R-410A Rooftop Unit: Undercharge Versus Liquid-Line Restriction
A 25-ton R-410A packaged rooftop unit serving a retail space is reported as not keeping up on a 95°F day. Measured values are: suction pressure 100 psig, suction-line temperature 50°F, discharge pressure 350 psig, liquid-line temperature 105°F. The sight glass shows flashing, and the filter-drier has a 1°F temperature drop. The condenser coil is clean and indoor airflow is at design. The unit has a thermostatic expansion valve and no receiver. The technician wants to adjust the TXV and add refrigerant.
1. Estimate the saturated temperatures, superheat, and subcooling.
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Using approximate R-410A saturation values:
- 100 psig suction ≈ 29°F saturation
- 350 psig discharge ≈ 108°F saturation
Superheat = suction-line temperature - suction saturation = 50 - 29 = 21°F.
Subcooling = discharge saturation - liquid-line temperature = 108 - 105 = 3°F.
Typical targets are roughly 8–12°F superheat and 8–12°F subcooling, depending on the manufacturer. The unit has high superheat and very low subcooling, with flash gas in the sight glass.
2. Diagnose the most likely cause and distinguish it from a liquid-line restriction.
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The combination of low subcooling, sight-glass flashing, high superheat, and low suction pressure points to a low refrigerant charge. A liquid-line restriction can also cause flashing, but it usually produces a measurable temperature drop across the filter-drier or a large subcooling upstream of the restriction. Here the filter-drier drop is only 1°F, so a restriction is unlikely. The low subcooling at the condenser outlet is the key: the condenser is not holding a liquid seal because the charge is low. Recommend a leak search, repair, evacuation, and charging by weight or manufacturer subcooling method-not blind addition of refrigerant.
3. The technician proposes opening the TXV to lower superheat and adding 2 lb of refrigerant. Evaluate this plan.
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Opening the TXV may temporarily lower superheat, but it does not correct the low subcooling or the flashing sight glass. With insufficient charge, increasing valve opening can make the evaporator starve or hunt and may drop suction pressure further. Adding 2 lb to a 25-ton system is a small, arbitrary amount; it may mask the symptom without fixing the leak and risks overcharge after the leak is repaired. The correct sequence is leak detection and repair, evacuation, then charging to the manufacturer's subcooling or weigh-in target. Confirm condenser airflow and TXV operation afterward.