Installation, Piping, Evacuation, and Dehydration
Piping and line sizing
Suction lines are sized to limit pressure drop while keeping velocity high enough to return oil to the compressor. Liquid lines are sized for pressure drop and to prevent flashing. Slope horizontal runs toward the compressor, add a trap at the base of a vertical riser and an inverted trap at the top, and insulate the suction line. Follow the manufacturer's long-line guidelines when the indoor and outdoor units are far apart or the outdoor unit sits above or below the indoor coil by more than the standard allowance.
Brazing
Flow dry nitrogen through the line while brazing to prevent copper oxide scale, which will plug the filter drier and damage the compressor. Use a flow regulator rather than a blast, remove Schrader cores, and protect the TXV/EEV bulb and any plastic components from heat.
Evacuation and dehydration
A micron gauge reads absolute pressure: one atmosphere is 760,000 microns, and 29.92 inHg is the corresponding barometric reading, so 500 microns is a deep vacuum (about 29.9 inHg) that only a micron gauge can resolve. Evacuate to 500 microns or below, isolate the pump, and confirm the vacuum holds. Triple evacuation - breaking the vacuum with dry nitrogen between pulls - speeds moisture removal.
Traps
- Judging vacuum with a manifold gauge, which cannot read 500 microns.
- Pressure-testing with oxygen or compressed air (oil plus oxygen equals fire or explosion).
- Leaving POE oil open to air; it picks up moisture in minutes.
- Skipping the standing decay test, which catches both leaks and residual moisture.
Rule of thumb
"500 and hold": pull to 500 microns, blank off for 10–15 minutes, and require the reading to stay under about 1,000 microns.