friction equation d equals the fifth-root-ish of 0.109 times Q to the 1.9 over the friction rate, and velocity equals 183.4 times Q over diameter squared. On the right, the result: a 16-inch round duct (or its rectangular equal-area equivalent) carrying the air at about 860 feet per minute, under the 900 fpm trunk limit. — Source: ASHRAE/Huebscher friction equation; ACCA Manual D

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A three-step flow. On the left, the inputs: 1,200 CFM of airflow and a 0.08 in.w.c. per 100 ft friction rate. In the middle, the ASHRAE friction equation d equals the fifth-root-ish of 0.109 times Q to the 1.9 over the friction rate, and velocity equals 183.4 times Q over diameter squared. On the right, the result: a 16-inch round duct (or its rectangular equal-area equivalent) carrying the air at about 860 feet per minute, under the 900 fpm trunk limit.

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