Errors that change the system result
| Mistake | Why it fails | Better check |
|---|---|---|
| Starting with duct size instead of airflow | Area is meaningless without assigned CFM for the run. | Establish room and branch airflow before selecting geometry. |
| Using floor area instead of room volume or load | Ceiling height, solar, internal gains, and airflow purpose are omitted. | Use a documented airflow or load basis. |
| Using one velocity everywhere | Mains, branches, terminals, route constraints, and acoustic goals differ. | Check actual velocity by run and the system pressure budget. |
| Ignoring fittings | Elbows, takeoffs, transitions, dampers, and terminals add local losses. | Include fitting and component losses in the complete path. |
| Calling straight-run loss total static pressure | Filters, coils, grilles, equipment, and other components are missing. | Assemble the entire system resistance before fan selection. |
| Applying a generic friction factor | Material, air properties, geometry, and method are not stated. | Record the data and assumptions used for the friction calculation. |
Example: area is only the first step
At 400 CFM and 800 FPM, the preliminary area is 0.50 ft². Selecting a real duct is not the end: calculate its actual area and velocity, then include the straight length, fittings, terminals, filters, and available fan pressure. A number that fits the ceiling can still fail the pressure or sound check.
Review before changing a duct
- Confirm the required CFM and the air-stream role.
- Confirm the selected geometry and actual velocity.
- Check pressure losses through the entire critical path.
- Check fan performance, balancing, noise, and distribution.
- Document assumptions for qualified design review.
FAQ
Can a calculator replace final HVAC duct design?
No. A calculator supports transparent preliminary checks; final design needs the complete system, equipment, applicable requirements, and qualified review.