Ductwork, Piping, and System Components
What this domain covers
This domain tests duct sizing, equivalent length, static pressure, refrigerant piping, evacuation, condensate management, and the function of major system components. Expect calculation questions on friction rate and practical questions on why a system loses capacity.
Core rules
- Duct sizing. Manual D is the residential duct design method. A common starting point is 0.1 in. w.c. friction per 100 ft of effective length; supply velocities are often 700–900 fpm and returns 600–700 fpm.
- Available static pressure. Total external static pressure (often 0.5 in. w.c. for a residential air handler) must cover the filter, coil, grilles, and duct run. Subtract device pressure drops before calculating duct friction rate.
- Equivalent length. Fittings, boots, and turns add resistance. Use total effective length (TEL), not tape-measure length.
- Piping and evacuation. Refrigerant lines are sized for capacity and oil return. Braze with nitrogen purge to prevent oxidation; evacuate to 500 microns and hold to confirm no leak or moisture.
- Condensate. Slope drain lines at least 1/8 in. per ft, install a trap when required, and provide a secondary drain or pan with a float switch.
Worked example
Available static pressure is 0.50 in. w.c. The filter, coil, and grilles account for 0.15 in. w.c., leaving 0.35 in. w.c. for the duct system. The longest run has a total effective length of 300 ft. Friction rate = 0.35 × 100 ÷ 300 = 0.117 in. w.c. per 100 ft. Use that rate with the duct calculator; using 0.1 in. w.c. per 100 ft without subtracting device drops undersizes the duct and raises static pressure.
Traps that cost marks
- Using measured length instead of equivalent length.
- Forgetting filter and coil pressure drop in available static.
- Compressing flex duct or leaving sharp bends, which raises resistance.
- Brazing without nitrogen purge, leaving internal oxide.
- Omitting a condensate trap or secondary drain protection.