One of our favorite “Go-to-a-licensed-professional-for-AC-system-charging” stories involves a guy we spent more time than we care to admit troubleshooting before it became apparent that he was releasing the vacuum before charging. His thinking: “I checked for leaks, now I’m moving on to the next step.” He was not only evacuating and charging without a license but without any understanding of what he was doing. Please, if you are not a licensed AC professional, do not evacuate/charge on your own. Go to a licensed professional. It’s worth it to get it done right. That said, we’ve run into many experienced car guys who don’t really understand evacuation and decided a quick summary would be helpful to even license AC professionals.
AC system evacuation serves three purposes:
- Initial leak check
- Remove air
- Remove moisture
Evacuation Purpose #1: Initial Leak Check
If everything is in order when you evacuate an AC system, the low side should reach a minimum of 28-30” Hg within a short period.
What does “low side” mean?
If you are asking this question, you shouldn’t be charging the system. Go to a licensed professional. But because you’re wondering, AC systems operate off pressure change. The high or high-pressure side is from the compressor to the expansion valve. The low or low-pressure side is from the expansion valve through the evaporator and back to the compressor.
After 30-45 minutes of evacuation, close the service valves and turn off the pump. Let the system sit for 10 minutes. If there is a vacuum loss of 2” or greater, there is probably a leak.
What if there is a leak?
Add your initial charge or fully charge the system and use your electronic leak detector to find the source. Focus on hose connections, crimp sleeves, service ports, compressor shaft seals, and assembly points. Since refrigerant is heavier than air, spend additional time on the underside of components. For hard-to-reach items, like the evaporator, check the condensation drain. Our advice is to start with the O-rings as it is not unusual for one to have been crushed or split by over-tightening during installation.)
Why do you say “initial” leak check?
Because despite what our customers thought, minimal initial vacuum loss is not a guarantee that your AC system is leak free. You should always run an electronic leak detector over the AC system after you charge it.
What is an electronic leak detector?
Again, if you are asking this question, our response is the same–you shouldn’t be charging the system. Go to a licensed professional who has an electronic leak detector.
Evacuation Purpose #2: Removing Air
Because there is only so much volume available within an AC system, it’s essential that air is removed from the system to allow for the refrigerant and oil. Our customer didn’t get this. When he released the vacuum, air filled the system. When he charged an air-filled system, he created pressures that were way too high. Another benefit of air removal when can charging is that it eases the initial charge of refrigerant into the system. (You need to get at least 50psi of refrigerant into the system before you turn the car on. That’s easier to do if all the air has been vacuumed out of the system before you pierce the can.)
Evacuation Purpose #3: Removing Moisture
Moisture removal is arguably the most important purpose of evacuation as moisture is the leading cause of AC system failure. When combined with refrigerant and lubrication, moisture will turn acidic (hydrochloric acid) and start to eat up the AC system components from the inside out. Aluminum components and seals are particularly vulnerable. To repeat, we recommend 30-45 minutes of evacuation at 28 – 30″ Hg of vacuum.
Why 28-30″Hg of vacuum?
Moisture won’t be drawn out by the vacuum pump unless it boils. A wonky example … at 24.04″ Hg of vacuum, water boils at 140 degrees Fahrenheit. You may be able to check for leaks, but even in Death Valley, you’re not getting out much moisture. At 29.87″ Hg of vacuum, water boils at five degrees Fahrenheit. We recommend 28-30″ Hg of vacuum to ensure you boil the moisture in the system so the vacuum pump can get it out.
Why 30-45 minutes?
The longer the evacuation, the more moisture will be removed. Note, running it longer than 30-45 minutes will likely remove even more moisture but you reach a point of diminishing returns. If you evacuate for 30-45 minutes, the filter drier should handle any residual moisture without a problem.
New Filter Drier Note
If your evacuation is done as part of a new system install, you’ll have already installed a new filter drier but if it is part of a system repair, don’t forget to replace the old filter drier. The filter drier should be replaced each and every time the air-conditioning system is opened up to the atmosphere. See our August 22, 2014 Tech Talk Post: My mechanic says I need to replace my AC filter drier. Do you agree?
To summarize proper AC system evacuation:
- Don’t do it if you are not a licensed professional
- New filter drier
- Evacuate for 30-45 minutes at 28-30″ Hg
- Let the AC system sit for 10 minutes. If vacuum loss is less than 2″ Hg, You’re ready to charge.
- After charging, the electronic leak test
I have a 1968 Mercedes 280 sl with an R12 Frigiking system. The original filter/dryers are no longer available. I would like to keep the system original. Can you open the can, replace the filter and dessicant and weld it up again. I believe the can is aluminum.
Good article, I am licensed, and journeyman level A/C refrigeration tech since 1983. If you are not EPA certified 608 and 609 at a minimum you should not attemp to service the system. Not only could you damage the system you can severely injur yourself to include death. Ignorant people do stupid things, check your technicians certifications before trusting your system to possible damage. The EPA also has stiff fines, and penalties for people who illegally work on A/C, and refrigeration systems weather you are a technician or not.
I’ll keep in mind what you said about AC professionals and that you shouldn’t try to charge your car’s AC without their help. My dad told me to do it myself. I’ll find a professional who can do it and get it done right the first time.