I've recently noticed a blue streak running down on the wall under my mini-split indoor unit. I cleaned it but it came back after a couple of weeks. So I disassembled the unit and found from where it was coming exactly.

I had water drainage issue in the past, but never from that side, so I suspected something else. I found that it was dripping from what I assume to be the refrigerant line. There's some sort of insulating foam around it and when I poked at the seam close to the streak it felt humid and a few drops fell off.

The drain pan seems okay, though it is hard to say with certainty that water isn't dripping from it to this line. I think this isn't refrigerant, but rather water with copper rust. So maybe just condensation on the line?

Is my hunch correct? How should I proceed in dealing with it? Is it a sign of something bigger and should I get a professional?

See the images below. I cleaned the streak so it doesn't show up much.

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  • 1
    Assuming you paid for installation by a legitimate installer, I'd call them back to make this right, as they should have done at install time.
    – Ecnerwal
    Aug 26, 2023 at 1:27
  • What @Ecnerwal said. See my answer for a description of what to do if this has been installed for a while and you need to fix it.
    – KMJ
    Aug 26, 2023 at 5:36

3 Answers 3


A tiny bit of corrosion on the copper lines is fine. You'll get less of it, and eliminate the liquid coming off the lines, if it's not possible for air to easily get in there to bring the moisture. For that reason, it's normal to wrap these lines and the insulation in tape once the leak testing is complete. Mitsubishi and other mini split manufacturers list it as a step in their installation guide, and often provide the tape. If your installer didn't install the tape, it's easy to add after the fact, at least to the ends of the line.

  • Get some appropriate tape. Non-adhesive, plastic impregnated cloth or PVC tape is common. It's sometimes called 'mummy tape' and should be available from your local HVAC supply, or online.
  • Unlatch the indoor unit and tip it up. A second person to assist with this task may be helpful.
  • Start all the way at the unit end of the refrigerant lines, as close to the unit as possible. Thread the tape between the two lines, then wrap it over itself a few times to hold it in place. Continue wrapping down the lines, pulling moderately tightly as you go and folding in any loose pieces of insulation. Once you get to the wall, wrap the tape under itself to tie it off in a knot.
  • Clip the unit back down in to place on the wall.

This should mostly stop air from getting in to the line set insulation, reducing the ability for water to condense in there. It will also improve the effective R-value of the insulation by preventing air exchange.

  • 1
    I went with this method to do it myself as the unit was installed 4 years ago. The refrigerant line was too tight to both the wall and unit to have a continuous tape rolling around it, but my HVAC tape was slightly adhesive so I did it bit by bit. Hopefully that fixes the issue, but I'm thankful for the instructions so that I could at least try myself! Aug 26, 2023 at 17:26
  • If you keep the air mostly away from the lines, you should get the expected effect.
    – KMJ
    Aug 27, 2023 at 2:44
  • And the unit pops free of the bracket to get better access to the lines - or at least I've never seen a unit that didn't, and since this one appears to use the standard 'lines across the back' install direction it should work that way.
    – KMJ
    Aug 27, 2023 at 2:44

The refrigerant and oil that goes in those mini splits are clear unless they contained dye used for finding a leak. If the unit is still cooling, chances are it's not the refrigerant. You, or a servie rep, should pull the unit away from the wall and find the problem. It could be that the insulation around the pipes has loosened. You need to get the corrected.


The most popular sealant that is used on the line connections is blue.

If the unit is cooling well, It is possible that the bit of condensation you found is mixing with some wayward sealant.

Insulating the lines better and wiping away the blue sealant should correct the problem.

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