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Load-calc confusion · no-solution · stock sizes · velocity flags
Fix common ductulator mistakes: treating this as a load calc, rectangular no-solution when the fixed side is too large, rounding up between stock sizes, and clearing red velocity flags.
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Most “errors” are scope misunderstandings or geometry limits — not broken math.
Open Duct Sizing Calculator →Confusing This With a Load Calculator
If you came here expecting it to tell you how many CFM a bedroom needs — that is Manual J territory. This tool only answers: given this CFM and friction rate, what duct size? Entering a guess CFM still produces a number; that number is only as good as the airflow you fed it. Pull CFM from a load calc, equipment schedule, or Manual D worksheet first.
Rectangular Solver Says “No Solution”
That message means the fixed side you entered is already larger than the required equivalent round diameter. Huebscher’s equation cannot invent a second side that makes a fat rectangle behave like a smaller round duct — the fixed dimension alone already exceeds capacity for that CFM/friction. What to do: reduce the fixed side (shallower joist bay insert, different chase), switch to round if clearance allows, lower the CFM on that run (split the branch), or accept a higher friction rate only if the fan and noise budget allow it.
Result Falls Between Stock Sizes
Exact calc often lands on 9.8", 11.3", etc. Suppliers stock 10", 12", … The tool rounds UP to the next standard galvanized size so you never undersize airflow. Undersizing raises velocity and static; oversizing a half-inch is cheap insurance. Order the suggested standard size (or the next larger if your house stock skips that increment).
Velocity Flag Shows Red / Warning
Velocity at the suggested stock size exceeds the guideline for the application you selected. Fix it by: (1) upsizing to the next larger stock duct (more area → lower FPM), or (2) lowering the friction rate (forces a larger required diameter before stock rounding). Either change drops velocity; both trade duct size for quieter air. Do not ignore a red flag on a bedroom branch — that is how you get whistle and complaint calls.
Still Stuck?
Run the built-in self-tests on the tool page. Re-read the guide for friction-rate context, or Round vs Rectangular if shape choice is the real issue.
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