When you install a standard mini split, the line set between the indoor and outdoor units is full of air and moisture. Both have to come out before you release the refrigerant stored in the outdoor unit. Moisture mixed with refrigerant oil forms acids that damage the compressor over time, and air takes up space the refrigerant needs.
Know the rules and your warranty. In the US, work that opens a refrigerant circuit is regulated under EPA Section 608, and many manufacturers require professional installation to keep the warranty. Pre-charged quick-connect systems are designed to skip this step entirely. Read your unit's manual first. Its procedure always overrides this guide.
Why a micron gauge matters
The low-side gauge on a manifold can't show a deep vacuum. It reads zero long before the moisture is gone. A micron gauge measures the vacuum precisely, and it's the only way to know when you're done.
The process
- Torque the flares to the values in the manual. Loose flares are the number one source of leaks. Over-tightened flares crack.
- Pressure test if you can. Pressurize the line set with dry nitrogen and watch for a drop over time. This finds leaks faster than a vacuum will.
- Connect the pump to the service port. Use the shortest, largest-diameter hoses you have. Removing the valve core with a core tool speeds this up a lot.
- Pull down below 500 microns. This can take anywhere from 20 minutes to well over an hour, depending on line length, moisture and humidity.
- Isolate and do a decay test. Close the valve between the pump and the system, turn the pump off, and watch the micron gauge for 10 to 15 minutes.
Reading the decay test
- Rises a little, then levels off below about 1,000 microns: tight and dry. You're good.
- Rises and levels off higher: moisture is still boiling off. Pull a longer vacuum and test again.
- Keeps climbing steadily toward atmospheric: you have a leak. Check the flares and hose connections.
Releasing the charge
Once it passes, open the service valves in the order your manual specifies with a hex key. Then remove the hoses and cap the ports and valve stems snugly. The caps are part of the seal on many valves, so don't leave them loose.
Fresh pump oil speeds it up. Moisture contaminates vacuum pump oil. Changing it before each job is cheap and can cut your pull-down time noticeably.