1. Oil Pressure Problems
What to look for

2. Compressor Oil Level
Check the compressor sight glass for sufficient oil.
The sight glass should be at least one-third full (for Copeland Compressors)
Too much oil can cause crankcase turbulence which can cause low oil pressure. Remove excess oil.
On start up, low oil pressure can occur in large systems with long piping runs or in an evaporator with a lot of tubing
If this is the case wait for the system to settle down and the oil level should recover
If the system is new you must wait 24 hours after start up to add any oil

3. Refrigerant Piping
Is there a suction riser? Does it have a trap at the bottom? If not, corrections must be made
Tube diameter must be consistent with the length of run and BTU capacity
If the tube is too large refrigerant velocity in the pipe becomes insufficient to carry the oil with it
Tubing should be slopped one inch per ten feet of run towards the compressor to aid in oil return and be properly insulated

4. Compressor Suction Superheat
This is the measure of how much above it’s saturated temperature the refrigerant is being heated
A high superheat causes low gas density which causes the system to run at low suction pressure
This causes the compressor to pump at an inefficient rate causing gas velocity to drop.
The gas is not dense enough to carry the oil back
To correct, open the expansion valve to allow more gas flow
Check the suction superheat and adjust to 20* at the compressor

5. Refrigerant Charge
If the charge is low there is not enough refrigerant to return the oil to the compressor
Determine why the system is low on charge.
When refrigerant leaks, oil will leak also which will reduce the oil level
Improperly set fan cycle control reduces system pressures affecting oil return

6. Low TXV Superheat
This condition causes floodback which washes oil out of the compressor
Oil washout causes bearing seizure resulting in compressor motor burnout which is often misdiagnosed as an electrical failure
Indicated by excessive foaming of the oil in the compressor sight glass
Causes can include improperly set TXV, iced evaporator, restricted air flow or an evaporator fan motor that is not running
Copeland recommends superheat of * for high temp, 5 – 8* for medium temp and 2 – 5* for low temp

7. Burned out Crankcase Heater
Boils off liquid refrigerant in the crankcase to prevent gathering
Without it, on start up, the compressor will attempt to pump a liquid which results in failure
Large amounts of liquid in the crankcase causes oil to be pumped out and violent oil foaming
This reduces lower crankcase pressure and the liquid is trying to boil off

8. System Defrost This is critical!
Iced evaporators will cause poor oil return
A pump down on defrost system helps avoid this problem by introducing the liquid refrigerant to the warm evaporator after defrost which increases it’s volume and helps carry oil back to the compressor

9. TXV Distributer
The distributer contains a nozzle that is sized for the application upon installation
Improper nozzle sizing causes refrigerant velocity loss with resulting in poor oil return
As a result it is uncommon to have the incorrect nozzle but it should not be overlooked

10. Oil Pump A worn pump will cause low oil pressure
The possibility of a worn pump is remote unless it is very old
It is possible that the pump is not getting sufficient oil due to a clogged oil pump inlet screen
To correct remove the screen from the pump, clean and reinstall
Reverse the compressor rotation to allow oil flow through the unused, clean oil port

11. Low Pressure Control
An improperly set LPC can cause the oil control to trip unnecessarily.
If the differential is set too close the compressor will short cycle causing the OFC to trip
Check the manufacturer’s recommended settings