How to Evacuate AC

man evacuating air conditioning system with tools
  • 3-4 hours
  • Intermediate
  • 300-500
What You'll Need
Refrigerant manifold gauge set
Vacuum pump
Hand tools
What You'll Need
Refrigerant manifold gauge set
Vacuum pump
Hand tools

Evacuating an air conditioner is necessary after any repairs that require opening the sealed components. Unsealing allows moist air to infiltrate the system, and that moisture must be removed for efficient cooling to continue. The following description will lead you through the processes required to correctly evacuate your air conditioner system.

Why Evacuate?

Air conditioner systems are evacuated to remove any air and/or moisture from the system components. This assures the best possible conditions for maximum cooling efficiency without freeze-up. Moisture in the system will freeze and most likely block flow of refrigerant, causing overly pressured conditions and possibly damaging AC system components. Air in the system will lower the ability of the air conditioner to cool efficiently.

Anytime an air conditioning system is opened to air exposure, it will need to be evacuated to assure best performance. The air conditioner desiccant (drying agent) contained in the receiver/dryer will collect quite a bit of moisture when opened and exposed to air. This moisture will take some time to boil away under vacuum. The desiccant will remove and trap quite a bit of moisture from the refrigerant, and will perform well if fully dried with vacuuming before refrigerant recharge.

A refrigerant purge will remove air from system, but it won't remove moisture from the desiccant, hence the need to use vacuuming for an extended period of time.

manifold gauge set


Air conditioner systems are evacuated by placing the connected components under vacuum. Most vacuum pumps will not achieve full vacuum of 29.6” Hg, but only manage 20-25” Hg. The lesser vacuum achieved by your pump will dictate the longer vacuuming times to achieve desired results. Once the desired vacuum range is achieved, you can shut off the vacuum pump and close the gauge valves for several minutes.

Observe the vacuum reading when the valves are closed, and check again in 30 minutes. If the vacuum reading goes up more than 2”/Hg on gauge, then check for leaks and repair as needed. The system must be able to hold vacuum for at least 30 minutes, or when it's recharged with refrigerant the refrigerant will escape and air conditioner will not operate.

You'll need a refrigerant manifold gauge set, a vacuum pump, and a set of basic hand tools. Some skill and knowledge is also necessary for the safe and proper use of refrigeration service equipment. Researching and reading the how to use manuals will help you use the tools safely and effectively.

vacuum air pump for evacuating air conditioning unit


Connect all hoses and components of auto air conditioner, install manifold gauge set by connecting the yellow hose to vacuum pump. Connect the blue hose to the suction side service port of your air conditioner system (larger hose) and the red hose to the pressure side service port (smaller hose).

Open all the valves and turn on the vacuum pump. When vacuum is achieved, close all valves and turn off the pump. Observe the reading on the vacuum gauge and check again in 10 minutes. If there's no change, turn on the vacuum pump and open all the valves to resume vacuuming. If the vacuum gauge has changed reading, find and repair any leaks.

man evacuating outdoor AC unit

When leaks are repaired, begin the vacuuming process again. Continue vacuuming for at least one hour and even longer in high humidity conditions or if your vacuum pump will not achieve a good vacuum in excess of 20” Hg. You want to vacuum out all the moisture possible.

The trapped moisture boils away slowly, thus the need for prolonged vacuuming. When the air conditioner system will hold vacuum for 30 minutes with all valves closed and vacuum pump turned off, it's ready for recharging with refrigerant.