Light Commercial System Installation and Piping
What this domain tests
Installation and piping questions test whether you can route and size lines so refrigerant and oil return reliably, and whether you can braze, pressure-test, and evacuate without contaminating the system. Light commercial work includes walk-ins, reach-ins, display cases, and rooftop condensing units.
Core rules and concepts
- Size suction, liquid, and discharge lines per the manufacturer. Too large a suction line lowers velocity and causes oil logging; too small a line causes excessive pressure drop.
- Pitch suction lines toward the compressor, commonly about 1/4 in per 10 ft, and install an inverted trap at the evaporator outlet when required.
- Use oil-return traps on long vertical risers as specified. A double riser allows oil return at low load while reducing pressure drop at full load.
- Purge with dry nitrogen while brazing to prevent internal oxidation and soot.
- Pressure-test with dry nitrogen, then evacuate to 500 microns or below, isolate, and confirm the vacuum holds. Triple evacuation helps remove moisture.
- Install the filter-drier, sight glass, solenoid valve, and vibration eliminator in the correct locations. Insulate suction lines and separate them from hot lines.
Worked example
A walk-in evaporator has a long vertical suction riser. At full load the line velocity is adequate, but at low load oil logs in the evaporator and the compressor loses oil. The riser was sized like a horizontal run. The correction is not a larger line-that makes velocity worse. Use a double riser or resize the riser per the manufacturer so the small riser maintains oil-return velocity at low load.
Traps that cost marks
Brazing without nitrogen purge; pressure-testing with refrigerant only; evacuating through long manifold hoses with Schrader cores installed; skipping the vacuum decay test; and adding traps indiscriminately instead of following line-sizing and riser rules.
Key points
- Line size balances pressure drop against oil-return velocity.
- Pitch suction lines and use traps and risers per design.
- Nitrogen purge prevents oxide and soot during brazing.
- Evacuate to 500 microns and verify a decay test.
- Filter-drier, sight glass, and solenoid belong in the correct sequence.