Line set
The pair of insulated copper lines that carry refrigerant between your indoor and outdoor AC units.
The line set is the two copper tubes — one larger and insulated (the suction line), one smaller (the liquid line) — that carry [refrigerant](/glossary/refrigerant) between the indoor coil and the outdoor condenser. During an AC replacement, installers either run new copper or flush and reuse the existing set, but reuse isn't always allowed: switching refrigerant types (such as the R-410A to R-454B transition in new 2025-and-later systems) can require new or thoroughly cleaned lines to avoid oil and pressure incompatibility. A kinked, undersized, or contaminated line set quietly robs efficiency, which is why it's a real line item — not an afterthought — in a [central AC replacement quote](/guides/central-ac-replacement-cost).