Refrigerant Piping, Brazing, Evacuation, and Charging
This domain tests line sizing, brazing practice, evacuation, leak testing, and charging. The exam focuses on preventing moisture and oxidation, achieving a deep vacuum, and using subcooling or superheat correctly for the metering device and operating mode.
Core rules and concepts
- Size lines per manufacturer tables. Undersized suction lines cause pressure drop and capacity loss; oversized lines can prevent oil return.
- Purge nitrogen through the piping while brazing to prevent internal oxidation. Use the correct filler alloy and avoid overheating valves and fittings.
- Evacuate to 500 microns or lower and perform a decay test. A rising vacuum indicates moisture or a leak; use triple evacuation with dry nitrogen if needed.
- Charge by weight when possible, adding for line-set length beyond the factory charge.
- With a TXV, use subcooling in cooling; with a fixed orifice, use superheat. In heating, follow the manufacturer’s charging chart or weigh-in method.
- Install a filter drier and protect it from heat during brazing.
Worked example
A fixed-orifice R-410A system has a suction pressure of 120 psig, which corresponds to about 41°F saturation temperature. The suction line temperature is 52°F. Superheat = 52°F − 41°F = 11°F. If the manufacturer’s target is 10°F ± 2°F, the charge is acceptable. If superheat were 25°F, suspect undercharge, a restriction, or low indoor airflow before adding refrigerant.
Traps that cost marks
Brazing without nitrogen purge; skipping the vacuum decay test; charging a heat pump in heating mode without the manufacturer’s chart; relying on pressures alone; and forgetting the additional charge for long line sets.