by Gerry Wagner
Is evacuation the weak link in the mini split installation? I think so…and I think for many of us, we have never had a good understanding of why we evacuate and how a vacuum pump works. Lets see if we can shed some light on it right now.
Most mini split outdoor units come pre-charged with some amount of refrigerant …it is segregated in the outdoor unit by the hex-head valves, one on the suction and another at the liquid connection of the outdoor unit. Thus the evacuation in a new installation is limited to the lineset and the evaporator (indoor unit).
The primary purpose of evacuation is to remove any non-condensables and other contaminates from the lineset and evaporator coil which we just installed. Let’s not assume we all know what non-condensables are:
Non-condensables are gases that will not condense into a liquid within the operating temperatures of the refrigeration system. Air and nitrogen are the most common non-condensables.
Why are non-condensables bad?
Non-condensables occupy condenser coil space that is normally used to condense refrigerants. Because of this wasted condenser space, the proper amount of heat cannot be rejected, causing a rise in condenser temperatures/pressures, higher compression ratios, and system inefficiencies.
How do non-condensables get into the lineset and evaporator coil? Well, there is no way to avoid it. The moment you release the nitrogen the manufacturer charged in the evaporator to test for leaks…and remember nitrogen itself is a non-condensable, and remove the plastic caps from the end of your lineset tubing, letting air into the tubing, you now have an invasion of non-condensables in your system!
Believe it or not, there is a school of thought that evacuation is completely unnecessary. I am using the words “school” and “thought” VERY loosely here!
I had an attendee of one of my GREE mini split design and installation classes tell me that he worked for a fellow that insisted evacuation was a waste of time and that he never evacuated a system and he never had a problem as a result.
The greatest HVAC instructor in the biz is my friend John Barba of Taco Comfort Solutions in Cranston, Rhode Island. John is the reason I am instructor…his enthusiasm is inspiring! John has a great saying when people tell him, “I have never had a problem”…his reply is always, “define problem.”
Yeah, the guy who never evacuates may think he is on to something because the un-evacuated system works…it blows cold, but for how long? How cold? How inefficiently?
I was shocked to learn of a mini split product that is marketed specifically to the weekend warrior, the “do-it yourselfer.” It is a complete system package including a lineset with something the manufacturer
is calling “quick-connect fittings.” The installation manual makes NO mention of an evacuation procedure!
I have to believe a majority of these products are installed with the weekend warrior thinking there is no problem, otherwise this company would be out of business but as my friend John Barba would say, “define problem.”
If we remember what Mrs. Gillacuddy taught us in 9th grade science class, she said that “everything in nature seeks equilibrium of pressure.”
I like to use the example of an automobile tire blowout. Air rushes from inside the tire to the atmosphere where the pressure is lower. This difference in pressure causes the air to seek equilibrium of pressure. Any time there is a greater pressure in one area than in another this differential in pressure exists. When an automobile tire has a slow leak, a greater amount of time is required for the equilibrium to take place.
The need for this differential in pressure emphasizes the importance of a high-vacuum pump, as well as the use of large diameter connecting tubing. The purpose of a vacuum pump is to remove contaminants from inside a closed system, thus reducing the pressure inside the system. In order for the mixture of gases to flow from inside the system to the pump, the pump must create a sufficiently lower pressure inside its cylinder than the one inside the system being evacuated.
Now, a common fault of installers when evacuating, one that I will admit to being guilty of in the past, is not changing the oil in their vacuum pump each and every time they use it. I frankly thought that this requirement in most vacuum pump instruction manuals was simply a ploy to sell more vacuum pump oil. The following comes from the manual of the Yellow Jacket “Bullet” vacuum pump:
“Oil may look clean, but still be contaminated. Looking clean is not enough. One job is more than enough to contaminate oil.”
The following is from the Yellow Jacket Buzz Newsletter:
“Vacuum pump oil is the versatile ingredient inside the pump that lubricates the pumping apparatus while also collecting moisture and contaminants from evacuated systems.
Simply put, it’s what keeps the pump, well…pumping.
Because vacuum pumps don’t have filters, the oil inside becomes saturated with contaminants, which reduces the pump’s efficiency. Once oil is saturated it can’t absorb any more system moisture.”
Often, installers and service techs think they are using the right tools to evacuate when in fact they aren’t.
One example is using your standard manifold gauge set to evacuate. Although most low-side analog gauges show a green shaded area for vacuum readings to 30 in. Hg vac. (500 microns) the fact is that standard manifold gauge sets are not suited for evacuation procedures.
I highly recommend the Yellow Jacket SuperEvac System! The system includes a 2-valve manifold and two 3/8″ vacuum hoses for a fast vacuum. Vacuum valves allow core removal and system isolation. This combination reduces vacuum time by over 50% compared to using a regular 1/4″ manifold.
Vacuum pumps were intended to create vacuum but vacuum pumps were never intended to hold vacuum. The pump must be segregated from the system once the proper vacuum level has been reached.
So, we should heed the words of Pearl Jam drummer, Matt Cameron and lyrist of the song Evacuation from the Binaural album… “time to take heed and change directions.”
ABOUT THE AUTHOR: Gerry Wagner is the Vice President of HVAC Technical Training for Tradewinds Climate Systems. He has 38 years in the HVACR industry working in manufacturing, contracting and now training.

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