Piping and Ductwork
Piping and Ductwork
This domain tests refrigerant line sizing, evacuation, brazing, pressure testing, duct sealing, and support. The exam wants field-ready installation judgment, not just component names.
Core rules
- Follow the manufacturer’s line-set table for tonnage, length, and lift. Suction risers need oil-return traps at the base and at intervals; horizontal runs need support and slope toward the compressor.
- Braze with nitrogen flowing to prevent internal oxidation. Pressure-test with dry nitrogen, typically to 500 psig for R-410A, and hold with a temperature-compensated gauge.
- Evacuate to 500 microns or lower and hold; a decay test confirms no moisture or leak.
- Seal duct joints with mastic or UL 181 tape, not cloth duct tape. Support flex duct at manufacturer intervals, keep bends gentle, and avoid compressing it.
Worked example
A 3-ton system with a 50-foot line set and 20 feet of lift must use the manufacturer’s table, not a generic rule. If the table allows a 7/8-inch suction line, a smaller line increases pressure drop and can reduce capacity; a larger line may not return oil. The correct choice is the table’s combination for capacity, length, and lift.
Traps
Brazing without nitrogen; skipping the suction-line trap on a tall riser; using flex duct with sharp 90-degree bends; and pressure-testing with oxygen or air instead of dry nitrogen.