Does Evacuating an AC System Remove the Oil?

If you have an air conditioning system that needs repairs, your technician may recommend evacuating the AC system prior to servicing it. But does evacuating the AC remove the oil inside it? Let’s take a closer look at what evacuating the AC involves and how it impacts the vital oil needed to lubricate the system.

What Does It Mean to Evacuate an AC System?

Evacuating an AC system means using a vacuum pump to remove all traces of refrigerant gas and moisture from the sealed system. This includes the following components:

  • Compressor
  • Condenser coils
  • Evaporator coils
  • Valves
  • Piping connecting the components

Technicians evacuate AC systems prior to making repairs to ensure no refrigerant gas or moisture gets released into the atmosphere. They also evacuate the system to verify there are no leaks by checking if the vacuum holds.

Most refrigerant in AC systems today is some type of fluorinated gas that can damage the ozone layer and contribute to climate change. That’s why proper disposal and evacuation procedures are critical.

Does Oil Get Removed During an AC Evacuation?

does oil get removed during an ac evacuation
Does Oil Get Removed During an AC Evacuation?

Now let’s look specifically at what happens to the oil during an AC system evacuation.

All air conditioning systems require oil to lubricate the compressor and other internal parts. Most use either mineral oil or synthetic polyol ester (POE) oil specifically designed for AC compressors.

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When refrigerant flows through the sealed system, it carries some of the oil along with it in circulation throughout the components. However, a portion always remains in the compressor to keep its mechanical parts lubricated.

During evacuation, only the refrigerant vapor gets removed from the system along with traces of moisture. The oil itself does not get evacuated along with it.

Here is why:

  • The oil has lower vapor pressure than the refrigerant, so it remains inside the system while the refrigerant evaporates and gets drawn out.
  • The molecules of refrigerant and oil do not adhere to each other, so they separate as evaporation occurs.
  • Much of the oil stays in the compressor and does not circulate throughout all the components.

Can Any Oil Loss Occur?

Most of the oil remains safely inside the sealed system during evacuation. However, a very small loss can occur over multiple evacuation processes. Here’s why:

  • Diffusion: There is a slight tendency for oil molecules to diffuse across any pressure gradient. This could allow trace amounts of oil vapor to make its way out of the system.
  • Hygroscopic: The oil itself and oil sludge present can be hygroscopic (attract and absorb moisture) to some degree. This moisture can carry away small amounts of oil when evacuated.
  • Adsorption: Refrigerant and moisture molecules can also adhere to or adsorb oil residues from inside the AC system components through normal intermolecular actions. Over many evacuations, this could result in a gradual reduction of oil.

How Much Oil Loss Can Occur?

For most evacuation and recharge procedures, oil loss is minimal and should not significantly impact lubrication or system performance.

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Under proper conditions, it is typical for no measurable loss in oil level to occur at all.

However, according to HVAC expert James E. Brubaker, oil loss of up to 10% can occur after several evacuation and recharging cycles depending on the quality of the procedures.

The worst-case scenario would be if all refrigerant and moisture is not properly removed prior to opening the AC system for repairs. Any trapped refrigerant can carry out more oil in liquid form when it evaporates in the presence of air.

Can Oil Loss Impact AC System Performance?

As long as oil loss is kept to a minimum, there should be no major impact on the air conditioning system.

The compressor pump is designed to work properly with oil fill levels slightly below or above the optimum range. Lower oil levels primarily reduce the cushioning effect inside the compressor, which could increase noise and vibration.

However, if oil loss exceeds 10-15%, the following can gradually occur:

  • Increased compressor noise and vibration
  • Reduced cooling capacity and efficiency
  • Mechanical wear inside the compressor
  • System overheating issues

Oil Reservoirs Help Maintain Lubrication

Fortunately, most modern AC compressors have an internal oil reservoir or sump that helps replenish oil circulating through the system.

As oil gradually depletes over time, the reservoir releases oil to maintain sufficient lubrication of internal parts. This minimizes the impact small amounts of oil loss can have on performance.

However, once the reservoir oil level drops too far, lubrication can suffer. That’s why technicians need to measure and check oil levels with gauges and sight glasses on the compressor after evacuation.

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Oil Replenishment Methods Help Restore Lost Oil

oil replenishment methods help restore lost oil
Oil Replenishment Methods Help Restore Lost Oil

There are also methods to safely replenish lost oil when necessary:

  • Adding compressor oil of the same type through the suction port – Technicians can add small amounts of fresh oil this way to bring levels up to the proper mark.
  • Oil injection pumps – These specialized pumps on recharging stations can inject new oil into the low-pressure side of the system after evacuation is complete. The circulating refrigerant carries it throughout the piping and components.
  • Compressor oil change – In severe cases of oil loss, a complete compressor oil flush and replacement may be needed. This requires recovering all refrigerant, splitting the AC system, and draining all old oil before reassembly.

With responsible evacuation procedures and oil level maintenance, compressors can last over 10-15 years. But oil depletion can reduce the lifespan. That’s why monitoring oil levels after evacuating AC systems is so crucial.

When Might an AC System Need Evacuated?

Now that you know evacuating AC systems removes refrigerant gas but not the vital oil, you may be wondering when evacuation is necessary or recommended.

Here are some common scenarios when HVAC technicians will evacuate an air conditioning system with vacuum pumps:

  • Prior to major AC repairs – This allows the sealed system to be opened without releasing refrigerant.
  • When replacing a leaking or damaged component
  • Before charging new refrigerant after a leak
  • Annually prior to seasonal recharging (mostly commercial units)
  • When relocating AC equipment to a new home
  • After an AC system accident, flood damage, or fire
  • When adding components like more evaporator coils

The process cleans moisture, non-condensables, and refrigerant remnants from existing leaks out of the system prior to reassembly and recharging. This helps maximize cooling performance, efficiency, and equipment longevity.

While mostly refrigerant leaves during evacuation, remember a slight bit of the vital oil can also escape over time. Checking oil levels and replenishing it when below optimum is crucial for preventing compressor wear after evacuating AC systems.

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