Duct System Design Fundamentals and Load Considerations
The Duct System Design Fundamentals and Load Considerations domain tests whether you understand how a load calculation becomes a duct design. Expect questions on Manual J loads, Manual D friction-rate sizing, available static pressure (ASP), total effective length (TEL), equivalent length of fittings, velocity limits, and pressure classification.
Core rules and concepts
- Size the equipment and ducts from a room-by-room load calculation, not from rules of thumb.
- Manual D uses friction rate = ASP ÷ TEL, expressed in inches of water column per 100 ft. ASP is the blower's external static pressure minus the pressure drops of the filter, coil, grilles, dampers, and other components.
- TEL is the longest supply path plus the longest return path, including equivalent lengths for fittings, takeoffs, and flex bends.
- Select duct size from a friction chart at the design cfm; check velocity and available space. Keep returns adequately sized.
- Duct pressure class must match the system: low-pressure residential ducts commonly use 1 in. wc or lower classes, while higher-pressure systems need heavier construction.
Worked example
A blower has 0.50 in. wc external static. The filter drops 0.08, the coil 0.15, and grilles and dampers 0.07, totaling 0.30 in. wc. ASP is 0.20 in. wc. If the longest supply-plus-return TEL is 100 ft, the friction rate is 0.20 ÷ 100 × 100 = 0.20 in. wc per 100 ft. At 400 cfm, the friction chart is used to pick a duct size that does not exceed that rate.
Traps that cost marks
Using total external static pressure as ASP, omitting the filter or coil, underestimating TEL by ignoring fittings, and applying the trunk friction rate to a long branch. Oversizing flex duct to hide poor routing also increases equivalent length and can worsen airflow.