Rooftop TXV Unit With Low Subcooling and High Superheat
A 5-ton R-410A packaged rooftop unit with a TXV serves a small office. Outdoor ambient is 95°F; indoor is 75°F DB / 63°F WB. Suction pressure is 98 psig, suction line temperature 62°F, liquid pressure 292 psig, and liquid line temperature 92°F. Superheat is 31°F and subcooling is 2°F. The sight glass shows bubbles. Temperature drop across the liquid-line filter drier is 1°F. The evaporator coil is clean, the filter is new, and the blower is delivering about 1,900 cfm according to the fan curve. Compressor amps are below RLA. The building owner says the unit ran fine last summer and no one has added refrigerant. A maintenance tech suggested “adjusting the TXV” to lower superheat. You notice oily residue at the flare connection on the liquid line at the air handler, and the space has been 78°F for three days.
1. The readings show low subcooling, high superheat, and bubbles. Explain how you would rule in or out a liquid-line restriction, and state the most likely fault.
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A liquid-line restriction creates a pressure drop and a temperature drop at the restriction. The best check is the temperature difference across the filter drier: 1°F here is normal, so a plugged drier is unlikely. A restriction would also often show frost or condensation at the restriction and high subcooling upstream of it. Instead, this system has about 2°F subcooling at the condenser outlet, bubbles in the sight glass, low suction pressure, and 31°F superheat. That combination means the TXV is starved because the liquid line is not full of liquid - an undercharge. The oily residue at the flare is strong evidence of a leak. Adjusting the TXV would only change the superheat setting; it would not restore subcooling or fix the leak.
2. Describe the repair, evacuation, and charging steps for this TXV system, including the target subcooling method and documentation.
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Recover the remaining refrigerant, repair or remake the leaking flare, and replace the filter drier since the system will be opened. Pressure-test with dry nitrogen, evacuate to 500 microns, and perform a decay test. Weigh in the nameplate charge as a starting point, then charge by subcooling because the unit has a TXV: add refrigerant until subcooling reaches the manufacturer’s target, typically 8-12°F at these conditions. Verify superheat is in the normal 8-12°F range, compressor amps are below RLA, and the indoor delta T is normal. Document the leak repair, the final weighed charge, and the subcooling/superheat readings. Do not rely on the sight glass alone; it is only a clue.