Duct comparison
Low velocity vs high velocity duct systems
Velocity choice is a system tradeoff: the same CFM can use more or less duct area, changing pressure loss, sound, routing, terminals, and the required fan performance.
Key differences
| Factor | Lower velocity approach | Higher velocity approach |
|---|---|---|
| Duct area | Usually needs more cross-sectional area for the same CFM. | Usually uses less area for the same CFM. |
| Pressure path | Can reduce velocity-related loss but still needs complete fitting/component checks. | Can increase straight-run and fitting loss, requiring pressure-budget review. |
| Routing | May be harder to fit in constrained spaces. | May be easier to route where physical area is limited. |
| Sound and terminals | Still depends on terminals, fittings, construction, and fan. | Needs careful acoustic and terminal review. |
| Final selection | Use assigned airflow, actual geometry, equipment, total resistance, acoustics, construction, and project requirements. | |
Example: the area tradeoff
At 400 CFM, 0.50 ft² produces 800 FPM and 1.00 ft² produces 400 FPM. The calculation illustrates the area relationship only. It does not establish the appropriate velocity for a particular main, branch, return, exhaust path, or terminal.
Choose from the system, not a label
- Confirm the required CFM and each air-stream purpose.
- Calculate actual area and velocity for the feasible route.
- Check straight duct, fittings, terminals, filters, coils, and fan performance.
- Review sound, balancing, room distribution, installation, and local requirements.
FAQ
Is high velocity automatically noisy?
No. Sound also depends on fittings, terminals, attenuation, construction, fan selection, and the overall design.
Is low velocity always more efficient?
No. Duct size, fittings, route, fan operating point, leakage control, and construction all affect the result. Compare the complete installed system.