The line set is the pair of copper refrigerant lines that connect the outdoor and indoor units of a split system. The larger line (suction line) carries low-pressure refrigerant vapor from the evaporator back to the compressor; the smaller line (liquid line) carries high-pressure liquid refrigerant from the condenser to the evaporator.
The suction line is insulated to prevent condensation and energy loss. Line sets typically run through wall cavities, attics, or crawlspaces between the indoor and outdoor units.
When swapping refrigerants (e.g. R-410A → R-454B), the existing line set must either be properly flushed of old refrigerant oil or replaced entirely — the oils used with different refrigerants aren't compatible and will damage the new compressor.
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