Deep Vacuum / Evacuation

For homeowners

A deep vacuum is pulled on a refrigerant system after pressure testing and before refrigerant is charged. It removes two things from the system that don’t belong there: moisture and non-condensable gases (mostly air).

Why moisture is bad.

Water in a refrigerant system causes multiple failure modes:

• Reacts with refrigerant and oil over time to form acids — especially aggressive with R-410A and POE oil
• Freezes in the metering device (TXV or piston) when refrigerant flashes there, blocking flow and stopping cooling
• Causes copper plating — copper dissolves in acidic refrigerant and plates onto bearings and other steel surfaces, destroying them
• Damages compressor internals through corrosion

Why air is bad.

Air is “non-condensable” at typical condenser pressures and temperatures. It can’t condense back to liquid like the refrigerant does, so it accumulates in the condenser coil, displacing refrigerant. Head pressure climbs, cooling capacity drops, and the compressor works harder than it should.

The procedure.

1. Connect a vacuum pump to the manifold center port.
2. Open both manifold valves so the pump pulls from both sides simultaneously.
3. Run the pump until the system reaches the target vacuum level (measured in microns).
4. Close the manifold valves to isolate the system from the pump.
5. Watch the micron gauge for 5–15 minutes to verify the vacuum holds (the “decay test”).
6. If it holds, the system is dry and tight — ready to charge.
7. If it climbs to a specific pressure and stops, moisture is still present — keep pumping.
8. If it climbs continuously, there’s a leak — find and fix it, then re-evacuate.

Target vacuum levels:

• Industry minimum: 500 microns
• Best practice: 300 microns or lower

Below 500 microns, water boils at ambient temperature even on a cool day, so the pump can pull moisture out as vapor. Above 500 microns, water remains as liquid and can’t be efficiently removed.

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For technicians

The physics of vacuum evacuation.

At atmospheric pressure (760,000 microns), water boils at 212°F. As pressure drops, the boiling point drops:

• At 500 microns: water boils at about 0°F
• At 1,000 microns: water boils at about 17°F
• At 5,000 microns: water boils at about 32°F
• At 25,000 microns: water boils at about 80°F

To remove liquid water from a system, the system pressure must be low enough that the water actually boils at ambient temperature. 500 microns is the target because at that pressure, water boils well below any reasonable ambient temperature — any liquid water in the system rapidly boils off and gets pumped out.

Micron gauge — the right tool.

Standard manifold gauges read in PSIG and inches of mercury. The vacuum scale on a typical gauge goes from 0 down to 30 inHg — which corresponds to about 25,000–30,000 microns, far above the 500-micron target. Manifold gauges cannot measure deep vacuum.

A micron gauge is a separate instrument calibrated specifically for deep vacuum readings. It’s a thermistor-based or capacitance-based sensor that gives accurate readings from a few microns up to 25,000 microns. Modern micron gauges are digital, battery-powered, and clip onto the line with a 1/4” SAE fitting.

Critical: the micron gauge must be installed close to the system, not at the pump. The pump can pull a deep vacuum at its own port while the system still has moisture trapped behind restrictions in the hoses, manifold, and Schrader valves. Reading vacuum at the pump shows you what’s happening at the pump, not what’s happening in the system.

Why pull through both sides.

When the manifold valves are both open, the pump can pull from both the high and low sides of the system simultaneously. This matters because:

• TXV systems have a metering device that restricts flow — pulling vacuum from one side only requires moisture to migrate through the TXV, which is slow
• The system has many internal volumes (suction line, evaporator, liquid line, condenser, accumulator) that all need to be evacuated

Pulling from both sides means every part of the system is connected directly to the pump. Evacuation time drops dramatically.

Core removal tools.

A core removal tool goes on a Schrader port and lets the tech remove the Schrader core while keeping the system sealed. With the core in place, the cross-section through the port is small — maybe 1/16” effective diameter. Pulling vacuum through such a small orifice is slow. With the core removed, the full port diameter (1/4”) is open — pumping speed is much higher and evacuation time drops by half or more.

The decay test.

After reaching target vacuum, the tech closes the isolation valve to separate the pump from the system. The micron gauge stays connected to the system. Then the tech watches:

• Vacuum holds steady within 50–100 microns over 5–15 minutes: system is dry and tight. Pass.
• Vacuum rises slowly and levels off (typically at 1,000–5,000 microns): residual moisture is still evaporating. The pressure level corresponds to the equilibrium vapor pressure of water at ambient temperature. Continue pumping.
• Vacuum rises continuously without leveling off: there’s a leak. Find and fix it per nitrogen pressure test and leak detection procedures.

Time required. Depends on system size, ambient temperature, and pump capacity:

• Small system (1–2 ton residential), warm ambient, dry: 30–60 minutes
• Medium system (3–5 ton residential), warm ambient: 60–120 minutes
• Contaminated system: 2–4 hours or more

Speeding up evacuation:

• Warm the system if possible (warmer = faster water boil-off)
• Remove Schrader cores using a core removal tool
• Pump from both sides (open both manifold valves)
• Maintain pump oil cleanliness (contaminated oil drops pumping efficiency)

Triple evacuation.

For systems known to contain significant moisture (system left open after a flood, very humid installation conditions, mineral oil system being converted to POE), a triple evacuation procedure is used:

1. Pull vacuum to 1,000–2,000 microns
2. Break vacuum with dry nitrogen, bring system back to 0 PSIG
3. Pull vacuum again to 1,000–2,000 microns
4. Break with nitrogen again
5. Pull final vacuum to 500 microns or below

The nitrogen breaks dilute the moisture concentration each cycle, making the next vacuum step more effective.

Vacuum pump basics.

Standard service pumps are oil-sealed rotary vane pumps, two-stage. Specifications:

• CFM — the pump’s free-air displacement. Residential service pumps are typically 5–10 CFM.
• Ultimate vacuum — quality two-stage pumps reach 25 microns or below in fresh oil.

Pump oil maintenance. Pump oil absorbs moisture and refrigerant during use. Contaminated oil lowers pumping efficiency, cannot reach deep vacuum, and damages the pump internally. Change oil after every major refrigerant recovery, whenever the oil looks cloudy or yellow, and per the manufacturer’s interval.

Common mistakes.

Reading vacuum at the manifold gauges only. Tech thinks the system is at “full vacuum” because the manifold needle pegs at 30 inHg, but actual vacuum is far above 500 microns. Always use a separate micron gauge.

Skipping the decay test. Tech reaches 500 microns, charges the system, and leaves. The vacuum was reading 500 microns because the pump was actively pumping out moisture as fast as it was evaporating — but moisture was still in the system. Charge introduces refrigerant on top of residual moisture. Acid forms over years; compressor fails. Always run a decay test.

Pulling vacuum on a closed system. If service valves are still back-seated, the pump only evacuates the small volume between the service ports and the valve seats. The line set and indoor coil remain at atmospheric pressure. Always crack service valves open before evacuation.

Pulling vacuum with contaminated pump oil. Pump oil that absorbed moisture from a previous job releases that moisture into the new system being evacuated. Change oil before critical work.

Always replace the filter drier after any service that opens the refrigerant circuit. A new drier absorbs whatever residual moisture remains after evacuation and continues protecting the system over its service life.

Florida considerations. High ambient humidity means a system that’s been open even briefly has absorbed significant moisture. Florida service techs often pull vacuum longer than the textbook 30 minutes — 60–90 minutes is more common for assurance. The triple evacuation procedure is sometimes the standard rather than the exception, especially for any system installed during summer.