How to Pull a Vacuum on a Mini Split Line Set

What the vacuum actually removes

Air in a refrigerant circuit is a non-condensable gas. It takes up room in the condenser that refrigerant should be using, so pressure goes up and capacity goes down. Moisture is worse. With the heat and pressure inside a running system, water reacts with the refrigerant oil and can turn into acid and sludge. Pioneer's installation manual is blunt about it: air and foreign matter in the circuit can cause abnormal pressure spikes, damage the unit, cut its efficiency and hurt someone.

A vacuum pump drops the pressure in the lines so low that water boils off at room temperature and gets pulled out with the air. That's why the target is so deep. You're drying the lines as well as emptying them.

Tools you need

• A vacuum pump. Mitsubishi's MSZ-GL manual calls for one with backflow prevention, or an adapter that stops pump oil from running back into the hoses if the pump shuts off under vacuum.
• A micron gauge. People try to skip this one, and it's the only thing that tells you whether you're finished. More on that below.
• A manifold and hoses rated for your refrigerant. R-410A runs at about 1.6 times the pressure of the old R-22, which is why Fujitsu's installation manual lists an R-410A gauge manifold and charge hoses as special tools, plus a vacuum pump adapter if your pump is an older one.
• Hex keys that fit each service valve stem. Check the sizes before you start.
• A torque wrench with flare crowfoot heads, so the flare nuts are at the manual's torque before you begin.
• Dry nitrogen and a regulator, if your manual breaks the vacuum with nitrogen the way Mitsubishi's does.

Why a compound gauge can't tell you when you're done

The blue low-side gauge on a manifold reads vacuum in inches of mercury (inHg), and full scale is about 30. A micron gauge reads in microns of mercury. One inch of mercury is 25,400 microns. Everything that matters for a dry system, from a few thousand microns down to a few hundred, is packed into the last sliver of that 30 inHg dial. On a compound gauge, a line set with a slow leak can look exactly like one that's properly dried.

Manufacturers don't agree on how much this matters, and it shows in their manuals. Both procedures below are official. They're very different.

Your own unit's manual comes first. If all it gives is a compound-gauge target, use a micron gauge anyway. It costs you a little time, and it's the only way to see what's really going on inside the lines.

Step by step

1. Finish and torque every flare first. Evacuation doubles as a leak test, and a loose flare will fail it. Mitsubishi's torque table gives 10 to 13 ft-lb for a 1/4-inch flare and 25 to 30 ft-lb for 3/8-inch. Use the table in your own manual.
2. Leave both service valves closed. They ship that way, with the charge held inside the outdoor unit. Take the cap off the service port on the larger gas-side valve and nothing else.
3. Hook up the hoses. The low-side (blue) hose goes to the gas valve's service port, and the center (yellow) hose goes to the pump. Pioneer notes that some units need a 1/4-inch to 5/16-inch port adapter. Put the micron gauge where it reads the system itself. A gauge at the pump only tells you about the pump.
4. Start the pump and open the low-side valve on the manifold. The high-side valve stays closed.
5. Pull down to your manual's target. If it uses nitrogen breaks like Mitsubishi's, stop at each intermediate reading, close off the pump, let nitrogen in to 0 psig, then evacuate again. The dry nitrogen soaks up moisture and carries it out on the next pull.
6. Isolate the system and watch. Close the manifold valve, shut the pump off, and time whatever hold test your manual calls for.

Reading the decay test

Once the pump is isolated, the micron reading will creep up a bit. What it does after that tells you what's in the lines.

• A small rise that levels off below your target means the system is tight and dry. Go ahead and open the valves.
• If it levels off above your target, moisture is still boiling off. Restart the pump and pull down again.
• If it keeps climbing and never levels, air is getting in. Put a little nitrogen pressure on the system, check each flare with soap solution, fix the leak and start over. Never release the charge into a line set that won't hold a vacuum.

Opening the service valves

Pioneer's manual adds one more check before you open the valves all the way. With the hose still on the gas port, you crack the liquid valve a quarter turn counterclockwise with the hex key, listen for refrigerant moving, and close it again after about 5 seconds. The low-side gauge should now read positive pressure. Watch it for a few minutes. If it drops, there's a leak to find before you go any further.

Then open both valves fully:

• Turn each stem counterclockwise until it seats against its stop. Pioneer warns against forcing it past that.
• Open them all the way. Mitsubishi's manual notes that running with them partly open lowers performance and causes trouble.
• Pull the hose off the service port quickly and cleanly.
• Put all three caps back on. Mitsubishi lists 15 to 22 ft-lb for the valve caps and 10 to 13 ft-lb for the service port cap. The caps are the final seal, so don't leave them finger-tight unless your manual says to.

Last, brush soap solution over every flare and valve while the system runs. Growing bubbles mean a leak.

When the line set is longer than the factory charge covers

Every outdoor unit ships with enough refrigerant for a stated line length. Past that, the manual gives an extra charge per foot. Pioneer's WAS/WYS manual lists 0.16 oz per extra foot on systems with a 1/4-inch liquid line. Adding refrigerant is regulated under EPA Section 608, and the EPA also prohibits venting refrigerant. If your run needs extra charge, have a certified tech weigh it in.

Sources

• Mitsubishi Electric MSZ-GL installation manual, section 4-1: purging procedures and leak test, flare torque table.
• Pioneer WAS/WYS installation manual: air evacuation, valve opening and additional charge.
• EPA Section 608 refrigerant management rules.

Mini split vacuum FAQ

How many microns should a mini split vacuum reach?

Whatever your own installation manual says. Mitsubishi's MSZ-GL manual, for example, has you evacuate down to 500 microns and says the system should hold 500 microns for at least an hour. Some other manuals only give a compound-gauge target like -30 inHg. That's too coarse to show microns, so you still want a micron gauge to see where you really are.

Can I use my manifold gauge instead of a micron gauge?

A manifold's compound gauge shows that there's a vacuum. It can't show how deep. One inch of mercury equals 25,400 microns, so a needle sitting on 30 inHg can't tell 500 microns from several thousand. Mitsubishi's manual says manifold gauges must not be used to measure vacuum and that a micron gauge must be used at all times.

How long should I run the vacuum pump?

Until you hit the manual's target. Don't go by the clock. Pioneer's WAS/WYS manual gives at least 15 minutes as a minimum, but long line sets, damp weather and skinny hoses all drag it out. The decay test afterward is what tells you you're done.

What does it mean if the micron reading rises after I close the valves?

If it climbs and then levels off, there's usually moisture still boiling off in the lines. Pull it down again. If it keeps climbing and never levels, you've got a leak, most often at a flare. Fix that before you open the service valves.

Do I need EPA certification to vacuum and open the valves on a new mini split?

Evacuating new, empty line sets and then releasing the factory charge from the outdoor unit doesn't vent any refrigerant. Adding refrigerant for a long line set, recovering it, or fixing a leak on a charged system falls under EPA Section 608, and that's work for a certified technician. Venting refrigerant is prohibited.