Do I Have to Replace My Old Ducts When I Replace My AC or Furnace?
No. Title 24 does not require existing ducts to be replaced when the equipment is. It requires the connected ducts to be sealed and tested: 10 percent total leakage, 7 percent to outside, or a smoke test if neither can be met. Choosing new duct work is optional and brings stricter rules.
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If a contractor lists new ducts as a code item, ask which section says so. Section 150.2(b)1E speaks of sealing the duct system connected to the altered equipment, and the manual's Examples 9-35 and 9-43 call for testing the existing ducts, not removing them.
Path one: keep the ducts
The installer tests and an ECC-Rater verifies, passing at any of three levels: total leakage of 10 percent of air handler airflow, leakage to outside of 7 percent, or, when neither is reachable, every accessible leak sealed and confirmed by visual inspection and a smoke test. Skipping the test are ducts documented as already sealed and verified, systems under 40 linear feet by visual inspection, and asbestos ducts.
Path two: replace part of them
More than 25 feet of new or replacement duct invokes Section 150.2(b)1D. New ducts in unconditioned space take the Table 150.2-D insulation (R-6 in zones 3 and 5 to 7, R-8 in the others), and the altered system is sealed and tested wherever it sits. Example 9-39 applies this even to a duct replaced only for deteriorated insulation. The arithmetic is in the 25-foot rule page.
Path three: replace the whole system
A complete replacement duct system must be at least 75 percent new material; up to 25 percent can be reused registers, boots, plenums or duct if accessible and sealable. The leakage target tightens to 5 percent of air handler airflow.
A note on credit
In the performance method the standard design for duct sealing is the 150.2(b)1D and 1E requirement (Table 150.2-G), so sealing old ducts earns no extra credit.
Questions worth asking the contractor
- How many linear feet of duct will actually be new, and is that above or below 25?
- If the ducts fail, is smoke-test sealing included in the price?
Equipment-specific lists live in the furnace checklist and the air conditioner checklist. For the report, see residential Title 24 reports.
Common Title 24 Questions
- When is Title 24 required for a home?
- Does a remodel need Title 24?
- Does a room addition need Title 24?
- What plans are needed for a residential Title 24 report?
- How much does a residential Title 24 report cost?
- How long does a residential Title 24 report take?
Frequently Asked Questions
Yes, Exception 2 to Section 150.2(b)1E excuses duct systems with less than 40 linear feet as determined by visual inspection.
The installer seals every accessible leak, then the rater confirms it by visual inspection and a smoke test. Replacement is not the only fix the Standards recognize.
Explore Ducts
- Title 24 Duct Insulation Requirements
- Duct Leakage Testing: When Is It Required Under Title 24?
- When Is Duct Leakage Testing Required?
- Duct Replacement 25-Foot Rule Under Title 24
- Ducts in Soffits and Conditioned Space: Do They Need Insulation?
View all Ducts guides → · HVAC and Water Heating · Title 24 guide