Ductwork and Air Distribution
Core concepts
Understand static pressure, velocity, friction loss, and equivalent length. Know the difference between return, supply, and transfer air, and how duct design affects fan performance. Be familiar with duct materials (galvanized, flex, duct board), sealing methods (mastic, tape, aerosol), insulation and vapor barriers, and support spacing. Understand the equal-friction and static-regain methods, and why Manual D uses a friction-rate target and available static pressure budget. Recognize the roles of dampers, turning vanes, and takeoffs in reducing turbulence and loss.
Traps in questions
- Flex duct installed with sharp bends, excess length, or compression dramatically increases resistance; questions often show a sagging run.
- Return air undersizing raises static pressure and reduces airflow even if supply ducts are correct.
- Duct leakage in unconditioned space wastes capacity and can cause pressure imbalances and backdrafting.
- Equivalent length of fittings often exceeds straight-run length; do not size on measured length alone.
- Vapor barrier placement matters in humid climates: the barrier goes on the warm side to prevent condensation.
Worked example / rule of thumb
A common residential target is about 0.1 in. w.c. friction rate per 100 ft of effective length, adjusted to the available static pressure. If the blower has 0.5 in. w.c. available and the coil, filter, and grilles consume 0.35 in. w.c., only 0.15 in. w.c. remains for the duct system - so the friction rate must be recalculated, not assumed.
Study guidance
Practice computing available static pressure by subtracting component losses from total external static pressure.