Air Distribution, Ventilation and Indoor Air Quality
Air carries the load
Sensible capacity moves with CFM = sensible BTUH / (1.08 x delta-T) for standard air. The constant 4.5 is used for total heat, and 0.68 appears in some altitude-adjusted sensible charts. Nominal 400 CFM per ton is a starting point, not a law. A favorite exam trap is feeding a total-capacity number into the 1.08 sensible formula; the result looks plausible but is wrong.
Static pressure and duct design
Total external static pressure is measured across the blower with the filter, coil, and ductwork in place. It includes the filter and coil drops; every accessory adds to it. Duct sizing uses a friction rate, typically 0.1 in. w.c. per 100 ft of equivalent length, with velocity limits to control noise: roughly 700-900 fpm in trunks and 500-700 fpm in branches. Equivalent length converts fittings and flex runs into straight-duct feet. Flex duct loses capacity when sagging, kinked, or compressed.
Ventilation and IAQ
ASHRAE 62.2 sets whole-building ventilation rates for dwellings, and exhaust fans must actually move air, so measure rather than assume. Filters are rated by MERV, and higher MERV means higher pressure drop, which can starve a PSC blower. Source control, dilution, and filtration are the three strategies; humidity control is part of IAQ.
Traps
Wrong delta-T (return air versus entering coil), static pressure read at the wrong tap, ignoring filter drop, treating a balancing damper as a cure for undersized ducts, and forgetting that a tightly sealed house needs makeup air.
Worked example
A 36,000 BTUH sensible load at a 20-degree delta-T needs 36,000 / (1.08 x 20) = 1,667 CFM.