Duct Leakage Test Failure at Rough-In
At rough-in, a 240-ft² rectangular galvanized supply duct system is pressure-tested at 1.0 in. w.c. in accordance with the project specification. The specification requires SMACNA Leakage Class 6. The measured leakage is 52 cfm. The installer has applied mastic to the transverse joints but not to the longitudinal seams. Flexible takeoffs are connected with cloth duct tape, and several plenum penetrations are sealed with silicone caulk. The test was run with all registers and dampers open, and the temporary test caps were taped in place. The general contractor is pressing to close the ceiling because the inspection is scheduled for tomorrow. The installer says the leakage is 'close enough' because the system will have a 20% safety margin. The test instrument calibration was current.
1. Calculate the allowable leakage and determine pass/fail. Show the formula and assumptions.
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Leakage Class 6 means 6 cfm of leakage per 100 ft² of duct surface area at the specified test pressure. Allowable leakage = (240 ft² / 100) x 6 cfm = 2.4 x 6 = 14.4 cfm. The measured leakage is 52 cfm, which is about 3.6 times the allowable. The test fails. A future 20% safety margin does not change the leakage class requirement, and the system should not be accepted until the measured leakage is at or below 14.4 cfm at 1.0 in. w.c.
2. Prioritize the likely leak locations and explain the corrections.
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The unsealed longitudinal seams are a major leak path and should be sealed with UL 181 mastic, with embedded mesh where required. The flex takeoffs connected with cloth tape are another likely source; cloth tape is not an approved permanent duct sealant, so remove it and use mastic or an approved UL 181 tape. Plenum penetrations sealed only with silicone caulk may also leak and should be sealed with approved mastic. Check the temporary test caps as well, since taped caps can leak and falsely raise the reading. Prioritize large openings first, then use smoke or pressure testing to find smaller leaks.
3. Describe the retest and documentation required before ceiling closure.
Read the worked answer
Correct the identified leaks, then retest at the same 1.0 in. w.c. pressure using a calibrated duct tester. Record the duct surface area, leakage class, allowable leakage, measured leakage, test pressure, date, technician, and instrument calibration. Document the corrections with photos and have the GC or inspector witness the retest if required. The ceiling should not be closed until the retest passes and the report is accepted. If the retest still fails, repeat the leak survey rather than accepting a marginal result.