Assembly, Fastening, and Joining
What this domain tests
The exam tests whether the joint you choose holds the pressure, the vibration, and the service environment, and whether it is the specified joint. Expect items on seam types, fastener spacing, seal class, and welding coated or stainless steel.
Seams and joints
- Longitudinal seams: Pittsburgh lock (the HVAC standard, mechanically locked and usually sealed), snap lock, button punch, welded.
- Transverse joints: slip-and-drive with S-cleat and drive cleat, standing S, TDC/TDF flanges, companion angles, welded flanges.
- Leakage is controlled by seal class. SMACNA assigns Seal Class A, B, or C by pressure class; Class A seals all transverse joints, longitudinal seams, and penetrations. Sealant, mastic, or listed tape (UL 181A-P for rigid duct) goes on a clean, dry surface.
- Reinforcement (angle, channel, TDC) is sized to pressure class and duct width from the SMACNA table.
Mechanical fasteners
- Self-drilling screws (#8, #10) with neoprene washers for exterior work; blind rivets for light gauge with one-side access.
- Minimum edge distance is about twice the fastener diameter. Spacing follows the specification or SMACNA table, typically 300 mm (12 in.) on centre maximum at joints, tighter at higher pressure.
- Isolate dissimilar metals with tape, gaskets, or coated fasteners.
Welding, soldering, brazing
- Galvanized: grind zinc back at least 50 mm (2 in.) each side, weld with ventilation, and expect zinc-oxide fume.
- Stainless: GTAW or GMAW with 308L/316L filler, argon purge on the back, stainless-only brushes.
- Aluminum: GMAW with argon and 4043 or 5356 filler; remove oxide just before welding.
- Copper: lead-free solder with the correct flux; brace the joint so it cannot move while cooling.
Worked example: rivet count at a joint
A 40 in. transverse joint uses 3/16 in. blind rivets at 2 in. on centre, with minimum edge distance of 2 x 3/16 = 3/8 in. at each end. Usable span = 40 - 2(0.375) = 39.25 in. Spaces = 39.25 / 2 = 19.6, so 20 spaces and 21 rivets. Confirm each rivet clears the flange and corner bracket, and that the rivet grip range matches the total material thickness.
Traps that cost marks
- Taping a high-pressure duct and calling it Class A.
- Screws or rivets too close to an edge, causing tear-out under pressure.
- Welding galvanized without removing coating or providing ventilation.
- Using a carbon steel brush on stainless.
- Assuming a Pittsburgh lock is airtight without sealant.