I have a Viessmann heat and A/C installation where a single large common outside unit located on the roof connects to split units on three separate floors.

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First question:

There is a pair of copper tubing leading to each floor (three floors, six connections). The diameters of the tubing are not identical. What is the reason for this? Also, what is the reason that there are two tubing connections required for each floor? (Provided my assumptions are correct that each pair connects to one floor.)

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Second question:

I don't particularly like the tape wrapped around the tubing (although the installation was performed by a certified Viessmann dealer). I am afraid there is heating/cooling loss taking place, and it also does not look good. Can you please comment? The installation is two-years old and the tape is fraying. Can you please provide pictures of how a professionally-sound installation would look like?

Thank you.

  • 1
    I do not have the expertise to give an actual answer ... but I feel like every time I've seen a split system install, the place where the lines enter the home has looked like total garbage, very similar in style to this, tape wraps and all. I don't know why. I've seen about 4 examples. Jul 27, 2023 at 22:33
  • 2
    @GlennWillen Because doing it properly takes longer, takes more effort, and they need to care.
    – Nelson
    Jul 28, 2023 at 2:00

1 Answer 1


A refrigeration system (air conditioning, and also heat pumps) pumps refrigerant. The refrigerant is in liquid phase in the smaller tube (the "liquid line") and in gaseous phase in the larger tube (the "suction" line). There are several reasons why the liquid line may be smaller than the suction line:

  • refrigerant is more dense in liquid form, so a greater flow can be accommodated by a smaller-diameter tube
  • copper costs money, so just-big-enough size is less costly
  • refrigerant costs money too, so it's desirable for the internal volume of the tubes to be minimized

The tape wrap is insulation -- in theory! Usually there is a foam sleeve placed over the suction line. Why only the suction line? That's the line that gets cold. Foam insulation helps keep humid air away from the cold copper surface, which helps reduce condensation on the surface of the tube. Condensation is liquid water which runs along the tube, drips somewhere, and causes water damage to the building.

If you search for something like "ac line insulation" online you'll find insulation products, installation methods, and maybe better key words to search for!

  • 1
    The installs I've done and had done have always insulated both lines on heat pumps. To add to this answer, I encourage OP to properly insulate these lines. This install is fine but lazy. There will be a wire with each pair of lines as well.
    – KMJ
    Jul 27, 2023 at 17:48
  • I cannot thank you enough for your high-quality answer. I was afraid that possibly, the exposed tubing was exposed for a purpose, and should not be covered up, but I surmise that's not the case, as you did not say so in your answer. If I may ask another question, and please humor my ignorance: Does the cooling take place in the outside unit, and is then transferred to the split unit? Thanks again.
    – Sabuncu
    Jul 27, 2023 at 18:21
  • @KMJ Many thanks for your clarification. You are correct; there are three wires leaving the outside unit heading to the floors. "This install is fine but lazy." That summed it up for me in the best way, it put my mind at ease, but I know there's room for improvement. Again, huge thanks.
    – Sabuncu
    Jul 27, 2023 at 18:23
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    Put 'how a heat pump works' in to your favorite search engine for better videos than any of us can put together in an answer. Short answer is that it takes advantages of the properties of the refrigerant to transfer heat from the air to the fluid in each machine, swapping the roles based on if you want heating or cooling.
    – KMJ
    Jul 27, 2023 at 18:31
  • 1
    @Sabuncu Yes, when indoor heating is wanted heat is extracted from the outdoors -- cooling happens out there -- and the heat is discharged at the indoor unit.
    – Greg Hill
    Jul 27, 2023 at 20:25

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