In a perfect scenario, the oil in a refrigeration or air conditioning system would stay inside the compressor’s crankcase to lubricate the compressor’s moving parts. However, because of ever-changing heat loads on the system and varying system conditions caused by refrigerant undercharges or overcharges, inoperative valves, faulty or misadjusted metering devices, dirty condensers, dirty evaporators, fan motor failures, or plugged filter driers, there is no such thing as a perfect system in the real world.

Inevitably, the compressor’s lubricating oil will escape the compressor’s crankcase and enter the system’s tubing. Hopefully, this escaped oil will eventually find its way through the condenser, receiver, liquid line, metering device, evaporator, and suction line and end up back in the compressor’s crankcase. It is the velocity of the refrigerant traveling through the system and the piping arrangements that bring the oil back to the compressor’s crankcase. That’s why it is so important to have the proper line and coil sizes to support the correct refrigerant velocity for proper oil return. Just as important are properly sloped lines and properly located P-traps to return to the compressor.


Refrigerant migration and system flooding conditions are often two of the reasons why system lubricant or oil escape the compressor’s crankcase and enter the system.

Refrigerant migration deals with refrigerant migrating back to the compressor’s crankcase during the off cycle. This migration of refrigerant is caused by a pressure difference between the oil in the compressor’s crankcase and the refrigerant. Oil has a very low vapor pressure and will attract the refrigerant in both the vapor and liquid states. Refrigerant migration can cause the compressor’s crankcase to lose its oil, thus circulating the oil throughout the refrigeration system. This oil in circulation usually gets caught in the evaporator and can cause an oil-logged evaporator.

Refrigerant flooding refers to liquid refrigerant entering the compressor’s crankcase during the on or running cycle. Flooding can cause flashing of the oil in the compressor’s crankcase because of the liquid refrigerant boiling under the oil, and it also can cause excessive pressures in the crankcase. This phenomenon can cause the compressor to lose its oil and circulate it throughout the refrigeration system. Excessive oil in the system will again get caught in the evaporator and cause an oil-logged evaporator.


Oil in a refrigeration or air conditioning system has many functions. These functions include lubrication, deadening noise, transferring heat (cooling), reducing friction, minimizing mechanical wear, and sealing valves to prevent blow-by.

There are a number of ways in which an evaporator’s inside tubing can become oil logged. These include the wrong type or viscosity of oil, too much oil in the system, a liquid refrigerant flooded compressor circulating oil at startup, liquid refrigerant migration during the off cycle causing crankcase oil foaming on startups, a thermostatic expansion valve (TXV) out of adjustment — too little superheat causing a refrigerant-flooded compressor — not enough defrost periods for low-temperature-application machines, and a system that’s not piped correctly (no oil traps or piping too large).

Oil usually logs in the evaporator because it is the coldest component with the largest tubes, thus has the slowest refrigerant velocity. Oil logged in the evaporator will coat the inner wall of the coil and reduce the heat transfer through the walls. This will cause a loss of capacity and poor performance. The compressor will be robbed of some of its crankcase oil and run with a lower-than-normal oil level. This may score or ruin mechanical parts in the compressor. Some compressors have an oil sight glass for visual inspection of the oil level in the compressor’s crankcase.

Too high of viscosity oil will also be hard to return from an evaporator and will surely cause oil logging. Usually, the heat from the defrost heaters will warm and thin the oil in the evaporator so it can be returned to the compressor once the compressor starts up. This will happen only if the correct viscosity of oil is used.

If a suction line is oversized, the refrigerant velocity will be decreased. This will prevent the oil from moving through the suction line to the compressor’s crankcase. Remember, it is the refrigerant velocity that will move the oil through the refrigeration system’s piping.


Listed below is a system check for an oil-logged evaporator. The system is a low-temperature R-134a system incorporating a TXV as a metering device. Pressures and temperatures will vary depending on the severity of the oil logging. This check can serve as a useful tool in helping the technician recognize that a system has this hard-to-detect problem.

Measured values for a system with a severely oil-logged evaporator:

Compressor discharge temperature: 190°F

Condenser outlet temperature: 78°

Evaporator outlet temperature: minus 18°

Compressor inlet temperature: minus 13°

Surrounding ambient temperature: 75°

Refrigerated box temperature: 10°

Compressor amperage: High

Low side (evaporating) pressure/temperature: 3.5 in. Hg (minus 20°)

High side (condensing) pressure/temperature: 104 psig (90°)

Calculated values:

Condenser split: 15°

Condenser subcooling: 12°

Evaporator superheat: 2°

Compressor superheat: 7°


• Low oil level in sight glass on the compressor’s crankcase.

• TXV having a hard time controlling superheat (hunting).

• Low evaporator and compressor superheat.

• Warmer-than-normal box temperatures with loss of capacity and lower-than-normal suction pressure.

• Noisy compressor.

Let’s take a closer look at each of these symptoms.

Low oil level in compressor’s sight glass — Because a lot of the oil is in the evaporator, the crankcase will be low on oil. In fact, the entire system’s components, excluding the compressor, may have too much oil. This would cause a low oil level in the compressor’s crankcase sight glass. Many times, a compressor that is flooding with refrigerant will turn into an oil pumper. The crankcase will be foaming from the liquid refrigerant flashing in it. Small oil droplets entrained in the oil will be pumped through the compressor. This will oil-log many components in the system. The velocity of the refrigerant traveling through the lines and P-traps will try to return the oil from the system to the crankcase.

However, oil will continue to get into the system if the compressor flooding situation is not remedied.

TXV having a hard time controlling superheat — The TXV also will see too much oil passing through it. The evaporator’s tailpipe will be oil-logged and the inside of the tubes will be coated with oil. The remote bulb of the TXV at the evaporator outlet will have a hard time sensing a true evaporator outlet temperature because of the reduced heat transfer through the line. The TXV will hunt and keep trying to find itself. A constant superheat will not be maintained. The TXV remote bulb may sense a warmer-than-normal temperature from the oil insulating the inside of the line. This could make the TXV run a low superheat and flood or slug the compressor with refrigerant. In many cases the sight-glass in the liquid line will be discolored with a yellowish or brown tint from refrigerant and oil flowing through it.

Technicians may confuse this low superheat reading with an overcharge of refrigerant. The difference is that an overcharge of refrigerant will give high head pressures and high condenser subcooling readings. TXV systems usually can tolerate a bit of an overcharge and still hold a good evaporator superheat, if set properly. However, once the head pressures get too high, the TXV will soon overfeed the evaporator and show low superheat.

Low compressor superheat — Because the TXV may be running low superheat, this will cause the compressor (total) superheat to run lower.

Warmer-than-normal box temperatures with capacity losses — Because of the reduced heat transfer in both the condenser and evaporator from the excess oil coating the inner tubing, capacity will be decreased. The compressor will run longer trying to maintain a desired box temperature. Evaporator temperatures and pressures may run low because of the reduced heat transfer from the oil insulating the evaporator tubes. This will cause reduced mass flow rates and low evaporator pressures.

Noisy compressor — The compressor may be noisy because of lack of oil. Metallic sounds may be heard from lack of lubrication or parts out of tolerance from excessive wear.   

Publication date: 4/3/2017

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