R-410A Split System With Low Superheat and High Subcooling
A 3-ton R-410A residential split system with a TXV is called back after another contractor replaced the outdoor unit. Conditions: 95°F outdoor, 75°F indoor dry bulb, 63°F wet bulb, 1,200 cfm measured airflow. Suction pressure is 118 psig with a 46°F suction line temperature. Liquid pressure is 390 psig with a 90°F liquid line temperature. Supply air is 61°F, return 75°F. Compressor draws 14.5 A against a 16.0 A RLA. The condenser coil is clean, the filter is new, and the indoor coil is not fouled. The homeowner reports the house cools slowly but does not freeze up. The installing contractor added 2 lb of R-410A because the sight glass was bubbling and the system 'needed a little more.' No weight tag or charging record is present. The nameplate charge is 9 lb 4 oz. You must decide whether the system is overcharged, restricted, or has a metering problem before recommending recovery or parts.
1. Using R-410A saturation temperatures, calculate superheat and subcooling. Rank the three most likely causes and explain what each reading rules in or out.
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Readings: R-410A at 118 psig has a saturation temperature of about 40°F, so superheat = 46°F - 40°F = 6°F. At 390 psig the saturation temperature is about 115°F, so subcooling = 115°F - 90°F = 25°F.
Interpretation: With 1,200 cfm, a clean coil, and a new filter, airflow is not the cause. Low superheat at the compressor means the evaporator is being fed more refrigerant than it can boil off; high subcooling means liquid is backing up in the condenser. Ranked causes: (1) overcharge, especially after 2 lb was added without a weighed charge; (2) TXV stuck open or bulb/equalizer problem; (3) liquid-line restriction, less likely because a restriction usually raises superheat and can create a temperature drop across the drier.
Rule-outs: A dirty evaporator or low airflow would usually lower superheat but not raise subcooling this much. A bad compressor would show low head and low amp draw, not 390 psig and 14.5 A. The normal-to-high amp draw supports excess liquid return or overcharge rather than a failed compressor.
2. What test separates an overcharge from a TXV stuck open, and what result confirms each?
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Recover the refrigerant, evacuate, and charge by weight to the nameplate 9 lb 4 oz. Then measure superheat and subcooling at the same indoor and outdoor conditions. If superheat returns to the manufacturer's target and subcooling falls into range, the original problem was overcharge. If superheat remains below target while subcooling is correct and the liquid line has no temperature drop, the TXV is overfeeding or stuck open. As a non-invasive check first, measure the temperature drop across the filter drier and at any liquid-line restriction; more than about 2°F drop points to a restriction, not overcharge. Also inspect the TXV bulb for tightness and insulation and compare bulb temperature with suction-line temperature. Do not use the sight glass as a charge target.
3. The homeowner asks you to add more refrigerant. Write your response and the steps you would take.
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I would explain that adding refrigerant would raise head pressure and subcooling further, increase compressor current, and may slug the compressor; the sight glass is not a charge indicator. Steps: (1) record baseline readings and amp draw; (2) recover all refrigerant into a recovery cylinder and weigh it; (3) compare recovered weight with nameplate; (4) leak-check if the recovered weight is low; (5) evacuate to 500 microns, charge by weight, and verify superheat/subcooling against the manufacturer's chart; (6) document readings, charge weight, and any parts. If the TXV is the fault, replace it only after confirming with the weighed-charge test. Safety: use PPE, avoid overfilling the recovery cylinder, and do not vent refrigerant.