How to Check an Existing Line Set Before Reusing It

Why this matters

On a changeout, the line set is usually the one thing the crew would rather not touch. It runs through a finished wall or a packed attic, replacing it adds hours, and it "worked fine yesterday." Sometimes reusing it is exactly right. Sometimes it is the reason the brand new system leaks in October, runs a high superheat forever through a crushed suction line, or loses its compressor to the acid the old one left behind.

The difference is about an hour of checks done before the new equipment is brazed on. After the new unit is connected, every defect in the old copper becomes a warranty argument about the new unit. Before, it is just a line set that passed or failed.

This article owns the reuse check. If the old system died of a compressor burnout, the oil decision belongs to Flush Vs Replace Lineset Vs Filter-Drier After A Burnout: Decision Matrix, and this procedure stops at step 4 and hands off.

Before anything is cut

The old charge is recovered first, by a certified technician, to the evacuation level the federal rules require for that equipment; see related: EPA Section 608 Rules on a Residential Service Call. Confirm 0 psig on both lines with gauges before cutting. A line set "recovered" by somebody else can still hold vapor or nitrogen, and a tube cutter opening a pressurized line sprays oil and refrigerant at your face.

Cut with a tube cutter, not a saw. Saw chips go into the line and stay there until they reach a metering device.

Wear safety glasses and nitrile gloves for every step that opens or blows the line. Old compressor oil can be acidic, and acid oil on skin or in an eye is a chemical burn; rinse skin with water and an eye for 15 minutes and get it looked at. Nitrogen comes off a cylinder at over 2,000 psig, so it goes through a regulator with a relief device on the outlet, every time, and is vented outdoors.

The checks, run on one job

The job used throughout: a 2.5-ton R-22 split system coming out, a 3-ton R-410A heat pump going in. The existing set is 3/8 inch liquid and 3/4 inch suction, 34 feet of tube with three elbows, and the customer says the old compressor "was never replaced."

Here is the run, with what the checks below turned up marked where it was found:

 attic
  [air handler]==X========+=========================+
     crushed suction,     brazed coupling,          |
     about 4 ft out,      open attic, reachable     | down
     at the access path                             | the
 ===================================== ceiling      | wall
                                        about 9 ft  |
                                        of lift     |
                           6 ft outdoor insulation  |
                           crumbling from UV  ======+
                                           [outdoor unit]

1. Confirm the sizes against the new unit's table, not the old one's. Measure the outside diameter at both ends with a caliper or a tube gauge; a 3/4 and a 7/8 suction look alike in an attic. Then read the new unit's line-set table, using equivalent length (tape length plus the maker's allowance for each fitting), per Line Set Sizing, Length and Lift Field Card.

On the job: calipers read 0.375 inch and 0.750 inch. Using the card's illustrative values, 2 feet per elbow: 34 + (3 x 2) = 40 equivalent feet. The new unit's table lists 7/8 inch as the standard suction and 3/4 inch as an alternate limited to 50 equivalent feet. 40 is under 50, so the 3/4 line is allowed; any capacity correction the table prints for the alternate size goes on the ticket so nobody is surprised by it later. Lift is about 9 feet, outdoor below indoor, inside the table's limit.

If it fails: a suction line smaller than the table allows for the length means high pressure drop and lost capacity, in both modes on a heat pump, where the same line carries the hot gas in heating. (Oversizing past the table is the oil-return risk.) No check below rescues a wrong size. Replace.

2. Walk the entire run and find every joint and every crush. Follow the line from end to end with a flashlight. You are looking for two things: joints you did not know about, and places the tube is no longer round.

A joint you can see is a joint you can test and repair. A joint inside a finished wall, under a slab or behind a soffit is one you cannot reach if it leaks, and on a line headed for higher operating pressure that is a real risk. Crush points live where something else happened to the house: under an attic walk path, at the wall penetration where a siding crew nailed trim, at a joist where a later electrician pulled cable over the line.

On the job: one brazed coupling in the open attic, reachable, and one flattened stretch of the suction line about 4 feet from the air handler, where it crossed the attic access path. Calipers across the flat read 0.66 inch against 0.75 inch across the round, so the tube had lost 0.09 inch, 12 percent of its diameter, in that spot.

Decision rule: a crush on the suction line you can measure, where you can reach it, gets cut out and replaced with new tube and two couplings brazed under a nitrogen purge per Brazing With a Nitrogen Purge Standard. A crush you cannot reach is a reason to replace the set. Visible rippling or a kink on the liquid line is treated the same way.

3. Check what came out of the cut ends. Wipe the inside of each cut end with a clean white paper towel and look at it in daylight, then smell it.

Clear to light amber oil with no sharp smell is normal for a mineral-oil system. Dark brown or black oil, a burnt or sharp acrid smell, or sludge are signs of a burnout at some point in the system's life, whatever the customer remembers. An oil acid test kit on a sample turns a judgment call into a reading.

On the job: a thin film of light amber oil on both towels, no smell, acid test negative.

4. Stop here if the oil says burnout. Do not flush, blow or test further on the strength of this article. Route the decision to Flush Vs Replace Lineset Vs Filter-Drier After A Burnout: Decision Matrix, which owns it.

A clean result also raises the oil question that a changeout always carries: the old system ran mineral oil and the new one runs POE. Most makers accept a small residual of mineral oil in a reused set without flushing; some state a limit or require a specific procedure. Follow the new unit's installation instructions, and see related: Compressor Oils: POE, PVE and Mineral on the Service Call.

On the job: the installation manual permits reuse of a mineral-oil line set with normal residual oil, no flush required. Noted on the ticket with the manual's page.

5. Blow it through with nitrogen. With the regulator set low, a short burst of nitrogen through each line with the far end open into a rag outdoors. Nobody stands in line with the open end, and safety glasses stay on; whatever is in the line comes out fast. You are checking that the line is open end to end and looking for slugs of oil, water, or debris. This is not cleaning; it moves loose material and leaves the oil film behind.

On the job: a short spit of oil from the suction line on the first burst, then dry gas; the liquid line ran clean. Both lines clearly open.

6. Pressure test the line set by itself. Cap both ends (braze-on caps purged with nitrogen, or flare caps where the maker uses flares), then run a standing nitrogen test as described in How to Run a Standing Nitrogen Pressure Test on a New Line Set, staged, soaped and temperature-corrected. The ceiling is the lowest test pressure stated by the new outdoor unit, the new coil and any accessory that will be in the circuit; the line set will live in that system, so it is tested to that system's limits and no higher.

Why test it alone rather than with the new unit attached: if it leaks, you learn it before a single new joint is in the circuit, and the leak belongs to the old copper on the record. Test it with everything connected and a leak at an old joint reads, to the customer, as a leak in their new system.

On the job: the crush repair and both couplings were brazed first, so the repair was inside the test. The new unit's manual states 450 psig; 150 psig and 300 psig stages both soap-dry at the old attic coupling, the two new couplings, and both caps. At 450 psig, the line set held through a 30 minute hold after settling, with no change beyond what the temperature correction accounted for.

If it fails: find the leak with soap at test pressure. A leak at a reachable joint is re-brazed and retested from the first stage. A leak you cannot find or cannot reach ends the reuse.

7. Inspect the insulation along the whole run. The copper can pass and the insulation still fail it: compressed flat under straps, split seams, waterlogged in a crawlspace, or chalky and falling off outdoors after years of sun. Insulation can be replaced without replacing the copper by slitting new sleeve onto the line and gluing every seam.

On the job: attic insulation intact apart from the cut-out section; 6 feet of outdoor insulation crumbling from UV. Both replaced, the outdoor run covered, per How to Install and Insulate a New Line Set.

The reuse decision

Replace the set when any one of these is true: the size fails the new table for the length; a joint or a crush sits where you cannot reach it; the oil shows burnout and the burnout matrix says replace; the pressure test fails and the leak cannot be found or reached; or the new unit's instructions do not permit reuse for this situation. For an A2L unit, also check what the maker and A2L Equipment Install Requirements Field Card say about existing joints inside the occupied space before deciding.

Otherwise reuse it, with the repairs and corrections written down.

On the job: reused, with a 3/4 inch suction at 40 equivalent feet under the 50 foot alternate limit, one crushed section cut out and replaced, outdoor insulation replaced, and the line set pressure-tested alone to 450 psig after the repair. Each of those findings went on the ticket with its number, so the next tech can see the set was checked rather than assumed.

What a skipped check looks like a year later

Skipped the caliper check: a 5/8 suction left in place on a 3-ton changeout. It charges and runs, but the extra pressure drop costs capacity every hot afternoon, and the next tech, seeing suction pressure at the outdoor unit lower than expected, adds refrigerant to chase it.

Skipped the walk: the crushed section stays. On a TXV system it looks like a restriction or a weak valve, and the valve gets replaced.

Skipped the standalone test: the old attic coupling weeps at R-410A pressures it never saw on R-22, and the warranty claim lands on the new outdoor unit.

References

• New outdoor and indoor unit installation instructions: line-set table, reuse and residual oil provisions, test pressure
• See related: Line Set Sizing, Length and Lift Field Card; EPA Section 608 Rules on a Residential Service Call
• See related: Flush Vs Replace Lineset Vs Filter-Drier After A Burnout: Decision Matrix; Compressor Oils: POE, PVE and Mineral on the Service Call
• See related: How to Run a Standing Nitrogen Pressure Test on a New Line Set; Brazing With a Nitrogen Purge Standard
• See related: A2L Equipment Install Requirements Field Card