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Cooling · InstallationAC Line Set Replacement
AC line set replacement swaps the two copper lines between the outdoor unit and the indoor coil: a small liquid line and a larger insulated suction line. Replace them when the diameter does not match what the new unit's manual calls for, when they carried R-22 and cannot be flushed clean, or when they are kinked, patched or fail a nitrogen pressure test. Lines that pass all three checks stay.
Line sets are sized, not guessed. Most residential systems use a 3/8-inch liquid line with a 3/4-inch or 7/8-inch suction line, and the installation manual gives the exact pair for each tonnage and run length. A suction line one size too small adds pressure drop, and the system loses capacity on the hottest days, when it has none to spare. One too large on a long vertical rise slows the vapour until oil stops travelling back to the compressor.
At changeout the old lines have to pass three tests. First, history: R-22 ran with mineral oil, which does not mix with the oil in current compressors, so those lines are flushed and proven clean or replaced. Second, condition: no kinks, flattened bends or patched sections. Third, a nitrogen pressure test held at the pressure the new equipment specifies.
Rub-through is the usual leak: copper touching a foundation sleeve, a duct strap or its partner line vibrates with every compressor start and wears a pinhole over years. Lines buried in a finished basement ceiling or a wall are the hard case: if they test tight and match the size, they stay. If they fail, the new set is usually routed along the outside wall under a line-hide cover and through a new sealed penetration, which costs less than opening and repairing drywall.
Insulation matters as much as the copper. The black foam on the suction line keeps it from soaking up heat and from sweating inside walls, and sunlight breaks it down at the outdoor unit until it crumbles and leaves bare pipe. Replacing that section with UV-rated insulation, or enclosing the run in line-hide, is a small job that can be done without opening the refrigerant circuit. In older brick houses with no ducts on the upper floor, where the lines climb an exterior wall to an attic air handler, the cover also protects a long exposed run.
Replace the line set when
- The new unit's manual calls for a different suction or liquid line diameter than what is installed
- The old system ran R-22 and the lines cannot be flushed and proven clean
- A leak search finds an oil stain or pinhole where the copper rubs on masonry, strapping or the other line
- The copper is kinked or flattened, often at the bend behind the outdoor unit
- The lines have been patched before, or the run has to be lengthened for a unit that is moving
How it works
Length, vertical rise, number of bends, where the lines pass through the wall and what they cross inside. The sizes are checked against the installation manual for your tonnage, and the existing lines are pressure-tested if reuse is possible.
Refrigerant is recovered by an ODP-certified technician. New lines are run with as few joints as the route allows, supported and kept off anything they could rub against, brazed with nitrogen flowing so no scale forms inside, and fitted with a new filter-drier.
The new lines hold a nitrogen pressure test, then a vacuum is pulled below 500 microns and proven to hold. Charge is weighed in with the allowance for line length from the manual, superheat and subcooling are checked, and the wall penetration is sealed.
Common questions
Can the old line set be reused with a new air conditioner?
Yes, when it is the diameter the new unit specifies, has no kinks or patches, and holds a nitrogen pressure test. R-410A lines moving to a new R-454B or R-32 unit are commonly reused on that basis where the manufacturer permits it; the A2L refrigerant transition explains the change. Lines from an R-22 system need flushing first, and a short exposed run is simpler to replace.
What does a new line set cost?
The price follows the route more than the copper. A short run through an unfinished basement to a unit on the same wall is the least work. Length, a second storey, a finished ceiling, a new wall penetration, line-hide cover and the refrigerant recovery and recharge all add to it. You get one written figure after the run is measured; pricing explains how quotes are put together.
How long can the lines be?
Every manufacturer publishes a maximum length and a maximum vertical rise for each model, and long runs can call for a larger suction line and extra refrigerant. Most condensers are factory-charged for a short standard run, with the added charge per metre listed in the manual. Past the published limit the unit is outside its design, so a longer route means a different location or different equipment.
The foam on the outdoor pipe has crumbled. Is that urgent?
Not urgent, but worth fixing before July. A bare suction line picks up heat between the wall and the unit, which costs a little capacity, and it sweats, which can stain siding or drip inside a wall cavity. New UV-rated insulation on the exposed section is a quick repair, often done during a spring tune-up. On a cooling-only system the small liquid line is normally left bare.
Do the lines get replaced when the unit is moved?
Sometimes. A move of a metre or two can use a brazed extension if the existing copper is sound and the total stays inside the manufacturer's limit. A move to another wall usually means a new set, because several joints in an old line are several places to leak. AC unit relocation covers the rest of that job, and a full air conditioner installation always includes the line decision. Ask for a quote.