HVAC Air Distribution
This domain tests how supply, return, and exhaust air is moved, sized, and delivered so that each space receives the design airflow without excessive pressure loss or noise. Expect questions on duct sizing methods, fitting and dynamic losses, fan/system interaction, terminal selection, and balancing.
Duct sizing and pressure loss
- Equal friction holds friction rate constant (often 0.08–0.15 in. wg per 100 ft) and is common for simple supply systems; it does not guarantee static regain at each branch.
- Static regain drops velocity progressively so regained static pressure offsets downstream friction, suiting long, high-velocity mains.
- Velocity reduction steps velocity down after each takeoff and is useful when noise or erosion limits govern.
- Total loss = straight-duct friction + fitting dynamic loss, where dynamic loss = C × velocity pressure. Velocity pressure is (V/4005)² in. wg for standard air.
Fans, terminals, and balance
Fan laws are exact only for geometrically similar fans at the same air density: flow varies with speed, pressure with speed squared, and power with speed cubed. Duct system effect-poor fan inlet/outlet connections-adds apparent resistance not shown on the bare fan curve. Terminal selection uses throw, drop, noise criterion, and ADPI; a high ADPI indicates acceptable draft and temperature uniformity in the occupied zone. Balancing dampers must have authority, and leakage class should match the specified duct pressure class.
Worked example
A 60 ft straight duct runs at 0.12 in. wg/100 ft. Its fittings total C = 0.8 at velocity pressure 0.25 in. wg. Friction loss = 0.12 × 60/100 = 0.072 in. wg. Dynamic loss = 0.8 × 0.25 = 0.20 in. wg. Total = 0.272 in. wg. Adding this to every run gives the critical-path pressure, which sets fan static pressure; adding the longest run’s losses to the shortest is wrong.
Traps that cost marks
- Confusing velocity pressure with static pressure, or applying standard-air formulas at high altitude without density correction.
- Ignoring system effect and then blaming the fan curve.
- Sizing terminals from nominal cfm rather than actual primary airflow and required throw/ADPI.
- Treating equal-friction sizing as automatically quiet-check velocity and NC at the terminal.
- Omitting fitting losses or double-counting the same elbow.