What's the accepted evacuation target and decay test before I release charge on a new residential split system, and where should the micron gauge be?

Evacuation Target:

• Pull vacuum to 500 microns or below (field standard).
• Hold for 15–30 minutes; maximum rise should be ≤100 microns (i.e., final reading ≤600 microns).
• If it climbs past 700–1,000 microns, suspect a leak or outgassing.

Decay (Hold) Test:

• Isolate the system with core tools or valve closures.
• Monitor for 15 minutes minimum (longer in humid conditions).
• Acceptable decay: ≤100-micron rise.

Micron Gauge Placement:

• Install the micron gauge at the service port on the low side (suction line).
• Remove the gauge hose from the vacuum pump side and connect directly to the low-side service port via a dedicated vacuum-rated hose.
• Do not leave the gauge connected to the vacuum pump during the decay test — isolate it.

Code Reference:

• ACCA AHR2-2018 and ASHRAE Standard 15 recommend deep vacuum to ≤500 microns for hermetic systems.
• Section 608 requires evacuation to remove noncondensables and moisture.

Pro Tip:

• Use a vacuum tree with a dedicated gauge port and core depressor for accuracy.
• Always replace filter-driers if the system was open to atmosphere.

If readings don’t hold, recheck for leaks, especially at flares and joints.

Evacuation Target: 500 microns (or less) is the industry standard per ASHRAE Guideline 3-2016 and manufacturer specs. Many OEMs now require 250–300 microns for POE oils (R-410A, R-32, R-454B). Check the install manual.

Decay Test (Standing Vacuum Test):

1. Pull system to target (e.g., 250 microns).
2. Isolate pump: Close core tool valves at the system ports. Turn pump off.
3. Wait 10–15 minutes minimum (30 min preferred).
4. Pass Criteria: Micron reading does not rise above 500–1000 microns (varies by OEM; 500 is safe baseline). If it rises steadily → leak. If it plateaus → moisture/outgassing; evacuate longer.

Micron Gauge Placement: At the system service ports (core tools), NOT at the pump.

• Use valve core removal tools (core tools) on both liquid and suction ports.
• Connect micron gauge to the side port of one core tool (usually suction side for larger volume).
• Connect vacuum pump to the other core tool.
• Why: Measures actual system vacuum. Pump inlet reads lower (pump vacuum), misleading you. Hose/core depressor restriction creates pressure drop; gauge at pump won't see system pressure.

Critical Steps:

• Remove Schrader cores (use core tools).
• Use 3/8" or 1/2" vacuum hoses (short, large ID).
• Nitrogen sweep (1–3 PSI) before evacuation if system was open >1 hr.
• Triple evacuation (nitrogen break) only if required by manual or extreme moisture suspected.

Release Charge: Only after passing decay test. Open liquid line service valve first (slowly), then suction.