When Is Duct Leakage Testing Required?

New systems: 5 per cent

For new ducts forming an entirely new — or a complete replacement — duct system directly connected to the air handler, measured leakage must be 5 per cent or less of air handler airflow.

That is a demanding number, and it is measured rather than assumed. It is why duct sealing on a new system is a deliberate exercise with mastic and mesh at every joint, not a roll of tape at the end.

Alterations scale by how much you touch

Replacement work does not all get held to the new-system figure. The targets step with scope:

• 5 per cent where 75 per cent or more of the ducts are replaced
• 10 per cent total, or 7 per cent to outside, for extensions over 25 feet
• 6 per cent for ducts in garages

The logic is proportionality: replace almost the whole system and you are effectively building new, so you meet the new-system target. Extend an existing system and you are held to a realistic figure for a system you did not build. The tighter garage number reflects that leakage there draws on air you particularly do not want in the house.

The 25-foot trigger — alterations only

The 25-foot threshold belongs to alterations, under Section 150.2(b)1D: on an existing house, installing more than 25 feet of new or replacement duct triggers the sealing and testing rule. Twenty-five feet is not much: a single run rerouted around a remodel can reach it.

An addition works differently. Exception 4 to Section 150.2(a) sets no length threshold at all: when any length of duct is extended from the existing system to serve the addition, both the existing duct system and the extension must meet the sealing requirements of Sections 150.2(b)1Di and 1Dii. A short run into a new room is covered just the same as a long one.

Replacing equipment triggers it too. A new air handler, outdoor unit or coil brings the connected duct system under the 10 per cent (or 7 per cent to outside) target, unless the duct system is under 40 linear feet, has already been sealed and tested, or is built with asbestos. This is the rule that surprises homeowners on what feels like a small job: a furnace changeout can pull a diagnostic test and a rater visit into a project nobody expected to need one.

New homes: tested at rough-in or at the end

On a new home, every forced-air duct system that supplies an occupiable space has to be sealed and tested, with no minimum length. If the test is done at rough-in, before the interior is finished, the target is 5 per cent with the air handler installed and 4 per cent if it is not yet installed.

The 10-foot figure that sometimes appears in this context belongs to a different rule: a system supplying an occupiable space through more than 10 feet of duct must have air filters meeting the code's filtration requirements. It is not a sealing threshold.

Who tests, and what it means for programme

The test is performed by an ECC-Rater — the role called a HERS rater until 1 January 2026 — and the result is registered rather than simply reported to you. A fail has to be corrected and retested.

So this sits on the critical path near the end of a job, and it needs the ducts accessible. Booking the rater after everything is closed up is how a duct test becomes an expensive problem instead of an afternoon.

Common Title 24 Questions

1. When is Title 24 required for a home?
2. Does a remodel need Title 24?
3. Does a room addition need Title 24?
4. What plans are needed for a residential Title 24 report?
5. How much does a residential Title 24 report cost?
6. How long does a residential Title 24 report take?

Explore Ducts

1. Duct Replacement 25-Foot Rule Under Title 24
2. Ducts in Soffits and Conditioned Space: Do They Need Insulation?
3. Extending Existing Ducts to an Addition: Title 24 Rules
4. Flexible Duct Installation Rules Under Title 24
5. Insulating Existing Ducts in the Attic: Required or Optional?

View all Ducts guides → · HVAC and Water Heating · Title 24 guide