When Is Duct Leakage Testing Required?
A new or completely replaced duct system has to measure 5 per cent or less of air handler airflow. Alterations have their own scaled targets, and replacing more than 40 linear feet of existing duct is enough to trigger a test.
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 40-foot trigger
A duct leakage test is triggered by any new ductwork, and specifically by replacing more than 40 linear feet of existing duct. Forty feet is not much — a single run rerouted around a remodel can reach it.
This is the threshold that surprises homeowners on what feels like a small job. A furnace changeout with some ductwork tidied up can pull a diagnostic test and a rater visit into a project nobody expected to need one.
Ten feet in occupiable space
Where ducts exceed 10 feet in occupiable space, they must be sealed and leakage tested, confirmed by field verification and diagnostic testing under the reference appendix procedure.
Ducts inside the conditioned envelope are sometimes assumed to be exempt on the reasoning that leaked air stays in the house. The code does not accept that: leakage still wrecks the airflow balance between rooms even where the air is not lost outright.
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.