Duct Design (Manual D)

Manual D begins where Manual S ends. The room-by-room CFM requirements from Manual J and the available static pressure from your selected equipment are automatically imported — no manual re-entry.

Choose a duct layout topology: trunk and branch (most common residential), radial (from a central plenum), or perimeter loop. Then define the trunk route and branch takeoffs to each room.

The equal friction method is the default for residential systems. It sizes all ducts to the same friction rate (in. w.g. per 100 ft equivalent length), calculated from the available static pressure divided by the total equivalent length of the longest duct run.

The velocity reduction method is available for noise-sensitive applications. It starts with higher velocity in the trunk and reduces velocity as branches take off.

The interactive static pressure budget tracks every component: equipment total external static pressure (from Manual S), minus filter pressure drop, minus coil pressure drop, minus register and grille losses, equals available pressure for ductwork.

If the duct layout exceeds available static pressure, you'll see a warning with suggestions: increase trunk size, reduce longest run length, use smoother fittings, or select equipment with higher static pressure capability.

The duct schedule lists every section with its identifier, size (round equivalent or rectangular dimensions), length, CFM, air velocity (FPM), friction rate, and cumulative static pressure. Sections exceeding recommended velocities (900 FPM trunk, 700 FPM branch, 600 FPM runout) are flagged for noise risk.

Export the duct schedule as part of the Manual D PDF report or as a standalone CSV for integration with other tools.