Frost at the TXV on a 5-Ton R-410A Split System
A 5-ton R-410A split system with a TXV is not cooling a conference room. Indoor return air is 75°F dry bulb and 63°F wet bulb; outdoor ambient is 95°F. Suction pressure is 95 psig, liquid pressure is 310 psig, suction superheat is 28°F, and liquid subcooling is 1°F. The evaporator is frosted at the TXV outlet and at the distributor, but the rest of the coil is only cool. The blower is on high speed, the filter is new, and the indoor delta T is 7°F. The condenser coil is clean and the outdoor fan is running. The system has a factory-installed filter drier and a TXV with an external equalizer. The owner says another technician added 2 lb of refrigerant last week, and the frost appeared after that.
1. Interpret the readings and identify the two most likely fault categories.
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The suction pressure corresponds to a saturation temperature near 30°F, so with 28°F superheat the suction line is around 58°F. The liquid pressure corresponds to about 92°F saturation, and 1°F subcooling means the condenser is barely stacking liquid. High superheat plus low subcooling points to a starved evaporator, not an airflow problem. The two main categories are: (1) low refrigerant charge, and (2) a restriction in the liquid line or at the TXV inlet. The frost concentrated at the TXV outlet and distributor is a strong clue for a restriction, because refrigerant boiling at a pressure drop creates local freezing. However, a low charge can also starve the coil. Airflow is less likely because the filter is new, the blower is on high, and low airflow would usually produce a low delta T with a colder coil, not 1°F subcooling.
2. Describe a step-by-step diagnostic sequence to distinguish between low charge and a restriction without adding refrigerant.
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First, measure temperature drop across the filter drier and any strainer; a cold or frosted outlet indicates a restriction there. Second, check the TXV inlet screen and the distributor tubes for frost or a temperature drop. Third, verify the TXV bulb is secure, insulated, and not loose or corroded; a lost bulb charge can overclose the valve. Fourth, pump the system down or isolate the liquid line and inspect the TXV inlet screen for debris. Fifth, compare the frost pattern before and after gently warming the TXV body; if the frost clears and superheat changes, the valve is sticking or restricted. Sixth, if no restriction is found, recover the charge, weigh it, and compare with the nameplate charge before recharging. That last step distinguishes a true low charge from a restriction. Adding refrigerant first would mask the diagnosis and risk overcharging.
3. Explain why adding refrigerant last week was likely the wrong action and what risk it created.
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Adding refrigerant without finding the cause treated the symptom, not the fault. If the real problem is a restriction at the TXV or filter drier, the extra refrigerant cannot pass the restriction and will back up in the condenser. Subcooling may rise later, head pressure will increase, and the compressor will work harder without improving the evaporator. If the problem was a low charge, adding only 2 lb may still leave the system undercharged, and the technician has no weight record to know the actual charge. Overcharging a system with a restriction can also flood the compressor on startup or cause high-pressure trips. The correct approach is to recover, weigh, leak-check, and repair the restriction, then charge to the nameplate or manufacturer's specified weight. The frost at the TXV outlet after the addition suggests the restriction became more pronounced as liquid backed up.