Air Distribution, Ductwork, and Ventilation
Static pressure is the budget
A duct system is a budget problem. Start with the blower's rated external static pressure, subtract every component drop (filter, coil, dampers, heat strips, grilles), and the remainder is available static pressure for the ductwork. Divide that by the total effective length to get the friction rate (inches w.c. per 100 ft) used to size each run. Long equivalent lengths from fittings and flex duct consume the budget quickly - a compressed flex run can lose a large fraction of its capacity.
Design methods include equal friction, velocity reduction, and static regain. Supply trunks commonly run 700–900 fpm and returns lower; excessive velocity produces noise (check the NC rating) and pressure loss. Balancing dampers, turning vanes, and generous return paths prevent rooms from being starved or pressurized.
Ventilation is separate from distribution. Whole-house ventilation standards set a target outdoor airflow based on bedrooms, floor area, and occupancy; exhaust-only, supply, and balanced systems (HRV/ERV) each have trade-offs. Filtration is a pressure-drop decision as much as a health one: higher MERV captures more but costs static pressure and blower energy.
Traps: adding fittings without recalculating equivalent length; verifying airflow by feel; ignoring return-air adequacy; neglecting duct leakage to unconditioned space; causing depressurization that backdrafts combustion appliances.
Rules of thumb: about 400 CFM per ton for cooling, 0.1 in. w.c. per 100 ft as a starting friction rate, and never accept a design without a pressure-budget check.