This article includes significant contributions from Neil Comparetto and Brian Chadwick, both of whom are HVAC technicians in Virginia. Brian is also the owner of Chadwick Air, and Neil is a co-owner of Comparetto Comfort Solutions and a longtime contributor to HVAC School. Thanks, guys!

DISCLAIMER: A lot of the information in this article is based on field experience and does not appear to have a peer-reviewed scientific basis. Please DON’T treat this article as a definitive source of information. It is merely a collection of experiences and possible explanations for those experiences.

Photo Credit: Brian Chadwick

The residential HVAC technicians in our audience may be familiar with the following scenario:

You’re on a service call for a ductless system that isn’t working properly. You take your superheat and temperature readings. The system appears to be low on charge. Of course, your first step is to do leak tests around the obvious places, like connections and flare fittings, but you find nothing.

Then, you rip the white line set insulation open and see dark brown or black copper. Maybe there are even some green spots. According to the customer, the system is less than five years old. However, the copper looks like it’s well over 50 years old. Something’s not right. You perform an electronic leak test along the line—it squeals. When you do a bubble test on the copper, you see bunches of microbubbles. (Look at the image up top.)

Why the heck was that happening?

I’m sure some of you already know the answer. We knew the system leaked, but why did it happen inside the insulation? Wasn’t the insulation supposed to protect the copper from precisely that?

The short (but unsatisfying) answer is that the copper line set has corroded. The corrosion causes pinholes to appear in the copper, which is where the leaks come from. Despite being protected by fancy polyethylene (PE) sheaths, copper line sets often corrode and leak beneath the squishy barrier. Seems a bit strange, right?

The even stranger part of the situation is that several ductless systems are only a few months to a few years old when they start leaking due to corrosion. In other words, these corroded tubes aren’t ancient, so we wouldn't expect them to have leaks and corrosion. So, why is it so common? What can we do to stop it from happening?

Although there isn’t much definitive information on the causes of these issues, we’ve still noticed some trends that can help us understand and repair line set corrosion.

Sticker situation

The first clue is right on the box. Some of you may have noticed that some mini-split line set manufacturers have an interesting sticker on the box:

Photo Credit: Neil Comparetto

The stickers are yellow and tell you to seal both ends of the line set. Maybe you’ve seen a similar sticker and found it odd. Perhaps you didn’t question it at all, or maybe you haven’t come across it. Regardless, that yellow sticker is our starting point.

Why would we need to seal both ends of a line set? That’s not typical practice, so why would the manufacturer give those instructions?

Sealing the line set prevents moisture from getting in. If we look a bit at the chemical properties of PE and copper, we might be able to figure out why sealing the line sets at both ends is so important and what it has to do with corrosion.

Basic [more like acidic] chemistry

Although oxygen can pass through pure polyethylene quite easily, moisture cannot. Remember, that’s why the manufacturers recommend sealing both ends; it’s to cut off moisture’s access points. Copper cannot corrode without oxygen and moisture.

However, moisture alone doesn’t cause corrosion to the degrees that technicians have been seeing. So, there must be a reason why moisture must be kept out of the system.

There’s a theory—again, it’s just a theory—that some line set manufacturers’ white PE insulation reacts with moisture to create an acidic environment around the copper line set. Remember, you can determine if a substance is acidic based on its pH. Water is neutral and has a pH of 7. Anything with a higher pH, like soap or bleach, is basic. Anything with a lower pH, like lemon juice or vinegar, is acidic. 

So, you have a clear idea of what an acidic substance is. Mild acids irritate your tongue at least a little bit, so you could probably imagine the damage that acids could inflict on copper. THAT is likely the primary source of severe corrosion, not moisture and oxygen alone.

How do we know that the insulation is the culprit?

That’s the problem. We don’t have a definitive way of knowing what the insulation contains and if it is responsible for the corrosion or not. However, one undebatable fact is that various line set manufacturers have starkly different rates of corrosion. 

The purpose of this section is NOT to disparage or endorse any manufacturers. This segment is an overview of real technicians’ experiences and research on a common issue.

Brian Chadwick has seen corrosion on the bodies of quite a few PDM-brand pre-insulated line sets, but the fittings sometimes have issues as well. In those cases, the symptoms are like those of leaky flares. The image below shows an O-ring failure on a pre-insulated line set. (The original fitting was from RapidLock while the brand was still under the ZoomLock name.) A new ZoomLock fitting ended up solving that issue. Other than the line set corrosion and O-ring failures, those systems that Brian worked on had no other apparent issues.

Photo Credit: Brian Chadwick

It's worth noting, however, that PDM does not just manufacture and sell pre-insulated line sets. They also produce and sell the PE insulation itself. That means that it is possible (but not certain) that other pre-insulated copper companies may source their PE insulation from PDM. (Alternatively, PDM and other companies could possibly source their insulation from a common PE manufacturer.)

When Neil Comparetto did some investigation, he discovered that Isoclima/Isopolar line sets also had many technician reports of corrosion. Considering that some manufacturers use other companies’ parts and that Isoclima is not an American company, the quality of their copper and insulation is difficult to judge. One thing is certain, though. Many Isoclima line sets have corroded very shortly after being installed. (They corrode within months, if not straight out of the box.)

Mueller, a different manufacturer, doesn’t appear to have these corrosion problems to NEARLY the same degree as Isoclima. However, Mueller isn’t perfect; you will still likely come across line set corrosion with their PE-insulated (white) line sets.

Another potential (but not proven) risk factor for corrosion is having the insulated line set near high concentrations of VOCs (volatile organic compounds). According to an article in ACCA’s Consumer Education series, VOCs contribute to formicary corrosion in copper evaporator coils in tight, poorly ventilated homes. Many VOCs in common household products, like formic or acetic acid in cleaners or solvents, interact with moisture and oxygen to corrode copper evaporator coils. It is possible that the same principle may apply to copper line sets.

How can we know if we’re dealing with corrosion without opening the insulation?

The telltale sign of corrosion is a leak. If a system has an unexplained low charge or other problems that could be caused by leaks, you’ll want to do your typical leak test procedures. If you don’t find any leaks in the obvious locations (such as connections), then it’s likely that the leak is inside the tubing itself, and you’ll have to expose the line set.

Acid makes little pinholes in the copper inside the insulation, many of which won’t be visible to the naked eye. You may see indications of leaky areas if you open the insulation and see green spots. A greenish color indicates the most severe instances of copper corrosion (think about old pennies). However, that doesn’t mean that there will be a leak in every green spot (or that leaks won’t occur in the dark brown regions).

Electronic leak detectors can usually pick up those tiny leaks from the pinholes. Neil likes to slit the insulation jacket and run an electronic leak detector over the opening. It can usually detect a leak in the line. From there, you can expose the line set and pinpoint the leak on the line.

Of course, once you do that, you can always spray the line set with some Big Blu or other leak detection fluid. Then, you just sit back and watch for bubbles to form.

So, what can we do now?

Most of the PE-insulated line sets seem to have fundamental issues. As such, we can only do so much to reduce corrosion risks during installation or maintenance. 

The best thing you can do is seal both ends of the insulation with caulk or sealant to prevent moisture from coming in. That way, moisture can't interact with the insulation and form acid inside. The sealant should be waterproof, UV-resistant, and have a strong adhesive. The yellow stickers recommend Polyken-936-30 tape. However, the adhesive isn’t usually strong enough to stop moisture and air from coming in. You’ll be better off if you try a heavier-duty caulk or sealant.

Photo Credit: Neil Comparetto

However, you can replace the line sets and use sturdier materials. Again, I should mention that this next practice is only based on personal experience and doesn’t have a scientific basis. 

Some members of the HVAC School Facebook group have said that the black insulation has had far fewer leakage and corrosion problems than the white jackets. Black sheaths are made of elastomeric insulation, which is completely different from white PE jackets. 

If possible, you may want to consider replacing corroded line sets with new ones and using black elastomeric insulation instead of the white PE-type. Neil has been using the Mueller Duraguard UV black line set insulation. (K-Flex is the labeled insulation manufacturer, and the insulation is branded as K-Flex Titan.)

As you can see, there isn’t a straight answer as to what is really going on with those PE-insulated line sets. Still, the trends and the collective frustration in the residential HVAC side of the industry don’t lie; there is a problem, and we’d like to find a way around it.

All we can do is be aware of the issues that certain line sets have and listen to what others in our industry have done to combat them. For now, it might just be worth your while to get your hands on a high-quality caulk or give the elastomeric black insulation a try.