I recently bought a 10 year old house and noticed the insulation on the outside HVAC tubing had deteriorated. I replaced it with Duck brand foam insulation about a month ago. Today I noticed that it seems to have melted down where the smaller tube has touched it (picture attached).

The old tubing had the smaller diameter tube zip tied to the insulated larger tube but it had not seemed to have melted the old insulation.

So I'm wondering if I need a special kind of foam insulation or does the smaller diameter pipe get hot enough to melt insulation and should be kept away from the insulation?

Thank you.

Melted insulation

  • how hot does the pipe get?
    – jsotola
    Commented Jul 16, 2023 at 4:01
  • What is that smaller tube made of, and does it really have multiple joins (repairs?) along its length? Commented Jul 16, 2023 at 9:47
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    @Criggie that's a great point, one that I neglected to check. This is what I purchased: Duck Brand Insulating Foam Pipe Covers, 0.75-Inch by 3-Feet, Pack of 4 Sticks, 1285241 a.co/d/51lef9C and on reviewing the product page, they don't specify indoor vs outdoor use so I think it's safe to assume it's indoor only :/. I'll get some outdoor rated foam insulation and hopefully that is the only issue
    – ijlalk
    Commented Jul 16, 2023 at 13:27
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    @ijlalk even so two joints along that fiddly little bit is... odd. You might in fact be chasing a red herring looking at it: foam can be collapsed like that by plasticiser etc. exuded from some other piece of plastic, e.g. that grey drain tube or whatever it is. Commented Jul 16, 2023 at 13:54
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    That's not a gray drain tube. It's a flexible, water tight electrical conduit that houses the wires that provide power to the outside unit. Vary common for this type of use.
    – SteveSh
    Commented Jul 16, 2023 at 15:32

1 Answer 1


That insulation should be OK. I believe you may have another problem about to get you.

That small tube is for liquid refrigerant that supplies the evaporator coil (cooling coil) in your home. It should be warm but not hot. You might want to check your condensing unit (the one outside that blows out hot air) probably where that line connects to and be sure it is clean. That line being hot possibly indicates a high head pressure which will waste energy and shorten the life of the compressor. This is not inclusive there are other possibilities.

You may want to have an HVAC tech check the system.

Yes tell them that and measure the temperature if you can. Also note if the outside fan is running and if the air coming out is hot and about what temperature it is. You can also measure the temperature of the big line as it has the gas returning to the compressor.

  • Thank you for your response, I just checked the pipes this morning with the HVAC running and the small pipe did not feel warm, it actually felt cool. I will recheck it when it is hotter outside later today and if it's hot, call an HVAC tech. When I do, should I just tell them the liquid line is hot?
    – ijlalk
    Commented Jul 16, 2023 at 12:43
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    Note that in the case of a heat pump system, the small refrigerant could be carrying liquid in either direction, depending on whether the system is heating or cooling.
    – SteveSh
    Commented Jul 16, 2023 at 12:50
  • That is good to know! I did not realize that. This is a heat pump system.
    – ijlalk
    Commented Jul 16, 2023 at 12:59

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