Construction
HVAC Duct Sizing
From BTU/h or CFM, get the right duct size. Round + rectangular equivalents. Use this for run sizing; final Manual D needs the full picture.
FAQ
Why velocity matters? +
Above 900 fpm in residential = noisy. Below 600 fpm = quiet but oversized + expensive. Sweet spot: 700-800 fpm in trunks, 500-700 in branches.
Round vs rectangular? +
Round is more efficient (less surface area per CFM) but harder to fit between joists. Rectangular fits but loses ~15% efficiency vs round at same CFM. Use rectangular only when round won't fit.
What's friction rate? +
How much pressure your blower can deliver per 100 ft of duct. 0.08 is the residential default. Bigger = smaller (cheaper) duct but louder + needs higher static pressure blower.