Reusing a 60 ft R-22 Line Set for a New R-410A Heat Pump
A 3-ton R-410A heat pump replaces a 3-ton R-22 air conditioner. The existing line set is 60 ft long with a 1/4 in. liquid line and a 5/8 in. suction line; the indoor coil is 25 ft above the outdoor unit. The new unit's installation manual specifies a 3/8 in. liquid line and allows up to 50 ft of vertical separation with a 5/8 in. suction line, but requires the manufacturer's long-line accessories and charge adjustment. The factory charge covers 25 ft of 3/8 in. line; additional charge is 0.6 oz per foot of 3/8 in. liquid line. Outdoor temperature is 45°F and falling; the homeowner wants heat tonight. The contractor proposes to braze the existing line set, pull a vacuum to 500 microns, and “top off” the charge by gauges.
1. Should the existing line set be reused? Explain the engineering and practical risks.
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Do not braze the existing line set as-is. The 1/4 in. liquid line is too small for a 3-ton R-410A system and will cause excessive pressure drop, flashing at the TXV, low capacity, and unreliable charging. Replace the liquid line with 3/8 in. copper.
The 5/8 in. suction line may be reusable if it is clean, undamaged, and within the manufacturer's length and vertical limits; 60 ft and 25 ft of rise are within the 50 ft vertical allowance stated, but confirm the manual's exact table. Because the old system was R-22, residual mineral oil and contaminants are a concern. If the line set is not replaced, purge and clean it per the manufacturer's approved method, and replace the filter drier. Do not reuse a line set with a hidden kink or a mismatched size just to save time. A new 3/8 in. liquid line and 5/8 in. suction line is the lowest-risk choice.
2. Calculate the additional charge and explain how it should be added at 45°F outdoor temperature.
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With a 3/8 in. liquid line, the factory charge covers 25 ft. The extra length is 60 − 25 = 35 ft. At 0.6 oz/ft, add 35 × 0.6 = 21 oz, about 1.3 lb of R-410A.
Weigh it in with a scale; do not “top off” by gauges, especially at 45°F, because suction pressure and superheat are poor charging indicators in heating mode at low ambient. If the unit has a heating-mode approach or subcooling target in the manual, use that only after the weigh-in and after the system has stabilized. Record the factory charge, the added charge, and the final total.
If the existing 1/4 in. line were somehow retained, the charge calculation would be different and the system would still be wrong; that is another reason to replace it.
3. Describe the brazing, pressure testing, and evacuation sequence, including what to do if the micron decay test fails.
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Braze with nitrogen flowing through the line to prevent internal oxidation; use 15% silver alloy and protect the TXV and service valves with heat sinks or wet rags. Pressure-test with dry nitrogen to the manufacturer's test pressure, typically around 500 psig for R-410A, and hold it long enough to prove no leaks.
Evacuate to 500 microns with a micron gauge at the farthest point, not at the pump. Isolate the pump and hold for 10 minutes; a rise above 1,000 microns indicates a leak or moisture. If it fails, do not just pull longer: find the leak or repeat the triple evacuation with nitrogen breaks, and replace the filter drier. Only then release the charge and start the unit.