Vacuum · 16 min
Pull a vacuum on a new unit
Cores out. Gauge on the system, not the pump. Decay is the test. Hitting 500 microns with the pump still running proves almost nothing.
Safety
Never use the compressor as a vacuum pump.
Do not start a compressor in a deep vacuum — motor insulation and start components are not built for that.
A2L new installs: evacuation before charge is required. Inert-gas purge of the lines is required, not a nice-to-have.
Recover any factory refrigerant holding charge. A nitrogen holding charge can be released — confirm which one the OEM shipped.
Tools: Vacuum pump that hits ~50 microns isolated (fresh oil) · Micron gauge · Vacuum-rated core-removal tools · Large-bore vacuum hoses (not the 1/4" charging set) · Nitrogen with a flow regulator (sweeps / triple evac) · Scale and A2L-rated gear if you will charge after
1. Prove the pump and the gauge before you blame the system
Fresh oil if the last job was wet. Cap the pump, hang the micron gauge on the pump, pull. HVACR School: the isolated pump should make ~50 microns. Then isolate your hose/CRT rig and know its own decay — a leaking core tool will fail every unit you touch.
Expect: Pump to ~50 microns isolated. Rig decay is known and small.
If not: Milky oil, a shaft leak, or a dying gauge. Fix the rig. Do not evacuate yet.
2. Open the system the OEM way, then pull the cores
Release or recover the holding charge per the outdoor I/O. Core tools on liquid and suction. Cores out. Valve the micron gauge onto the side port of a core tool at the system — never at the pump. Large hoses. Indoor and outdoor both open.
Expect: Both service valves open to the lines. Cores out. Gauge reading the cabinet, not the pump port.
If not: A closed king valve or a core still in will fake a great vacuum at the pump and a wet system in the coil.
3. Pull deep, then isolate and watch decay
New systems (HVACR School / Kalos field process): below 300 microns, then valve off the pump. In 10 minutes the rise should stay at or under 500 microns. In-service repair: 500 microns, decay not above 1000 in 10 minutes. OEM or commissioning sheet wins if it is tighter. AC Service Tech: if it does not rise above 500 in 10 minutes after a deep pull, the system is clean, dry, and tight enough to charge.
Expect: Deep pull, then a decay that flattens under the limit.
If not: Continuous climb = leak (or a leaking rig). Fast jump that levels off = moisture / outgassing — keep pumping or nitrogen-sweep and pull again.
4. Wet or dirty? Nitrogen sweep / triple evac, then charge
Break the vacuum with dry nitrogen, flow through, pull again. Some A2L OEMs specify triple evacuation — do that when they say so. When decay passes, close the core tools, reinstall cores or leave tools on for the weigh-in, and charge immediately. Do not leave a blank-off vacuum overnight on a new A2L and call it done without a morning decay.
Expect: Passed decay. Charge weighed in the same visit.
If not: Failed second decay → leak-search with nitrogen + tracer / bubbles / electronic. Do not ‘charge it and see.’
- A micron reading at the pump only tells you the pump is running. Jim Bergmann / HVACR School: gauge away from the pump, cores out, decay after isolation.
- Change vacuum pump oil when it milks. One wet job contaminates the next.
- Document the pull and the 10-minute decay (measureQuick, MyNAVAC, BluVac, or the ticket). Commissioning without a number is a story.