How to Pull a Deep Vacuum: Micron Targets and the Decay Test

Evacuation removes air (non-condensables) and moisture. Moisture in a POE oil system turns into acids and sludge, and air raises head pressure. The micron gauge and the decay test are how you prove the job is done, not the clock.

Step 1: Pressure test with nitrogen first

Find leaks under pressure, not under vacuum. Pressurize with dry nitrogen (never oxygen or compressed air) to the manufacturer's test pressure, and watch for a pressure drop, correcting for temperature changes. Fix leaks now. Hunting for leaks in a vacuum pulls moisture-laden air into the system.

Step 2: Set up the rig for speed

• Core removal tools on both service ports. Valve cores are the biggest restriction.
• Short, large-diameter vacuum hoses (3/8" or 1/2" where you can), not standard charging hoses.
• Micron gauge at the system, as far from the pump as practical, often on the side port of a core tool. A gauge at the pump reads the pump, not the system.
• Fresh pump oil. Contaminated oil can't reach a deep vacuum. Blank-off test the pump: it should pull well below 50 microns on its own.
• A2L systems: use a vacuum pump that's rated or approved by its manufacturer for A2L refrigerants.

Step 3: Evacuate

Pull down below 500 microns. Many manufacturers and trainers recommend going lower on POE oil systems. HVAC School, for example, recommends pulling below about 300 microns and confirming the system holds under about 500. Time depends on system size, moisture and rig. Don't stop at a time limit; stop at a reading.

Triple evacuation (pull down, break with nitrogen, pull down again) helps on wet systems or when you're cleaning up after a burnout.

Step 4: The decay test

Close the core tools to isolate the system from the pump and hoses. Watch the micron gauge for 10–15 minutes:

What you see	What it means
Rises a little and holds below about 500 microns	Dry and tight, ready to charge
Rises, then levels off higher (about 1,000–1,500)	Moisture still boiling off. Keep evacuating
Keeps climbing toward atmosphere	Leak. Go back to the nitrogen test

Some rise is normal as the pressure evens out between the system and the gauge. What matters is whether it levels off and where.

Step 5: Release the charge or weigh in

Once the decay test passes, open the service valves (new install) or weigh in the charge per the installation instructions. Break the vacuum with refrigerant, not with air.

FAQ

Is 500 microns good enough?

500 microns is the common minimum, and many manufacturers specify it. Many experienced techs pull deeper (300 or lower) and require the system to hold below 500 during the decay test, especially on POE oil systems.

Why won't my vacuum go below 1,000 microns?

Usual causes are moisture in the system, a leak, contaminated pump oil, leaking hoses or gaskets, or valve cores left in. Isolate the pump and gauge to find out which side the problem is on.

How long should a decay test be?

10–15 minutes is common for residential systems. Larger systems may need longer to stabilize.

Sources

• HVAC School: evacuation basics
• HVAC School: how long should I evacuate a system
• measureQuick: how long should you evacuate a system
• ACHR News: why deep evacuation is system protection