Installation, Piping, and Sheet Metal
This domain tests line-set practice, brazing and flaring, condensate removal, duct gauge, support spacing, and sealing.
Line sets and piping
- Size the line set from the manufacturer table using equivalent length, adding the published allowance for each fitting and bend.
- Add an oil trap at the base of tall risers and follow the manufacturer interval on long vertical runs.
- Braze with a nitrogen purge; use 15 percent silver alloy for copper-to-brass or copper-to-steel joints.
- Torque flare nuts to the published value. Overtightening crushes the flare and causes leaks.
- Insulate the suction line fully and support horizontal runs about every 6 feet.
Condensate
- Slope the drain at least 1/8 inch per foot, install a trap on draw-through units, and never reduce the drain size.
- Provide a secondary pan with a float switch where required, and never tie a drain into a vent.
Sheet metal and duct
- Gauge comes from the SMACNA table for the duct size and pressure class; lightly constructed rectangular duct commonly runs 26 gauge.
- Hangers at 8 feet maximum for rectangular duct and 10 feet for round.
- Seal joints with mastic or UL 181A tape; flex duct stays fully extended, supported every 4 to 5 feet, with bend radius at least equal to duct diameter.
Worked example
Available static is 0.50 in. w.c., registers and grilles drop 0.10, and the longest run has a total effective length of 400 ft. (0.50 - 0.10) / 400 x 100 = 0.10 in. w.c. per 100 ft. That friction rate drives every duct size on the drawing.
Traps that cost marks
- Brazing without nitrogen, then finding scale in the TXV screen.
- Leaving out the condensate trap on a draw-through air handler.
- Sagging or sharply bent flex duct that chokes airflow.
- Mixing duct gauge classes without checking the SMACNA table.