Evacuation — pulling a deep vacuum on a system before charging — is one of the most important and most rushed steps in the trade. Done properly, it removes air and moisture that would otherwise cause acid formation, corrosion and poor performance.
Why evacuate?
Moisture in a system reacts with refrigerant and oil to form acids, damaging the compressor and blocking the expansion device with ice. Non-condensable air raises head pressure and reduces efficiency. A proper vacuum removes both.
The process
- Connect a vacuum pump to the system (ideally to both service ports for a faster, deeper pull)
- Pull the system down to a deep vacuum, measured with a micron gauge (not just the compound gauge)
- Perform a vacuum decay test — close the valves and confirm the vacuum holds, which proves it’s dry and leak-free
- Only then charge the refrigerant
Key tips
Use a quality vacuum pump with fresh oil, large-bore hoses for speed, and a micron gauge for an accurate reading. A vacuum that won’t hold means moisture or a leak — investigate before charging.
FAQ
How low should the vacuum be? Pull to a deep vacuum measured in microns and confirm it holds with a decay test — a single low reading isn’t enough on its own.
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