Pump-Down Solenoid Leak-By Causes Short Cycling
A walk-in cooler uses a pump-down cycle with an R-404A condensing unit. The thermostat opens the liquid-line solenoid at setpoint. The low-pressure control is set to cut out at 12 psig and cut in at 30 psig, with an 18 psig differential. During pump-down the suction pressure falls from 45 psig to 12 psig in 25 seconds and the compressor stops. The thermostat remains open. Within 75 seconds the suction pressure rises to 30 psig and the compressor restarts. It pumps back to 12 psig in 10 seconds and repeats the cycle. When the thermostat calls, the system runs at 40 psig suction and cools normally. The solenoid coil is de-energized while the thermostat is open, and the contactor and overload are not tripping. The owner reports the compressor short-cycles overnight when the cooler is empty.
1. Explain why the compressor restarts while the thermostat is open and identify the possible leak paths.
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In a correct pump-down circuit, the thermostat opens the solenoid, the compressor pumps the evaporator down to the low-pressure cut-out, and the compressor stays off until the thermostat closes again. The low-pressure control should not cut in because the closed solenoid should isolate the evaporator and hold it at low pressure.
Here the pressure rises from 12 to 30 psig in 75 seconds with the solenoid de-energized. That means refrigerant is entering the low side. The most common path is a liquid-line solenoid that leaks through a dirty seat, worn seat, or trapped debris. Other paths include a manual bypass around the solenoid, a leaking check valve, or a compressor discharge valve that leaks into the suction side when the compressor is off. The low-pressure control itself is not the root cause unless it is miswired or defective; the pressure rise is real and must be explained.
2. Describe a step-by-step method to isolate the leak-by without recovering the entire charge.
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With the system off and the thermostat open, front-seat the liquid-line service valve upstream of the solenoid. Watch the suction pressure for several minutes. If the pressure still rises, the leak is downstream of that valve, such as the solenoid seat or a bypass. If it stops rising, the leak is upstream or the solenoid is the only path and is leaking by.
Next, isolate the compressor with the suction service valve and observe the low-side pressure. A continued rise with both the liquid line and suction valve closed points to a leaking valve in the compressor or a gauge/control issue. If pressure holds, the leak is in the solenoid or evaporator circuit. Use an electronic leak detector and a soap bubble test at the solenoid, bypass, and check valve. Do not leave the system isolated in a way that traps refrigerant; restore valves after testing.
3. Recommend repairs and low-pressure control settings to prevent recurrence.
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Replace or rebuild the leaking liquid-line solenoid with the correct refrigerant-rated valve and seat. Remove any manual bypass. If a check valve is leaking, replace it. If the compressor discharge valve is leaking into the suction side, repair or replace the compressor valve as required.
Set the low-pressure control so the system pumps down fully without pulling into a vacuum. The cut-out should be low enough to evacuate the evaporator but above 0 psig; the cut-in should be below normal running suction pressure and above the cut-out. Do not use a wider differential to hide a leaking solenoid. After repair, verify that the suction pressure holds steady with the thermostat open and the solenoid de-energized for at least 30 minutes.