An outdoor refrigeration condensing unit will normally operate no matter how cold it gets outside. If the room thermostat simply de-energized the contractor to cycle the compressor off we would run into refrigerant migration problems. During cold winter months, the refrigerant will “migrate” to the coldest place in the system. The coldest place is the outdoor condensing unit and compressor. The refrigeration oil in the compressor is “miscible” with the refrigerant which means they mix well. The liquid refrigerant will mix with the compressor oil or collect underneath it in the compressor crankcase. When the compressor starts and the liquid refrigerant leaves the compressor the oil will leave with it. Liquid slugging and lose of oil may be the result.
In a pump down system, the room thermostat closes a solenoid valve that is located on the liquid line. When the room thermostat is satisfied the solenoid valve will close stopping the liquid refrigerant flow to the TXV. The compressor will continue to operate until most of the refrigerant in the low side boils off and is pumped through the compressor into the condenser and receiver. As the suction pressure falls below the low-pressure control setting the compressor will cycle off. Most of the refrigerant is now stored in the condenser and receiver. A crankcase heater will prevent the small amount of vapor that’s left from condensing in the compressor oil. When the room thermostat calls for cooling again, the solenoid valve energizes allowing refrigerant to flow to the TXV. As the suction pressure rises the low-pressure switch closes and starts the compressor. If a defrost timer or pump down switch is utilized on the system they should be wired in series with the thermostat so that the system will only shut off after pumping down.
The solenoid coil, defrost timer and room thermostat can be powered from the evaporator so no electrical between the indoor and outdoor units may be required. Multiple evaporators can be connected to one condensing unit. All of the evaporators may operate off one solenoid valve or each evaporator may have its own solenoid valve and thermostat.