Apprentice is pulling a vacuum on a new mini-split with a standard manifold set, both Schrader cores in, 1/4 in.

What went wrong:

1. Moisture/Non-condensables Not Removed: A 300-micron vacuum is insufficient for a mini-split. Industry standards (ACC A117.1, manufacturer specs) require ≤500 microns after 30+ minutes of pumping, followed by a decay test (hold vacuum for 10–15 minutes; rise ≤100 microns). At 300 microns, moisture likely remains, leading to poor performance and potential compressor damage.

1. Improper Evacuation Procedure: - The apprentice didn’t verify a proper decay test. - Closing the gauges and opening service valves without isolating the system or confirming a stable vacuum introduces air/moisture.

1. Hose/Core Issues: - Leaving Schrader cores in during evacuation restricts airflow, slowing dehydration. Cores should be removed for faster evacuation. - 1/4 in. hoses are acceptable but ensure they’re rated for deep vacuum (some standard hoses leak air under vacuum).

1. Refrigerant Loss: Opening service valves after a poor vacuum may have allowed air into the system or caused improper refrigerant migration, reducing charge.

Corrective Action:

• Remove Schrader cores.
• Evacuate to ≤500 microns, then perform a decay test.
• Use a dedicated vacuum pump and micron gauge (not just the manifold).
• Recharge only after confirming a stable vacuum.

Code Reference: ACC A117.1 and manufacturer guidelines mandate proper evacuation procedures. Failure to follow these voids warranties and risks system failure.

Verdict: The apprentice skipped critical steps (decay test, core removal), leading to moisture retention and inadequate performance. A licensed technician should re-evacuate and recharge the system.

Three critical errors:

1. Schrader cores left in hoses. Standard 1/4" hoses with cores act as severe restrictions. You cannot achieve a deep vacuum or verify it with cores installed. Remove cores with a core tool before evacuating.
2. Micron gauge on pump inlet. This reads pump capability, not system vacuum. The gauge must be on the system (liquid line Schrader port via core removal tool) to measure actual system pressure.
3. No decay test. Closing valves at 300 microns without a rise test guarantees moisture/leaks remain. Per EPA 608 / industry standard, hold < 500 microns for 10–15 min (decay test) before opening valves.

Result: System evacuated only the hoses/pump. Moisture/non-condensables remained → acid formation, TXV freeze-up, capacity loss.

Fix: Recover charge. Pull cores. Connect micron gauge to system port. Evacuate to < 250 microns. Valve off pump. Decay test: hold < 500 microns for 15 min. Only then open service valves. Weigh in exact charge.