Reusing an R-22 Line Set for an R-410A Changeout
A contractor is replacing a failed 3-ton R-22 condensing unit and coil with a new R-410A system in a two-story house. The existing line set is 40 feet long with 20 feet of vertical rise from the outdoor unit to the indoor coil, a 7/8-inch suction line and a 3/8-inch liquid line, and it is routed through finished walls with no accessible joints. The old system had a slow leak that was never located, and the line set has no filter drier and no oil traps. The homeowner wants to keep costs down and asks whether the old lines can be reused. The new unit's installation manual lists a 3/4-inch suction line for a 3-ton system at this length and requires a filter drier and evacuation to 500 microns.
1. Should the existing line set be reused? Analyze the technical and cost factors.
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Reuse is possible but conditional, and the slow leak is the deciding factor. R-22 systems circulate mineral oil; R-410A requires POE oil, which is hygroscopic and will carry residual mineral oil and acid if the old lines are contaminated. The 7/8-inch suction line is one size larger than the manual's 3/4-inch recommendation, which reduces pressure drop but also lowers refrigerant velocity, so oil return over 40 feet with a 20-foot rise becomes a concern. The undocumented leak may be in the buried or concealed line set, in which case reuse simply moves the failure forward. The honest answer is that reuse can be approved only after the line set passes a nitrogen pressure test, is cleaned with an approved flush, and the manufacturer's line-set table permits the size. If the lines cannot be verified, replace them.
2. If the lines are reused, list the required procedures and explain why each matters.
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Pressure-test the line set with dry nitrogen to find the old leak, since a leak that is not located will reappear. Flush with an approved flushing agent and blow out with nitrogen to remove mineral oil and debris. Braze with nitrogen flowing through the lines to prevent internal oxidation, which forms flakes that clog the TXV and filter drier. Install a new bidirectional liquid-line filter drier rated for R-410A, because the old system had none. Add an inverted trap or oil-return consideration at the top of the 20-foot rise if the manufacturer requires it, given the oversized suction line. Evacuate with a micron gauge to 500 microns or lower and hold, then weigh in the charge by nameplate rather than guessing. Each step targets a documented failure mode: leak, contamination, oxidation, moisture.
3. What startup checks prove the installation is correct before the technician leaves?
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Verify the charge by subcooling at the liquid line and confirm superheat responds, rather than relying on gauge pressures alone. Measure the temperature split across the coil; for a 3-ton system with correct airflow it should land near 18-20°F. Check total external static pressure against the air handler's rated maximum and confirm cfm near 1,200. Measure compressor amperage against nameplate, check the suction line for oil return and frosting at the riser, and confirm the condenser has adequate clearance. Verify the filter drier is installed in the correct direction, the condensate drain traps and drains, and the disconnect and breaker are correctly sized. Document the micron level, final charge, and all readings, and give the homeowner a written explanation of what was replaced and why.