Ductwork, Air Distribution, and Ventilation
Static pressure and resistance
The blower moves air because it creates a pressure difference. Total external static pressure (TESP) is measured across the equipment - return static plus supply static - and is compared to the blower performance table. Excess static means low airflow, high amp draw, noise, and poor capacity. Friction losses come from duct length, fittings, and velocity; use equivalent length (TEL) to combine them, and remember that flex duct runout, sagging, and sharp turns multiply resistance.
Sizing methods
Equal-friction (about 0.1 in. w.c. per 100 ft), velocity-reduction, and static-regain methods are the standard approaches. Registers, grilles, dampers, and balancing devices must be selected for the required CFM at an acceptable velocity and throw. Trap: a large return grille with a restrictive filter grille can starve a system just as effectively as an undersized supply trunk.
Ventilation and indoor air quality
Know ASHRAE 62.2 mechanical ventilation requirements, HRV and ERV operation (ERV also transfers moisture), MERV ratings versus pressure drop, and combustion air requirements for atmospherically vented appliances. Seal ducts in unconditioned space, and never use building cavities as return ducts.
Worked example
Return static is −0.25 in. w.c., supply static is +0.30 in. w.c. TESP = 0.55 in. w.c. If the blower table shows 1,200 CFM at 0.50 and 1,100 CFM at 0.60, the system is delivering roughly 1,150 CFM - below nominal for a 3-ton unit, so duct corrections come before any charge adjustment.