Ductwork and Sheet Metal Fabrication and Installation
Sizing, fabricating, and sealing
Duct systems are sized by a recognized method, usually equal friction or the static pressure budget. Compute available static pressure (fan total minus drops across coil, filter, grilles, and heat exchanger), divide by total effective length, and you get a friction rate in inches of water per 100 ft. Traps: using the measured straight-run length instead of total effective length; forgetting that equivalent lengths for fittings often exceed the straight pipe; ignoring how much of the budget a tight filter and plenum consume.
Fabrication follows SMACNA standards: gauge chosen from duct dimension and pressure class, Pittsburgh lock seams, approved transverse joints, reinforcement, and hangers at the required spacing. Seal to the specified seal class with mastic or UL 181 tape. Leakage is capacity you paid for and lost.
Velocity limits keep systems quiet and clean: supply trunk roughly 600 to 900 fpm, branch runs 400 to 600 fpm, returns 600 to 900 fpm. Too fast means noise; too slow means poor throw and dirt fallout in the run.
Worked example: available static 0.30 in. wc and total effective length 300 ft. Friction rate = 0.30 x 100 / 300 = 0.10 in. wc per 100 ft. Use the duct slide rule or chart at that friction to select each size, then check that resulting velocities fall in range before you cut metal.