PVC, CPVC, and FRP all replace metal duct in corrosive service, but they are not interchangeable. The gap between them shows up in the field — in service temperature, chemical resistance, weight, and installed cost.
Getting the material right early prevents the two most expensive mistakes: over-specifying an exotic material where a simpler one would last, or under-specifying and watching a duct run soften, sag, or corrode years ahead of schedule. The decision almost always comes down to temperature, chemistry, size, and code — weighed together.
Start with operating temperature
Temperature is the clearest dividing line. Standard PVC handles continuous service to about 140°F (60°C); CPVC’s extra chlorination pushes that to roughly 200°F (93°C). FRP (fiberglass-reinforced plastic) can be engineered for still higher temperatures depending on the resin system. For a deeper look at the thermoplastic ceilings, see our guide on PVC duct temperature limits.
| PROPERTY | PVC | CPVC | FRP |
|---|---|---|---|
| Max continuous temp. | ~140°F | ~200°F | Resin-dependent, often higher |
| Corrosion resistance | Excellent | Excellent | Excellent (resin-matched) |
| Relative weight | Light | Light | Heavier |
| Large diameters | Good | Good | Best |
| Relative cost | Lowest | Moderate | Highest |
Chemical compatibility and concentration
All three resist a broad range of acids, bases, and salts, but compatibility is concentration- and temperature-dependent. A chemical PVC shrugs off at ambient temperature can turn aggressive when hot or concentrated — exactly where CPVC or a matched FRP resin earns its place.
- Dilute acids, bases, salt-laden air near ambient temperature — classic PVC territory.
- Hot or concentrated acids and oxidizers — favor CPVC, which holds properties as the stream heats.
- Very large ducts, high pressure, or extreme temperature — where FRP’s engineered strength wins.
Design to the worst case.
Rated temperatures assume continuous duty. A brief spike during a process upset or steam clean can still deform thermoplastic. Size the material to the peak the duct will actually see.
Weight, cost, and fabrication
PVC and CPVC are light, easy to hang, and fast to fabricate with hot-gas or extrusion welding — which keeps installed cost down and joints leak-resistant. FRP is stronger and better for very large diameters, but it is heavier, needs more robust supports, and is fabricated differently. Fittings such as 45° elbows, reducers, and blast gates are readily available in PVC and CPVC to match the duct.
A practical selection sequence
- Define the worst-case temperature — continuous and peak, including cleaning cycles.
- List the exhaust constituents with concentrations, and check the most aggressive against resistance data at temperature.
- Confirm size and pressure — large diameters or high loads may point to FRP.
- Check code and insurer requirements for the installation location.
- Right-size the material — PVC where it comfortably meets every requirement, CPVC or FRP where temperature, chemistry, or strength demand it.
When a stream sits near a threshold — or mixes hot and aggressive constituents — that’s the case worth reviewing with a fabricator before the submittal is locked. See also our framework for selecting corrosion-resistant duct.
