Air Distribution, Duct Design, and Indoor Air Quality
This domain tests duct sizing, static pressure, airflow, ventilation, and indoor air quality. Expect Manual D concepts, equivalent length, and pressure budget questions.
Duct design
Available static pressure is what remains after filters, coils, grilles, and registers. Friction rate = available static × 100 ÷ total equivalent length. Use equivalent length for fittings, not just measured length. Size trunks and branches, then verify with a duct calculator. Return air must be sized like supply air.
Worked example
External static pressure is 0.50 in. w.c. Deduct filter 0.10, coil 0.20, and grilles/registers 0.10; available static is 0.10 in. w.c. Total equivalent length is 200 ft. Friction rate = 0.10 × 100 ÷ 200 = 0.05 in. w.c. per 100 ft. Use that rate on the duct calculator.
IAQ and ventilation
ASHRAE 62.2 addresses residential ventilation; exhaust bath and kitchen fans to the outdoors. Filtration MERV 8–13 is common; higher MERV increases pressure drop. Keep indoor humidity roughly 30–50%. Combustion appliances need adequate combustion and dilution air, and return-air designs must not depressurize a combustion zone.
What costs marks
- Using 0.10 in. w.c. as a universal friction rate.
- Ignoring equivalent length and fitting losses.
- Compressing flex duct or using sharp turns.
- Undersized returns causing high static and low airflow.
- Adding a high-MERV filter without checking static pressure.