Ductwork Design, Air Distribution, and Ventilation
Manual D, Pressure Budgets, and Outdoor Air
Duct design follows ACCA Manual D. The method begins with available static pressure: the blower's rated external static pressure minus the pressure drops of every device in the airstream, including the evaporator coil, filter, supply and return grilles, balancing dampers, and any electric heat section. Divide that remaining pressure by the total effective length of the longest supply and return path, which includes straight duct plus equivalent lengths for every fitting, then multiply by 100 to get the friction rate in inches of water column per 100 feet. The familiar 0.1 inch per 100 feet is a starting point only, not a rule; long, fitting-heavy systems need a lower friction rate and larger ducts.
Air distribution requires generous return capacity, low-velocity branches, and dampers at each branch for balancing. Flex duct must be fully extended, supported at manufacturer intervals, kept free of sharp bends and compression, and sealed with mastic rather than duct tape. Florida's energy code governs duct insulation R-value, sealing, and leakage testing for ducts in attics, where summer gains can be substantial. Ventilation is also in scope: ASHRAE 62.2 mechanical outdoor air for tight homes, energy or heat recovery ventilation in a humid climate, and local exhaust for kitchens and baths.
Worked example: available static pressure of 0.30 inch water column with a total effective length of 250 feet gives 0.30 times 100 divided by 250, or 0.12 inch water column per 100 feet.