TXV Starvation After a Liquid-Line Restriction
A 10-ton R-410A commercial split system with a TXV and a scroll compressor serves a conference room. The room is 78°F instead of 72°F, and the compressor runs continuously. Outdoor ambient is 90°F. Return air is 75°F dry bulb and 63°F wet bulb. Measured suction pressure is 115 psig; suction line temperature at the evaporator outlet is 56°F. Liquid pressure at the condenser outlet is 375 psig, and liquid line temperature there is 104°F. At the liquid-line filter drier, the temperature is 104°F entering and 98°F leaving. The sight glass downstream of the drier is clear. Airflow is 3,600 cfm; the unit is rated at 4,000 cfm. The TXV bulb is clamped to the suction line and insulated. The compressor amp draw is 17.5 A against a nameplate RLA of 22.0 A. A previous technician added 1 lb of R-410A because 'the superheat was high,' but the room temperature did not improve. The owner wants to know whether the unit is still undercharged or whether the TXV is bad.
1. Calculate the superheat and subcooling, then interpret the filter-drier temperature readings. What diagnosis best fits all of these measurements?
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At 115 psig, R-410A saturation is about 38°F. Superheat is 56°F minus 38°F, or about 18°F, which is high for a TXV system. At 375 psig, saturation is about 112°F. Subcooling is 112°F minus 104°F, or about 8°F, which is within a normal TXV subcooling range.
The key additional clue is the filter drier: 104°F entering and 98°F leaving is a 6°F temperature drop. A temperature drop across a filter drier generally indicates a restriction; a clean drier should not produce a significant temperature drop. The clear sight glass downstream does not rule out a restriction, because the refrigerant may flash after the restriction and still appear clear at that point. The most likely diagnosis is a restricted liquid-line filter drier starving the TXV, not a simple undercharge. That also explains why the previous 1 lb addition did not improve cooling: adding refrigerant upstream of a restriction does not restore proper flow through the TXV.
2. The owner asks whether the TXV is defective. What tests or observations would distinguish a bad TXV from the drier restriction?
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A bad TXV is possible but should not be assumed until the restriction is confirmed and corrected. The technician should measure pressure drop across the filter drier and any accessible strainer, and check the TXV inlet screen for debris. A temperature drop greater than a few degrees across the drier, especially with a starved evaporator, strongly supports a restriction. The technician can also observe whether the TXV responds to bulb temperature changes; if the bulb is properly clamped and insulated and the inlet screen is clean, but superheat remains high with normal subcooling, the power element or internal parts may be faulty. The low airflow of 3,600 cfm versus 4,000 cfm should be corrected as a separate issue, but low airflow alone would not explain the drier temperature drop. The correct sequence is to recover refrigerant, replace the restricted filter drier, inspect and clean the TXV inlet screen, leak-check, evacuate, and weigh in the charge before condemning the valve.
3. After the restriction is repaired and the system is recharged, what verification steps show that the original diagnosis was correct?
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Verification should show that superheat has returned to the TXV target range, typically about 8°F to 12°F depending on the manufacturer, and that subcooling remains in the normal range, often about 8°F to 12°F. The temperature drop across the new filter drier should be near zero. Suction pressure should rise to a normal evaporating pressure for the indoor wet-bulb condition, and the compressor amp draw should increase toward a normal load value without exceeding RLA. Airflow should be corrected to approximately 4,000 cfm, and the supply-to-return temperature difference should be within the unit’s design range. The technician should also confirm that the TXV bulb is securely clamped, insulated, and located on a clean section of suction line. If superheat remains high after the restriction is removed and airflow is correct, then the TXV or its power element becomes the primary suspect. Documenting the before-and-after readings is important because the original diagnosis depended on the combination of high superheat, normal subcooling, and a drier temperature drop.