Testing, Evacuation, Charging, and Startup
Sequence, then numbers
Order of operations is test, evacuate, charge, start. Pressure-test with dry nitrogen to the manufacturer's limit, often 300 to 550 psig, and hold it. Leak-check with an electronic detector or bubbles, repair, and retest before evacuation.
Evacuate with a micron gauge connected at the system, not at the manifold gauges, and isolate the pump with a valve so you can perform a standing vacuum decay test. A typical target is 500 microns or lower, holding below about 1,000 microns after isolation. Triple evacuation with nitrogen sweeps helps when moisture is suspected; moisture forms acid and acid destroys compressors.
Charge by weight whenever the line set or conditions differ from the nameplate, and otherwise by subcooling on TXV systems or superheat on fixed-orifice systems, once conditions have stabilized. Verify airflow first, because no charge adjustment fixes a duct problem.
Startup checks: rotation on three-phase, amp draw versus nameplate, temperature split, static pressures, condensate drainage, and crankcase heater energized for the manufacturer's required period before running the compressor.
Traps: reading vacuum at the pump instead of the system; skipping the decay test; charging by feel; adding refrigerant before checking subcooling.
Worked example: target subcooling is 10 F. Measured liquid line is 95 F with 110 F condensing saturation, giving 15 F subcooling. Excess subcooling suggests overcharge, a restriction, or non-condensables. Recover and weigh in; do not guess.