In a pump-down system, a liquid line solenoid closes when the box reaches temperature and the compressor keeps running until it pulls the refrigerant out of the evaporator and into the receiver. A low-pressure switch then stops the compressor. This protects the compressor from liquid and reduces starting stress.
Pump-down is common on outdoor condensing units and freezers, where refrigerant tends to migrate to the coldest part of the system during an off cycle. Without pump-down, liquid can reach the compressor at start-up.
The cycle step by step
- The thermostat is satisfied. Power to the liquid line solenoid is cut and the valve closes.
- The compressor keeps running. It pulls refrigerant out of the evaporator and suction line.
- Suction pressure falls. The low-pressure switch reaches its cut-out setting.
- The compressor stops. Most of the refrigerant is now in the condenser and receiver.
- The box warms. The thermostat calls, the solenoid opens and pressure rises.
- The compressor starts. Liquid feeds the evaporator through the expansion valve.
Parts that make it work
| Part | Role |
|---|---|
| Liquid line solenoid | Shuts refrigerant off from the coil |
| Low-pressure switch | Stops the compressor at the end of pump-down |
| Receiver | Stores the refrigerant removed from the coil |
| Check valve | Stops refrigerant flowing backward into the compressor |
| Thermostat or controller | Starts and ends the cycle |
Why it is used
- It keeps liquid refrigerant out of the compressor on start.
- It reduces oil dilution in a cold crankcase.
- It reduces the number of hard starts.
- It gives the system a defined end to each cycle.
Signs the cycle is not working
- The compressor short cycles.
- The compressor never stops after the box is cold.
- The compressor will not start although the thermostat calls.
- Low-pressure trips that need a manual reset.
- Frost on the suction line after the off cycle.
- Noise from the compressor at start-up.
Common faults
| Fault | Effect |
|---|---|
| Solenoid leaks by | Pressure rises during the off cycle and the compressor short cycles |
| Switch set wrongly | Compressor stops too early or never |
| Low charge | Suction pressure falls before pump-down completes |
| Weak check valve | Refrigerant migrates back to the compressor |
| Controller fault | Solenoid does not open or close on command |
What you can do
Notice how the compressor behaves at the end of a cycle. A run that stops within a minute or so after the thermostat is satisfied is typical of a healthy pump-down. If it never stops, or restarts repeatedly, tell the technician what you saw. Adjusting pressure switches or valves without gauges can damage the compressor, so leave settings alone.
Read more on the systems guide, compressor short cycling and the parts guide. For winter behavior, see low-ambient operation.
Living with a pump-down system
Operators rarely need to do anything, but a few habits help. Do not switch the disconnect on and off casually, because it interrupts the cycle and lets refrigerant migrate. After power returns from an outage, give the system time to stabilize before checking the box. When the unit is serviced, ask the technician to note the low-pressure cut-out and cut-in settings and keep them in your records. Those two numbers explain many puzzling behaviors later.
Questions about this topic
Do all walk-ins use pump-down?
No. Many small systems use a simple thermostat and compressor cycle. Larger outdoor and low-temperature units often use pump-down.
Is pump-down bad for the compressor?
No. It is designed to protect it, if the settings are correct.
Why does my compressor run after the box is cold?
It may be completing pump-down. If it runs for long periods, have it checked.