Re-charging a Fujitsu AOU18RLQ Mini-split

WARNING DISCLAIMER! This post is about a DIY configuration of an AC system that is intended for educational purposes only. Any liability is on you.

The methods described herein can not only damage the environment by releasing dangerous gasses, cause catastrophic damage to you by coming into contact with these gasses which can just about instantly freeze fingers or eyes, but also destroy your equipment. Be smart and call a local professional who knows about these things. Read the comments for some really good advice and further warnings. 

We’ve had our Fujitsu AOU18RLQ Minisplit for about 7 years.

It’s one of those things that you always mean to get to, but prioritize something else. I knew it wasn’t cooling as well as it should, but us humans just get used to things sometimes. It was time to do something about it.

There are two major components to maintaining mini-splits:

  1. Cleaning the indoor head unit
  2. Recharging the refrigerant gas (R410A) in the outside condenser unit

I will be covering how I re-charged the condenser unit only.

The best thing in the world to have is a remote temperature gun. You point the laser at the target and you read the temperature. I own this one. There are dozens of them out there.

When you get it you will spend at least an entire day taking temps of everything. You can also turn the laser off to temp your pets. The laser will blind, so be careful.

It was 98 degrees with about 80% humidity and I temped the inside of the exhaust of the head unit.

The cool air was 82 degrees! Something was definitely wrong because I had a delta of 16 degrees between outside and inside. So no wonder our electric bill was so high last month!

Something had to be done! I did a ton of research and then called for estimates.

“It’s out of warranty. The problem isn’t gas pressure.”

“It’s gas pressure. $875 to re-charge the system plus time on site, etc, etc. It will be two weeks.”

On it went. People in the ‘burbs will pay a lot less, but spending over $800 was not going to happen.

After much googling, and watching endless youtube watching, I decided that I would buy a full tank of R410A and an AC manifold – there was nothing really about DIY minisplit charging, so getting semi-pro equipment seemed the way to go.

I spent much time looking for these, hemming and hawing. If I was going to spend $800 it would be on equipment that I can maintain the AC system with and DIM – Do It Myself.

I stumbled on this site and searched R410A and luckily went to page 2. And there it was:

A mini-sized bottle of R410A for $44!

I then found this to connect it to the condenser.

r410a gauge

After doing much more reading, I wasn’t sure of the size of the coupler I might need, so I called and spoke to the owner – sorry, I can’t remember his name, but he was so nice and helpful. He said that buying it without it is a mistake, so just get it. For $4 I agreed.

5/16 to 1/4 adapter

A week later I had my whole kit (can only be shipped ground so YMMV).

Here is the process he told me to use:

  1. Take off the cover to reveal the charging port. Minisplits do not have a high pressure charging port, only a low pressure one. It should be a three-way valve:
  2. Screw the tank and charging valve together.
  3. Wear heavy gloves
  4. Take off the cap (where the arrow is pointing) with a C-wrench
  5. Test if you need the coupler. If you do, attach the coupler to the charging hose. Use pliers and a wrench to make a good connection. It’s brass, so don’t go crazy. Practice in your mind how you will screw this on to the valve. It is a schraeder valve like a car tire valve, so pressing in will open the valve.
  6. Attach the charging assembly to the port as quickly as you can and turn on the unit. The gauge you are using may only show the correct pressure once it is running!
  7. Look at how low it was!
    1. I called my SO who was inside and had her turn on the unit. It must be running on High Cool (not auto – High and low temp setting, maybe 65 degrees).
    2. After 5 minutes she temped the output of the inside unit (aim the laser deep inside the bottom part): 75 degrees (outside temp was close to that)
  8. Fully depress the charging button (you can see it to the left of the gauge for a ten count. The pressure might jump into the blue or yellow at this point.
  9. Stop. Let the unit run for 2 minutes. Output temp was now 72 degrees and I could feel a rise in the condenser fan heat. The pressure gauge will come back down.
  10. Repeat this process until the pressure stabilizes in the blue area. The sound of the compressor will change as more gas is added to the system. Don’t rush it. 10 seconds, let it run, repeat.
  11. My system took the entire tank:
  12. The proof is in the pudding: After running the system for a good ten minutes I temped the output: 58 degrees. FIFTY EIGHT! That’s a significant difference. A couple times I read 55!
  13. Your gloves are still on when you quickly unscrew the valve and replace the cover.
  14. Run the system for a good 30 minutes (if you plan to turn it off). Otherwise, enjoy much higher efficiency and cold air instead of lukewarm.

You’re welcome.


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