Evacuating a Refrigerant System

Air and moisture do not belong in a refrigerant circuit. Moisture reacts with refrigerant and oil to form acids that attack compressor windings. Air does not condense, so it raises head pressure and wastes energy. Evacuation means pulling a deep vacuum to boil away water and remove air before refrigerant goes in. The job is simple, but it is one of the easiest to do badly.

What You Will Be Able to Do

• Explain what a deep vacuum does to moisture in a system.
• Set up a pump, hoses and a micron gauge for fast, accurate evacuation.
• Run a decay test and interpret the result.
• Tell a leak from moisture by how the gauge moves.
• Break a vacuum the correct way.

What a Micron Is

A micron is a very small unit of pressure. Atmospheric pressure is about 760,000 microns. A manifold gauge cannot read these low pressures. It may show 30 inches of vacuum, and that tells you nothing useful at this level. Only a micron gauge shows whether you have a real vacuum.

Why 500 Microns

Lowering pressure lowers the boiling point of water. At atmospheric pressure it boils at 212°F. In a deep vacuum, water boils below room temperature, so it turns into vapor and the pump can remove it.

The usual target is 500 microns or lower, held after you isolate the pump. Some manufacturers require a lower number, so check the manual. The 500 micron target has three good reasons behind it:

• Water has boiled out of the system at this level.
• It is low enough that a decay test can separate a leak from leftover moisture.
• Field equipment and hoses cannot hold much lower for long, so a lower target wastes time.

Setting Up for a Fast Pull

1. Recover any refrigerant first, and confirm the system is empty.
2. Use short, large-diameter hoses. Standard hoses with Schrader depressors slow the job.
3. Remove the Schrader cores with a core removal tool if you can. The cores are the biggest restriction in the setup.
4. Connect a two-stage vacuum pump with clean oil. For most residential work a pump of about 4 to 6 CFM is enough. Bigger systems need bigger pumps.
5. Connect the micron gauge at the system side, as far from the pump as you can. A gauge at the pump shows how the pump is doing, not how the system is doing.
6. Pull from both the high and low sides.

For A2L refrigerants, use a pump rated for that use.

Evacuating Step by Step

1. Start the pump and open the valves.
2. Watch the gauge fall. It will drop quickly at first and then slow as moisture boils off.
3. If the gauge stalls between about 1,000 and 3,000 microns, moisture is still boiling. Keep pulling.
4. Once the gauge is at or below 500 microns, keep pulling a few more minutes.
5. Close the valve between the pump and the system, so the pump is isolated.
6. Watch the gauge for at least 10 minutes. This is the decay test.

Reading the Decay Test

Breaking a Vacuum

What you use depends on what comes next:

• Between evacuations, or to open a system that is under vacuum after recovery: use dry nitrogen. Nitrogen is dry, so it picks up moisture and sweeps it out, and it keeps air and wet outdoor moisture from being pulled into the system.
• At the end of a good evacuation: let refrigerant into the vacuum to begin charging, by weighing it in or by opening the service valves on a factory-charged unit.

Never break a vacuum with air. Air brings back the moisture you just removed. Do not break a vacuum with refrigerant if you plan to open the system again, since that adds refrigerant you would then have to recover.

Triple Evacuation

If a system was open a long time or had moisture or a compressor burnout, a sweep helps:

1. Pull to about 1,500 microns.
2. Break the vacuum with dry nitrogen to a few psig.
3. Pull again, then break again.
4. Pull down a final time to 500 microns or lower, and do the decay test.

Pump Care

Vacuum pump oil soaks up moisture and contaminants. Dirty oil cannot reach a deep vacuum. Change it when it turns cloudy, and after any system with acid or a lot of moisture. Keep the pump capped when it is not in use.

What Not to Do

Warning: Do not start a compressor while the system is under vacuum. Motor windings can arc or be damaged at very low pressure, and a hermetic compressor has its terminals inside the shell. Do not use the compressor itself as a vacuum pump.

Do not leave the pump running unattended on a system with a leak or on a poorly secured setup. If the pump stops while connected, its oil can back up into the hoses, so close the valve before you shut it off.

Common Mistakes

Check Your Understanding

The micron gauge reads 450, you isolate the pump, and it rises to 700 and stops there for 10 minutes. What does that mean? The system is tight and dry, because it stayed below 1,000 microns.

The gauge rises to 2,500 microns, levels off and holds there. What should you do? That points to moisture. Pull longer, or do a nitrogen sweep and pull again.

The gauge keeps climbing past 5,000 microns and does not slow down. What should you do? Treat it as a leak. Pressure test with nitrogen and find it before you go on.

Why is the micron gauge placed at the far end of the system? So it reads what the system sees, not just what the pump is doing.

Key Takeaways

• Moisture forms acids that destroy compressors, and air raises head pressure.
• Use a micron gauge, because a manifold gauge cannot read a deep vacuum.
• The usual target is 500 microns or lower, followed by a decay test of at least 10 minutes.
• A plateau above about 1,000 microns means moisture, and a steady climb means a leak.
• Break a vacuum with dry nitrogen, never air, unless you are charging into a finished vacuum with refrigerant.
• Never run a compressor under deep vacuum.