Duct Sizing

Notes

• **12″ is the smallest stock size that carries 400 CFM at 600 fpm.** The arithmetic gives 11.1″; ducts come in whole inches, so the real answer rounds up and the real velocity drops to 509 fpm. Rounding *down* to save a size is the most common way a system ends up noisy.
• **Velocity is a trade, not a target to minimise.** Slower air means bigger duct, more sheet metal and more ceiling given up. Faster air means smaller duct and a system you can hear from the sofa. At 509 fpm this run sits in the quiet band.
• **Flex is not rigid with a different label.** The corrugated inner wall raises friction loss substantially over smooth metal of the same diameter, and flex that is not pulled tight — the normal condition in a real attic — is worse again. If this run ends up in flex, size it as flex, not as metal.
• **The duct is half the answer; total static pressure is the other half.** Every elbow, boot and takeoff adds equivalent length, and a system can have every duct correctly sized and still fail because the total run is too long for the blower. Size the ducts here, then measure static pressure at the air handler.
• Rectangular equivalents are matched on area at 8″ deep. Flatter ducts of the same area carry less air than round: the more the shape departs from a circle, the more wall there is per unit of flow, and friction follows wall.