Who performs the duct leakage test?

Section 120.5(b) assigns acceptance tests to a certified mechanical acceptance test technician. The compliance manual also describes an ECC-Rater verifying a sample of sealed ducts, so ask the building department which route it expects.

Is there a leakage class number to meet?

Part 6 states the limit as a percentage of nominal air handler airflow, 6 or 15 percent. Seal Class A applies to the construction of pressure-class ducts.

Commercial Duct Leakage Acceptance Testing: What to Expect

Duct leakage is tested on constant-volume single-zone systems under 5,000 square feet with over 25 percent of duct surface outdoors or unconditioned: 6 percent of nominal airflow for new ducts, 15 percent for extended existing ducts. A certified mechanical acceptance test technician pressurizes the system to 25 Pa and records NRCA-MCH-04-A.

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Which duct systems are tested

All four conditions must hold: a constant-volume single-zone system, serving under 5,000 square feet of conditioned floor area, with more than 25 percent of total duct surface outdoors or in unconditioned space, and not in a healthcare facility. New systems meeting them follow Section 120.4(g)1. Entirely new replacement systems in an alteration, at least 75 percent new material, follow the same Section 120.4(g) rule. Ducts extended from an existing system are tested under Section 141.0(b)2Dii. Systems outside the criteria fall to the leakage testing in CMC Section 603.10.1 for new ducts.

Before the test

Seal Class A is required for pressure-class ducts. The technician checks that joints are not sealed with cloth-back rubber tape unless mastic and drawbands back it up, and that the ducts are still accessible, because the test must be done before access is blocked.

How the test runs

The whole system including the air handler is pressurized to 25 Pa (0.1 inch of water) with registers and grilles sealed except one, and outside air dampers and economizers sealed. Measured leakage is divided by nominal air handler airflow: 400 cfm per ton for a split or packaged system with one indoor unit, 350 per ton for each indoor unit of a multiple-split system, 250 for small-duct high-velocity, and 21.7 cfm per kBtu/h of output for heating-only. A 4-ton packaged unit has a nominal 1,600 cfm, so 96 cfm passes at 6 percent and 240 cfm at 15 percent. Flow gauges must be accurate to 3 percent and pressure sensors to 0.2 Pa.

If an altered system fails

The installer seals every accessible leak and retests. A system that still cannot reach 15 percent passes a smoke test, with no visible smoke from accessible duct, and a visual inspection of five sealing locations.

Replacement jobs trigger this in the first place, as the duct systems page explains. Related sizes and exceptions are on the small commercial replacement page and the rooftop unit questions. The signer rules are on who performs testing, and commercial work is quoted per project.

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