Long-Line-Set Startup and Weigh-In Charge
New construction startup: a 3-ton R-410A split system with a TXV, 3/8 in. liquid line, and 3/4 in. suction line. The line set is 75 ft long, and the outdoor unit is 20 ft above the indoor coil. The manufacturer’s installation manual states: factory charge is 9 lb 0 oz for 25 ft of 3/8 in. liquid line; add 0.6 oz per foot of 3/8 in. liquid line beyond 25 ft. Maximum vertical separation is 25 ft with an inverted trap at the indoor coil. The system has a crankcase heater and a bidirectional filter drier. After evacuation to 500 microns and a decay test, you weigh in the factory charge plus the additional charge. At startup: outdoor 90°F, indoor 75°F DB / 63°F WB, suction 118 psig, suction line 48°F, liquid 310 psig, liquid line 86°F. Subcooling is 14°F and superheat is 8°F. The space is not yet occupied, and the thermostat is set to 70°F.
1. Calculate the additional refrigerant required by the manufacturer’s table and the total charge. Show your work.
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The line set exceeds the 25 ft factory-charge length by 75 - 25 = 50 ft. The manual allows 0.6 oz of additional R-410A per foot of 3/8 in. liquid line beyond 25 ft. Additional charge = 50 ft × 0.6 oz/ft = 30 oz. Convert: 30 oz ÷ 16 oz/lb = 1.875 lb, or 1 lb 14 oz. Total charge = factory 9 lb 0 oz + 1 lb 14 oz = 10 lb 14 oz. That is the weight to charge after evacuation, assuming the line set is actually 3/8 in. liquid and the manual table applies. Record the final weighed-in charge on the unit label.
2. At startup, subcooling is 14°F and superheat 8°F. Using the manufacturer’s target subcooling of 10°F, is the charge correct? Explain your reasoning and what you would do.
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Subcooling of 14°F is 4°F above the 10°F target, so the system appears slightly overcharged - but first confirm steady-state conditions, correct liquid-line pressure and temperature, and no bubbles. Also verify that the additional charge was actually weighed correctly and that the line-set length is 75 ft. If the target is truly 10°F, recover refrigerant in small increments until subcooling is 10°F, then recheck superheat and compressor amps. Do not use superheat to set charge on a TXV system; the TXV will hold superheat near 8°F even when subcooling is wrong. A 20 ft vertical rise is within the 25 ft limit, but confirm the inverted trap, crankcase heater operation, and oil return.
3. List three long-line-set installation checks beyond charge.
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Check that the inverted trap is installed at the indoor coil and that the suction line is insulated and supported. Verify the crankcase heater is powered and functioning, especially because the outdoor unit is above the indoor coil. Confirm line sizing and pressure drop are within the manufacturer’s long-line guidelines, check oil level in the compressor, and ensure the bidirectional filter drier is correct for the line set. Also verify the vertical separation does not exceed 25 ft and that no additional oil is required per the manual. Finally, record subcooling, superheat, and compressor amps after the system stabilizes.