R-410A Rooftop Charging With Conflicting Superheat and Subcooling
A technician is commissioning a 5-ton R-410A packaged rooftop unit with a TXV and a single compressor. Outdoor ambient is 95°F; return air is 75°F dry-bulb and 63°F wet-bulb. The unit has a clean condenser, a new filter, and the blower on high speed. Measurements: liquid-line pressure 340 psig, liquid-line temperature 95°F, suction pressure 105 psig, suction-line temperature 50°F, compressor current 18 A against a 22 A RLA, and supply-air temperature 58°F. The sight glass shows a few bubbles. The manufacturer’s charging chart calls for 10°F subcooling at these conditions. The installer wants to add refrigerant until the sight glass is clear. The building owner is complaining that the space is not cooling fast enough.
1. Calculate superheat and subcooling, and assess whether the system is undercharged, restricted, or airflow-limited. Show your reasoning.
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R-410A at 340 psig has a saturation temperature of about 100°F. Subcooling = 100°F − 95°F = 5°F. R-410A at 105 psig has a saturation temperature of about 32°F. Superheat = 50°F − 32°F = 18°F.
The measured subcooling is below the 10°F target, and superheat is high. That combination usually points toward undercharge, but it is not enough by itself. The delta T is 75°F − 58°F = 17°F, which is reasonable, so airflow is probably not extremely low. Compressor current is below RLA, consistent with low load or low charge. Bubbles can indicate undercharge, but they can also come from a restriction or from flash gas caused by a liquid-line pressure drop. Before charging, check for a restriction and verify airflow, because adding refrigerant to a restricted system can overcharge the condenser.
2. What checks must be done before adding refrigerant, and how should charge be added?
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Verify gauge accuracy and take stable readings. Check the evaporator coil and filter for blockage, confirm blower speed and airflow, and measure return and supply wet-bulb as well as dry-bulb. Inspect the liquid line for a restricted filter-drier, kinked line, or partially closed valve. Check the TXV bulb for tightness, insulation, and correct location. Confirm the condenser is clean and the fans are running correctly.
If no restriction or airflow problem is found, add refrigerant in small increments while watching subcooling, superheat, compressor amps, and supply-air temperature. Allow the system to stabilize between additions. Stop when subcooling reaches the manufacturer’s target, not when the sight glass is clear. A clear sight glass alone is not a valid charging method for a TXV system.
3. After adding charge, subcooling reaches 10°F but superheat remains 22°F. What are the likely causes and next steps?
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With subcooling now correct, the condenser has enough liquid, so do not add more refrigerant. High superheat at this point points to the evaporator side or the TXV. Likely causes are a TXV that is restricted, misadjusted, or has a failed power element; a blocked inlet strainer; a bulb that is loose, incorrectly located, or poorly insulated; low evaporator airflow; or a distributor problem.
Next, verify airflow and the evaporator coil condition, then inspect the TXV bulb and compare superheat at the evaporator outlet with superheat at the compressor. If the TXV is not feeding properly, adjust it only after confirming the bulb and strainer. If adjustment does not bring superheat into the manufacturer’s range, replace the TXV power element or valve. Recheck subcooling after any change because the system will rebalance.