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On every ductless mini-split air conditioner I've ever seen, both the high and low pressure lines are insulated between the compressor and the building.

It seems like the liquid refrigerant coming out of the condensing coil can never be cooler than the outdoor ambient air temperature because the outdoor air is what is cooling it in the condenser coil.

It can, however, be warmer than the outdoor ambient temp. Therefore, it seems like leaving that high pressure line coming out of the compressor uninsulated would at worst save costs for some insulation, and at best give the high pressure refrigerant some additional cooling before it gets back to the indoor unit.

What am I missing here?

NOTE: I can think of some reasons why you'd want to insulate that line on the inside of the building where the high pressure liquid refrigerant would be warmer than the ambient indoor air temp.

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Many AC units can be run in reverse as a heat pump. When that is the case, the high and low pressure sides are swapped.

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I can see the outside of an airconditioner, bought more than ten years ago. It is not an inverter, but it does heat in the winter. There is one well insulated tube entering the wall ( and a water tube coming out).

For heat pumps it makes senseto insulate well both lines, since the use is reversed in winter, and I do not think they any longer sell non heat pump air conditioners.

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There is no metering device in the evaporator coil. The outlet line on a mini-split does not contain subcooled liquid, it contains vapor. The metering device (TXV) is in the oudoor unit, so it vaporizes the refrigerant in the outdoor unit, pumps the saturated vapor to the indoor unit, where it absorbs heat (superheat) and moves to the compressor to be condensed in the coil. This configuration allows the indoor coil modules to operate on refrigerant or chilled water.

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    $\begingroup$ I don't really think this answers the question about insulation. Can you take a look and update as needed? $\endgroup$ – Brandon Enright Apr 9 '15 at 17:30

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