Packaged Rooftop Unit With Low Subcooling and High Superheat
A 5-ton R-410A packaged rooftop unit with a TXV serves a small office. On a 90°F outdoor day, return air is 78°F dry bulb/62°F wet bulb. Measured suction pressure is 100 psig, liquid pressure 300 psig, superheat 24°F, subcooling 2°F. Compressor current is 14.8 A against a 22.0 A RLA; supply air is 62°F, return 78°F, delta T 16°F. Total airflow is 1,800 CFM. The filter is new, the condenser coil is clean, and the evaporator coil is clean. A sight glass just downstream of the filter drier shows continuous bubbles. The liquid line temperature drop across the filter drier is 1°F. The building owner says the unit just needs a little refrigerant because a prior technician added 2 lb last season and the problem returned.
1. Diagnose whether the system is undercharged or has a liquid-line restriction, using the superheat, subcooling, sight glass, and filter-drier temperature drop.
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The system shows low subcooling (2°F), high superheat (24°F), low suction pressure, and a reduced delta T, which are classic signs of undercharge. A liquid-line restriction would typically create a noticeable temperature drop across the filter drier or restriction; here the drop is only 1°F, so a restricted drier is unlikely. Continuous bubbles in the sight glass can indicate flash gas from undercharge, not necessarily a restriction. A restriction upstream of the TXV could also cause low suction and high superheat, but it would usually back up liquid and raise subcooling at the condenser outlet, not leave subcooling at 2°F. The prior 2-lb addition and return of the symptom make a leak the leading explanation.
2. Explain why adding refrigerant without leak repair is an efficiency and compliance problem.
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Adding refrigerant without finding the leak only temporarily restores capacity. The system will lose charge again, and the operating efficiency will degrade as suction pressure falls and the compressor runs longer to meet the load. Low charge reduces mass flow and capacity, so the 16°F delta T is likely below design and the unit cannot keep up on design days. Low suction pressure also reduces the evaporator's ability to remove moisture. From a compliance standpoint, knowingly venting or repeatedly topping off a leaking system is not acceptable; refrigerant emissions must be minimized and leaks repaired according to applicable regulations. The correct response is a leak search, repair, evacuation, and weighed-in charge, not another top-off.
3. Outline the diagnostic and repair sequence, including how to verify charge after repair.
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Perform a leak search using electronic detection, pressure testing with dry nitrogen, and inspection of common leak points such as flare fittings, Schrader ports, brazed joints, and the evaporator and condenser coils. Repair the leak, replace the filter drier if the circuit is opened, and evacuate to the manufacturer's micron level. Weigh in the nameplate charge, adjusting for any line-set or factory charge differences. Start the unit and stabilize at the measured indoor and outdoor conditions. Verify subcooling against the manufacturer's target, superheat, suction and liquid pressures, delta T, compressor amps, and airflow. Document the leak location, repair method, evacuated micron level, weighed-in charge, and final operating readings. If the leak cannot be repaired immediately, isolate or recover the charge and advise the customer of the options rather than topping off.