Hiring an HVAC Contractor to Replace Ductwork: Quotes and Red Flags

The number of supply runs and returns with their sizes; total linear feet; the material (flex, duct board or sheet metal) and insulation R-value; whether the supply and return plenums are replaced or reused; removal and disposal of the old ducts; mastic sealing of every joint; a duct-leakage test with the target number; new boots and grilles or reuse; the permit and inspection; any drywall repair; and a workmanship warranty. If rooms have been uncomfortable, it should also say whether a Manual J/D calculation was done. Get three quotes on the same scope so they compare.

A licensed HVAC (mechanical) contractor. Ductwork is part of the mechanical system, most jurisdictions require a mechanical permit for a whole-house replacement, and a few restrict the work to licensed trades. A handyman can't pull the permit, won't do a leakage test, and typically won't size anything. For sheet metal systems, some HVAC contractors sub the trunk fabrication to a sheet metal shop — that's normal, but the HVAC contractor should hold the contract.

One day for a 1,500 sq ft house with flex ducts and a crew of two or three; one to two days on a crawl space; two to four days for a sheet metal trunk system or ducts in finished walls. Add the inspection, which can be same-day or a week out. Your system is off during the work, so book it for mild weather.

Spring or fall — contractors are less booked, attics and crawl spaces are bearable, and you're not without heating or cooling on the worst day of the year. If you're replacing the furnace, AC or heat pump within a few years, do the ducts at the same time: the crew, permit and shutdown are already paid for, and contractors discount the combination 15–30%.

For a whole-house replacement, almost always: a mechanical permit, an inspection, and in many jurisdictions a duct-leakage test at final. Replacing a few runs often doesn't need one, but ask. The contractor pulls the permit and the fee ($50–$400) should be a line in the quote. A contractor who suggests skipping it is telling you they don't plan to be inspected.

A duct blaster — a calibrated fan sealed to the system — pressurizes the ducts and measures how much air escapes, reported as CFM per 100 square feet of floor area or as a percentage of system airflow. New work should test at or below the local code number (often 4 CFM25 per 100 sq ft to outside). Ask for it whether or not code requires it; it's the only objective check that the sealing was done, and it's $150–$400 on its own.

A per-foot price with no count of runs or registers; no mention of sealing or a leakage test; copying the old layout when you've told them rooms don't heat or cool; flex duct on a two-story trunk system where metal belongs; cloth duct tape in the truck; 'no permit needed' for a whole house; and a demand for full payment before the inspection. Also watch for the opposite pressure — a contractor pushing full replacement when a leakage test would show sealing is enough.

One to five years on workmanship — connections, supports, sealing, and airflow to every register — plus the manufacturer's warranty on the duct itself (often 10 years on flex, longer on metal). It should say the system passed its leakage test at the number in the quote. Don't make the final payment until the forms are gone, the inspector has signed, and you've felt air at every register.