TDC Joint Leakage in a Return-Air Plenum
A crew is assembling 1200 mm × 500 mm rectangular galvanized duct, 24 ga, in a commercial building. The duct is specified as 500 Pa pressure class and runs through a return-air plenum. Sections are joined with TDC flanges, EPDM gaskets, and a UL 181A listed mastic. After 20 joints, a smoke test shows leakage concentrated along the two 1200 mm sides of the transverse joints. Some gaskets are compressed to about 1 mm from a nominal 3 mm; several cleats are missing; mastic is smeared only on the outside of the flange. The foreman wants to wrap the joints with foil tape and continue, because the system will be balanced later. The job is behind schedule, and the building inspector has not yet inspected the duct.
1. Diagnose the likely causes of leakage and state whether foil tape is acceptable.
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The leakage pattern points to joint stiffness and assembly defects, not random sealant failure. The 1200 mm sides are the long sides of the TDC joint, so they deflect more under 500 Pa unless the cleats are correctly spaced and fully engaged. Missing cleats allow the flange profiles to separate, which opens the gasket line. Gaskets compressed to 1 mm from a nominal 3 mm have lost their elastic recovery and cannot seal minor gaps. Mastic applied only to the outside of the flange does not seal the mating faces, and it may bridge the gap rather than fill it.
Foil tape is not an acceptable repair for this joint. In a return-air plenum, the repair must use a sealant listed for the application and compatible with the duct; the specified UL 181A mastic is the benchmark. Tape may be permitted for minor closure seams in some specifications, but it is not a substitute for correct TDC assembly, gasket compression, and cleat placement at 500 Pa. The foreman's plan would hide the defect, not correct it, and would likely fail inspection and leakage testing.
Correct the joint by disassembling the affected sections, replacing damaged gaskets, installing the required cleats, aligning the flanges, and applying mastic to the mating surfaces and the inside of the joint as specified.
2. Describe the correct assembly sequence and inspection criteria for these TDC joints.
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Clean the flange profiles and the duct ends. Install a continuous EPDM gasket of the correct size and hardness on the flange face without stretching it around corners. Align the two duct sections so the flanges seat squarely and the gasket is not pinched. Engage the TDC cleats at the spacing required for a 1200 mm side at 500 Pa, and place cleats within 50 mm of each corner. Do not hammer the profiles into place; use the proper TDC tool so the flanges lock without deforming the gasket.
Apply mastic to the joint as specified. For a low-pressure system, sealing the inside of the joint is usually more effective than a surface smear on the outside; follow the project specification and the sealant manufacturer's instructions. Tool the mastic continuously without voids. Inspect every joint for gasket continuity, flange alignment, cleat engagement, corner tightness, and visible gaps. A feeler gauge or visual check should show no gap greater than about 1 mm at the gasket line. Check that the mastic is not cracked or pulled away. Any joint with a missing cleat or compressed gasket must be reworked, not taped over.
3. What re-test and documentation should be performed before closing the plenum?
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Repair all identified joints, then re-test the duct section before the plenum is closed. Use a smoke test for locating leaks and a pressure leakage test for acceptance. Test at the specified pressure, typically 1.25 times the design static pressure but not less than the specification minimum, so for a 500 Pa class the test pressure would be about 625 Pa unless the project states otherwise. Compare the measured leakage with the specified leakage class using the SMACNA leakage tables or the project's acceptance criteria. A leakage rate of 8% of design flow, if that is what the earlier test showed, would not meet a 4% allowance.
Document the repairs: joint locations, gaskets replaced, cleats added, mastic product and batch, test pressure, measured leakage, and test instrument calibration. Record who performed the test and who witnessed it. Take photographs of the repaired joints before closing the plenum. Obtain the inspector's sign-off and keep the test report with the commissioning documents. Do not close the plenum until the leakage test passes and the inspection is complete.