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.
Best Answer
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.