Duct comparison
Flexible duct vs rigid duct
Both can be part of an HVAC air-distribution system. The actual route, support, bends, connections, and installation instructions determine how the air path performs.
Key differences
| Factor | Flexible duct | Rigid duct |
|---|---|---|
| Routing | Can help make limited connections and route changes when installed correctly. | Uses fittings and transitions for route changes. |
| Air path | Must be stretched, supported, and routed according to manufacturer instructions. | Requires verified joints, fittings, surface condition, and transitions. |
| Resistance check | Do not assume straight rigid-duct performance. | Still needs fitting and component losses beyond straight length. |
| Installation review | Check compression, sag, bends, support, and connection quality. | Check supports, sealing, insulation, joints, and access. |
| Selection | Use the documented system design, manufacturer requirements, and the complete airflow path. | |
Use-case decision
Choose the material and route that can be installed and verified without creating unsupported bends, compression, excessive local losses, leakage, or service problems. The decision should be checked against the system’s required CFM, total pressure budget, acoustic criteria, construction details, and local requirements.
What not to assume
- Do not assume a nominal diameter guarantees the same pressure loss across materials or installations.
- Do not use a short geometry calculation as proof of final fan performance.
- Do not treat flexible duct routing as an exemption from support, bend, length, or manufacturer instructions.
FAQ
Can flexible and rigid duct use one friction assumption?
Not without confirming material, installation, length, support, bends, and the selected design data.
Is rigid duct always the lower-resistance path?
Actual route, fittings, joints, transitions, size, and construction determine the pressure result. Compare the complete installed path.