Re-charging a Fujitsu AOU18RLQ Mini-split

Published on Author JF25 Comments

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.

25 Responses to Re-charging a Fujitsu AOU18RLQ Mini-split

  1. Stupid question…..I have 4 indoor units connected to my single outdoor unit. Do I have to fill each low pressure connection, or do I only need to worry about filling one, and it’ll disperse the coolant to each indoor unit on it’s own?

  2. Holy smokes.

    I’m a huge DIYer and IT guy by trade but this is so stupid wrong it’s unbelievable.

    Mini splits are charged by weight and WEIGHT ONLY. You need a can of 410a and a scale. The system should be recovered and a vacuum pulled. The base charge will be on the sticker on the side, and any additional charge is calculated by additional feet of lineset ( I was googling for that spec and stumbled across this treasure trove of bad advice).

    Secondly, these things don’t need a recharge unless something is wrong and there’s a leak. And if there is, you should hit it all with some soapy water and find out where the leak is and then rinse off the soapy water.

    The most important part of all this is that the system should be pulled down with a vacuum pump, but your issue is that if there’s a leak, it’s pulling contaminants into the system. That’s why you should leak test with nitrogen first, fix any leaks, and then vacuum to a low micron count like 500 or so.

    This all is why a proper job from an hvac person costs 4-800 dollars and why I got into doing it myself and for my company internally. That said, I’ve got about $2500 tied up in manifolds, scales, vacuum pumps, nitro tanks, etc.

    Or do it the wrong way and potentially burn up a $1500 plus minisplit, and then have to pay double that to have a pro replace it correctly.

    • Disregard E. as he’s evidently doing too many “Holy smokes”.
      The system was low on refrigerant and still functioning…there is no way for it to “pull contaminants into the system” as the internal pressure is higher than atmospheric and contaminants can’t flow against pressure.
      Minisplits CAN be charged by weight. They can ALSO be charged by superheat calcs. The gauge is a poor-man’s version of that…albeit not perfect. JF got the better bang for the buck than E. Or maybe E just wanted to rant because he’s proud of how much he spent on tools.

      • Please DO NOT disregard E. I am in the field and have actually been through Fujitsu training. He is absolutely correct Fujitsu should ONLY be charged by weight. There is no such thing as charging them by superheat because they do not have fixed metering devices they modulate depending on many factors resulting in a change in superheat constantly. If you have to add there is certainly a leak and you will be adding again shortly if you do not address and fix the issue.

        • So far, so good a year later. Again, the cost in NYC of this, especially during the Summer and where the unit is located is just not feasible.

          I’m not arguing with you, though – you are correct that this should be done by a professional, but sometimes it just doesn’t make any sense. If the unit was under warranty we would, but it is getting on 9 years old.

  3. To add, because I’m sure you believe I’m wrong and that the neat little gauge told you that it’s right – that gauge is fooling you. There are metering devices on both the high and low side of a minisplit whereas a traditional split (central ) only has a metering device on the liquid side at the indoor coil. So the low side is always going to have some off and low reading, and then boom there’s too much liquid and it hits the compressor and locks it up.

    Granted you probably cannot do that with that little can, but again this is why they are charged by weight because you don’t know how much is sitting behind the compressor. You also typically charge with liquid and have the can upside down, but who knows what’s actually in that little can.

    • I get that you’re laughing at his tools and supplies, but I don’t get what your legitimate concerns are. His A/C is demonstrably cooling much better after his DIY; are you concerned that the system will have less life going forward, compared to if it had been done as you describe? Or is there some kind of immediate danger to his safety?

    • I get that you’re laughing at his tools and supplies, but I don’t get what your legitimate concerns are. His A/C is demonstrably cooling much better after his DIY; are you concerned that the system will have less life going forward, compared to if it had been done as you describe? Or is there some kind of immediate danger to his safety?

  4. And to answer Joe’s question. No, only one port gets charged and there is a single accumulator with a single compressor. Again, each outlet has a metering device.


    • All good points and buyer beware.

      Where I live, the maintenance would have been $1,500, minimum, and a 2-3 week wait. In addition, the original installation has lines running on the side of a building 5 stories up.

      Since the unit is getting on 8 years old and we had another burn out (on it’s own – no servicing done), we opted for the cheap fix. So far, so good.

  5. Hi JF, thanks for sharing how to charge the Fujitsu condenser unit. I have a 8y rs old condenser unit AOY30LMAW4 and I need to charge R410a gas every 6 month. Obviously there is a leak but sadly the leak seem to be coming from a section of the copper tubing concealed in my false ceiling. I own a hydraulic piping company and does high and low pressure testing and flushing services and so I consider myself qualified to troubleshoot a leak. I search the internet to find scraps of information on how to charge the condenser unit. What you describe is exactly what I did except that I didn’t checked the temperature difference.

    Whenever the Fujitsu technician come to service my condenser, I would observe what they are doing and asked questions on how they do the charging. I questioned them why they were charging my condenser not based on weight but rather on the pressure gauge reading. Their answer was always ‘by right’ you should do this but ‘by left’ they charged based on pressure reading! After paying expensive ‘tuition fee’ to learn the procedures of charging, I took the plunge and bought the 25lbs R410a gas cylinder and basic charging manifold kit. I did what you did except that I stop charging at about 120 psi. This simple DIY saved me a few hundred dollars a years!

  6. I have just installed a 2 zone Fujitsu mini split yesterday, A friend of mine who had installed 3 on his home (single zone units) 5 years ago gave me a hand. We vacuumed the system shut off the pump and slowly opened the units refrigerant values. We heard the refrigerant hiss from one of the zone valves but not the other, then we leak tested all the lines with very soapy water no leaks. Point is the zones are not blowing cold air and no condensation is dripping from drain tubes. Could someone reply with trouble shooting tips.

    • Greg: did you open both valves for each zone? There is one in both high pressure and low pressure lines. Can you hear the pump running and is the fan running on the outside unit?

  7. An accurate charge is VERY important in any AC or Refrigeration system. Even more so now with 410A systems. Older R-22 units had some wiggle room regarding charge, but in 410A systems, it is critical to not be over or undercharged. Either one can cause damage to the compressor. Overcharging will flood the compressor.
    That is why most manufacturers require you to charge by weight (using the precharge and calculating the extra based on line set length).

    On ductless systems, the most common culprit for leaks, is the flared fittings. Either they were not flared correctly, not tightened enough, or over tightened… and the majority of the time its the fittings that connect the indoor head to the lineset.

  8. I did the full vacuum (2hrs), tested for leak with nitrogen and filled to weight spec (7.4 lbs) Not getting as I cold as it should. Do any of you know how much extra refrigerant is too much on a 36k unit? How much refrigerant is lost when diconecting the hose on average?

  9. Critical Charging ( weighing the refrigerant in ) and doing the proper math per additional feet of lineset is the only way to charge a minisplit or multi zone minisplit. Yes you may have gotten the air to come out at 55 or 58 degrees. But it may be missing refrigerant or over charged if that is what you are basing it off of. You may have a compressor burn out soon if its running to high on head pressure or a unit that doesnt cool that great when its 100 degrees outside. All manufacturer manuals are available online at their website just look for them.

    • Hi John,

      As I’ve said before: I totally agree.

      However, based on where we are and the life of the system, how shoddily it was installed, and the unbelievable maintenance and installation costs of a new system here in a big city, we’re gambling and going this route.

      2 years on and things are going well.

      If I lived in the ‘burbs I would have let a professional deal with it as, quite frankly, anyone reading this article should.

  10. My only question with the method you used to recharge it, how does the air that is in the blue hose connected to the can not get into the system? I understand once you connect the hose to the charging valve the pressure comes into the hose, but where does the air that was in the blue hose go? It seems when you start to do your ten second pulses to add the refrigerant, the air that was in the hose would be pushed into the line set. Am I missing something about the function of these charging cans?

    • Did you mean, “…. blue hose connected to the can not get into the system? I only know the basic mechanics (as I said, I’m not a pro): The “low” side is the refrigerant that is coming from the evaporator (the head unit inside your house in this case) and is on its way back to the outside compressor.

      So, I assume that the pressure of the bottled refrigerant is higher than the low side AND that since the condenser creates a vacuum of sorts, it is “sucking” the added refrigerant into the system (that’s why the system needs to be running when adding the refrigerant ). Here’s a good reference for it:

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