Guides
Rust in Ductwork Replacement Cost: Repair Options to Compare
Rust in ductwork replacement cost is often a hidden line item. Here are the failures that stay invisible for a season and what they cost to fix.
What to take away
- Rust in ductwork replacement cost runs from about $450 for patching a short accessible run to $9,000 or more when a corroded trunk line and plenum come out of a Florida or Louisiana attic.
- The expensive mistakes are the invisible onespinhole leaks under insulation, condensate pooling in a low run, and coil-adjacent corrosion that eats metal for two or three cooling seasons before anyone smells it.
- Repair buys time when corrosion is under roughly 20 percent of a run's surface and the metal still holds a screw. Replace when the plenum, the first ten feet, or the pan is involved.
- A $150 to $400 static pressure and leakage test before you accept a bid tells you more than any visual inspection.
The failure that runs for three summers
A homeowner in Tampa gets a quote for a full duct replacement because the technician smelled a musty odor. Nobody cut the insulation open. Under that jacket, galvanized steel had been sweating for three cooling seasons.
The mechanism is ordinary. Cold supply air meets warm humid attic air, the vapor barrier fails at a seam, and condensation sits against bare metal. Corrosion needs only moisture, oxygen and a bare surface, and an attic in July supplies all three.
The consequence arrives slowly. Pinholes weep into the insulation, which stays wet, which feeds more corrosion. By the time a contractor opens the jacket, the run is scrap and the repair quote was never real. Prevention is a $150 to $400 leakage test and a moisture reading at the plenum before signing anything.
The ones that look fine at first
A surface bloom of white or brown powder on galvanized steel is often cosmetic. It sits on the outer skin, wipes off, and the metal underneath is sound. Contractors call it white rust, and it is common in coastal Louisiana where salt air reaches outdoor units and returns.
Two other cases look minor and are not.
- Rust at the plenum collar, where the trunk meets the air handler. This is a wet zone, and a $300 patch there often fails within a year.
- Rust along the bottom seam of a horizontal run in a crawlspace. Water pools in the seam, and the metal thins from the inside.
- Rust on flex duct collars and takeoffs. The collar corrodes, the boot loosens, and the leak is invisible behind drywall.
A visual inspection catches all three only if someone removes insulation at the plenum and the lowest point of each run. Most bids do not include that step.
Example: repair versus replace in a Louisiana attic
Repair versus replace in a Louisiana attic
Illustrative cost
- Patch one accessible run
- $450 to $900
- Seal and re-insulate a trunk
- $1,200 to $2,500
- Replace plenum and first ten feet
- $2,800 to $5,500
- Replace full trunk and branches
- $6,000 to $9,000+
Typical trigger
- Patch one accessible run
- Surface rust, under 20 percent of surface
- Seal and re-insulate a trunk
- Pinholes, moisture barrier failure
- Replace plenum and first ten feet
- Coil-adjacent corrosion, wet pan
- Replace full trunk and branches
- Widespread rust, failed leakage test
These figures are illustrative ranges quoted by HVAC companies in Florida and Louisiana, not a national average. Labor is the swing factor. BLS data on HVAC mechanics and installers shows wage variation across states that shows up directly in these bids.
The ones that only show up later
A homeowner accepts a repair bid in October. The system runs fine through a mild winter. The failure surfaces the following August, when the patch lifts, the leak returns, and the attic smells again.
This is the seasonal delay that makes rust expensive. Corrosion does not stop because the repair was cosmetic. If the moisture source, usually a pan or a coil, was never addressed, the new metal corrodes on the same schedule.
A second delayed failure is insulation. Wet insulation loses its rated value, and thermal insulation performance depends on staying dry. Homeowners pay twice: once for the failed repair, once for the energy lost in between.
Ask any contractor whether the bid includes fixing the water source. If the answer is no, the repair is a rental, not a fix.
What they have in common
Every expensive rust case shares one trait: nobody measured. The bid was written from a glance at the outside of the duct.
A leakage test, a moisture meter, and a look under the insulation at the plenum cost under $400 combined.
Skipping them turns a $600 repair into a $7,000 replacement. Contractors who document the test in the bid win more jobs. The quoting template with real numbers shows how to present that line item without scaring the customer.
Coastal and humid regions carry more of this risk. If your market mixes US and Canadian pricing assumptions, the US ductwork rates comparison explains why the numbers do not travel.
