Reusing an Acid-Contaminated Line Set for an R-410A Replacement
A contractor is replacing a failed 2-ton R-22 heat pump with a new 2-ton R-410A unit. The existing line set is 35 ft long with a 20-ft vertical rise; it is routed inside a finished wall and cannot be visually inspected. The line sizes are 3/8 in. liquid and 3/4 in. vapor, which match the new unit's published allowance for a 35-ft run. The old system used mineral oil and had a burned-out compressor; the circuit had a significant acid smell. The homeowner wants to reuse the line set to avoid opening the wall. The new outdoor unit has a factory charge for 15 ft of line. The manufacturer permits line-set reuse only if the lines are clean, pressure-tested, and evacuated to 500 microns with a decay test. The contractor has nitrogen, a vacuum pump, a micron gauge, and a replacement filter drier.
1. Should the contractor reuse or replace the line set? Justify the decision using the contamination and accessibility facts.
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The contractor should replace the line set. The old compressor burned out and the circuit had a strong acid smell, so residual acid and mineral oil are likely trapped in the lines. R-410A systems use POE oil, which is hygroscopic and can carry acid and debris through the new compressor. The line set is inside a finished wall and cannot be visually inspected, so the contractor cannot verify cleanliness or wall thickness. The manufacturer's reuse allowance is conditional on clean lines, and that condition cannot be proven here. Avoiding wall work is a cost concern, not a technical justification. Reusing an inaccessible, contaminated line set risks repeating compressor failure and would not satisfy the manufacturer's installation requirements.
2. If the line set is replaced, list the commissioning steps and calculate the additional charge beyond the factory 15-ft charge.
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Install the new line set with the correct sizes, use nitrogen purge while brazing, and install a new filter drier. Pressure-test with dry nitrogen at the manufacturer's test pressure, then evacuate to 500 microns and perform a decay test to confirm no rise. Break the vacuum with the system refrigerant or nitrogen as allowed, then release the factory charge. The 35-ft line set is 20 ft beyond the 15-ft factory allowance. If the manufacturer's additional charge is 0.6 oz per foot, additional charge = 20 ft × 0.6 oz/ft = 12 oz, or 0.75 lb. Total charge = factory charge + 0.75 lb. Record the weigh-in, line length, and final subcooling/superheat after stabilization.
3. If the homeowner refuses replacement and the manufacturer approves a documented reuse procedure, what must be done before the new unit is started?
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Only proceed if the manufacturer's written reuse procedure is followed. Flush the line set with an approved flushing agent or use a method approved by the manufacturer, then blow out with dry nitrogen. Replace the filter drier, pressure-test, and evacuate to 500 microns with a decay test. Because the lines cannot be inspected, the contractor should document the contamination, the flush method, and the final vacuum results. If the vacuum does not hold or debris is recovered, stop and replace the line set. The judgement point is that a documented procedure reduces risk but does not guarantee cleanliness in an inaccessible burned-out line set; the safer decision remains replacement.