TradesHow this works
The equal-friction method sizes a duct so the air loses pressure at a steady rate along the run. You pick that rate, the tool works out the round duct that matches it for your airflow, and it shows how fast the air is moving. Fast air is loud and hard on the blower, so the tool also shows a quieter size up.
This sizes one duct at a friction rate. It is a way to reason about a run and check a number, not a substitute for a full Manual D design that looks at the whole system.
Common questions
What friction rate should I use?
For a typical residential system a friction rate around 0.08 to 0.10 inches of water per 100 feet is common. A lower rate gives bigger, quieter ducts. A higher rate gives smaller, faster, louder ones.
Why does velocity matter?
Air moving too fast is noisy and adds static pressure that the blower has to fight. A branch run is usually kept under about 900 feet per minute and a trunk under about 1200. The tool flags a size that pushes past that and offers a quieter size up.
Is this a Manual D?
No. This sizes a single duct at a friction rate. A full Manual D design looks at the whole system, every fitting, and the blower curve. Use this to reason about a run, then confirm the design.
Sources: Equal-friction method, standard galvanized-duct friction fit (planning) · ACCA Manual D is the authoritative duct-design method (not implemented here). Engine version 1.0.0. Risk tier 2.