Freezer Floodback: Overcharge or Stuck-Open TXV
A 1.5 hp low-temperature walk-in freezer using R-404A will not pull below 10°F; the thermostat is set for 0°F. The box is 8 ft x 10 ft x 8 ft with 4 in. panels. At 85°F ambient, suction pressure is 34 psig and discharge pressure is 235 psig. Evaporator superheat is 2°F; compressor superheat is 4°F. Liquid-line subcooling is 22°F. The sight glass is clear, the condenser coil is clean, and the condenser fan is running. The evaporator fans are running and the coil is mostly clear with frost only at the bottom. The nameplate charge is 14 lb; a previous technician added 3 lb to make the sight glass clear.
1. What do the subcooling, superheat, and service history suggest? Distinguish overcharge from a stuck-open TXV.
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The most likely cause is overcharge, but a stuck-open TXV must be ruled out because both produce floodback.
- Subcooling of 22°F is high for this ambient and indicates excess liquid stacking in the condenser. The previous technician added 3 lb, over 20% above the 14 lb nameplate charge, which supports overcharge.
- Evaporator superheat of 2°F and compressor superheat of 4°F show liquid is reaching the compressor. A clear sight glass does not prove correct charge; it only proves no flash gas at that point.
- A TXV stuck open or a lost bulb charge also causes low superheat. With a stuck-open TXV and correct charge, subcooling would typically be normal, not 22°F, unless someone overcharged while trying to raise suction pressure.
- The high suction pressure and poor box temperature are consistent with flooding and reduced effective capacity, not with a simple undercharge.
2. What steps would you take to correct the charge and verify the repair?
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Recover, weigh in, and then evaluate the TXV under known conditions.
- Recover the refrigerant into a recovery cylinder and record the amount removed. Do not vent R-404A.
- Replace the liquid-line filter-drier if there is any indication of moisture, acid, or restriction.
- Weigh in the nameplate charge of 14 lb, or the manufacturer's revised charge for the line set and receiver, rather than charging by sight glass.
- Start the system and allow it to stabilize. Check subcooling, evaporator superheat, compressor superheat, suction and discharge pressures, and box pull-down.
- For a low-temperature freezer, verify evaporator superheat is in the range specified by the valve/equipment manufacturer and compressor superheat is high enough to prevent floodback, typically well above the 4°F observed here.
3. After weighing in the correct charge, subcooling is 10°F but evaporator superheat is still 2°F. What is your next diagnostic step?
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With charge now known correct, the remaining low superheat points to the expansion valve or its sensing bulb.
- Verify the TXV bulb is tightly clamped to a clean suction line, insulated, and located where it senses true evaporator outlet temperature, not compressor or header heat.
- Check for a restricted or partially plugged TXV inlet screen and for a bulb charge that has been lost. A valve stuck open will keep feeding refrigerant and hold superheat low even at correct subcooling.
- Pump the system down or isolate the valve per the manufacturer's procedure and inspect the valve pin, seat, and power head. Replace the TXV if it does not modulate.
- After replacement, recheck superheat, subcooling, compressor superheat, and box temperature before releasing the freezer to the customer.