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Practical considerations for line-set length?

lightnb | Posted in General Questions on

Given the choice of a 15′, 48′ or 60′ HVAC lineset run (this is new construction), obviously the 15′ would be the “best”. Except, that puts the compressor right outside the master bedroom window. If length were irrelevant, I would place the compressor in a spot where it would need the 60′ line-set. The 48′ spot is a compromise. Could you please tell me what practical effect line-set length has on performance when you go from 15′ to 60′? Besides initial cost of the copper plus the refrigerant, what effect will the longer line have on my daily life? An extra $1 per day in energy? Do I need to upsize the compressor? Or…? I’m looking for something more specific than “shorter is better”. I’d like to know the (not-necessarily monetary) costs of putting the compressor 45′ (of line length) further away.

Thanks!

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Replies

  1. Walter Ahlgrim | | #1

    You have stumbled into a very complex subject in fact the installation manual for my unit devotes 9 pages to this subject and not once do they get into efficiency, capacity or operating cost. Of course the shortest biggest line with the fewest bends will work best. Understand the contractor is incentivized to use the smallest shortest as the cost difference is pure profit. Please note if your vapor line is too large the velocity of the flow will be too low to move the oil that circulates with the refrigerant and the compressor could run dry and be damaged. If the liquid line is too small the unit will move enough refrigerant to work. That is what the 9 pages are all about.

    My wild guess is that the size of the tubes has a very small if any affect on operating costs.

    With that said the most logical place for my heat pump happened to be as far as possible from the master bed room and I could not be happier.

    http://pts.myrheem.com/docstore/webdocs/Public/ServicePublic/Trouble2a/pdfs/HP/IO/92-105074-05-04_RP17.pdf

    Walt

  2. Ethan Foley | | #2

    The obvious answer is "it depends" like virtually every question like this. :D But that's not very helpful. The capacity penalty (and energy) is going to range from 0-10% usually. If your heat pump (or is just A/C?) is sized very close to the load, that could be an issue. I certainly wouldn't sacrifice aesthetics, sound, and maintenance to save a few kWh.
    If you post the make and model number of your heat pump, we might be able to give you a more specific answer, depending on the unit.

  3. Walter Ahlgrim | | #3

    Also please tread lightly if you choose to discuss this fine point with your HVAC man. Note the salesman is unlikely to have read the install manual and is likely make up an answer so as to not look like a fool. The installer tends to get insulted if someone tells them how to do their job. Try to ask questions and not make statements.

    Yupater do you have a link to lab tests for the same equipment with different line sets?

    Walta

    1. Ethan Foley | | #6

      Mitsubishi has a capacity correction chart for their minisplits, not many (any?) of the split DX systems do. See attached.

  4. lightnb | | #4

    From the manual it looks like 75' is ok up to 3/8" liquid line and 50' for 7/16" lines, without being considered a "long line set".

    4 90' bends adds 3.6 feet of equivalent length

    So if the equivalent length is between 50 and 75', it shouldn't make any difference at all as long as the correct line size is installed? Is line diameter chosen based on length, or based on compressor size? I would need to know if we need a 3/8" or 7/16" line to know if 50 or 75 is the limit.

    Also, are lines allowed to run diagonally through the attic across trusses (with a "substantial guard strip", like electrical wires)? Can they come in contact or be buried in attic insulation? That would affect the run path.

    This is heat pump and ac together. Probably around 2 ton for one unit and half to one ton for the other. Climate zone 4a.

  5. Expert Member
    Akos | | #5

    The expansion valve in most min splits is in the outdoor unit, this means the gas in the line set is near room temperature.

    For example if you are heating, there could be something around 50F to 60F delta between the line and outside air. With a standard 3/8" thick insulation around a line set, you are probably loosing around 10BTU/ft, so a 50ft line set might loose 700 to 1000BTU. Not a lot but something.

    There is nothing wrong with running line sets through the attic and burying it in insulation, this is a good thing as it reduces your losses.

    The one thing you need to watch for with long line sets is that they can expand a fair bit. This is more of an issue if you are running them through studs as you can get a fair bit of noise if the compressor cycles. Make sure there are either bends or bushings to allow for movement, most installation manuals show these details.

    I would not mount an outdoor unit near bedrooms and never mount them onto stud walls or roofs. They should be on solid masonry or on the ground.

    1. steven765 | | #7

      "I would not mount an outdoor unit near bedrooms and never mount them onto stud walls or roofs. They should be on solid masonry or on the ground."

      Rationale? I'm thinking of mounting mine on the back wall of a hallway just above a back porch roof to avoid snow complications and minimize the rise and drop of the lines.

      1. Expert Member
        Malcolm Taylor | | #8

        Stephen,

        Structure borne noise.

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