Air Distribution System Components and Terminology
The Components and Terminology domain checks whether you can identify every element of a forced-air distribution system and use pressure and airflow terms correctly. Questions often ask you to distinguish a trunk from a runout, a register from a diffuser, and static from velocity pressure.
Supply and return path
A typical system moves air from the blower into a supply plenum, then a trunk duct, branch ducts or runouts, a boot, and a terminal device. Return air enters a return grille or filter grille, travels through return duct or a panned joist/plenum, and returns to the blower. Fittings include wyes, tees, elbows, transitions, and takeoffs. Dampers are not interchangeable: volume dampers balance airflow, fire dampers protect rated assemblies, smoke dampers control smoke, backdraft dampers prevent reverse flow, and barometric dampers relieve pressure.
Pressure vocabulary
CFM is volumetric flow; FPM is velocity. Static pressure acts against duct walls; velocity pressure exists only because air is moving; total pressure is static plus velocity pressure. Equivalent length expresses a fitting's resistance as straight duct. Aspect ratio is width divided by height. Free area and face velocity describe grille performance; throw, drop, and spread describe diffuser air patterns; NC describes generated noise.
Worked example
A return plenum reads -0.18 in. wc and a supply plenum reads +0.22 in. wc. Total external static pressure is 0.22 - (-0.18) = 0.40 in. wc. If the blower is rated at 0.50 in. wc ESP, 0.10 in. wc remains for added filter or duct resistance. The signs matter: a below-atmospheric return reading is negative, so subtract it.
Traps
Do not call velocity pressure static pressure or read a pitot tube as if it were a static probe. Do not subtract the return reading from the supply reading without handling signs. Do not confuse a fire damper with a volume damper, or a register (has a damper) with a diffuser.