Evacuation is often called “vacuum” or “pulling a vacuum,” and it’s one of the most important parts of the HVACR installation and repair process where the refrigerant circuit is involved.

Our goal should be to keep the closed refrigeration circuit clean, dry, and tight — just like I was taught since the very first week of HVAC school.

The only things we want inside the system are the proper refrigerant and the correct oil in the factory-designated quantities. Things like air, moisture, dirt, copper shavings, oxides, or anything else that isn’t refrigerant or oil should be kept out.

This means we use good practices while assembling the field connections and doing repairs by following these important steps.

  • Keep tubing sealed with original plugs/caps, or tape if needed, prior to assembly.
  • Confirm fittings and refrigerant circuit components are clean and free of debris.
  • Eliminate contaminants from the tubing by cleaning before cutting.
  • Ream in such a way that shavings won’t fall into the lines.
  • Protect open pipework so that no rainwater or condensation can enter the lines.
  • Install and/or replace filter/driers as appropriate.
  • Purge with nitrogen before and after fitting and brazing the tubing.
  • Flow nitrogen while brazing to prevent a buildup of cupric oxide (nasty black flakes) inside the copper tubing.
  • Inspect joints all the way around with a mirror.
  • Perform a standing pressure test with high-pressure nitrogen, according to manufacturer recommendations.

These practices are all part of keeping the system clean, dry, and tight.

Evacuation won’t remove solid debris from the system, and it is not a substitute for using best practices. Most issues with evacuation can be attributed to improper workmanship prior to the connection of the evacuation rig.

Once the repair or installation is complete and the system is fully assembled and pressure tested, it is time to connect the vacuum pump and evacuation rig.

We’ll make some recommendations that may differ from the methods you are accustomed to, but first, let’s cover what evacuation is and what it does.



The atmosphere all around us is pressurized to around 14.7 psi (at sea level). Pulling a vacuum on the system is simply removing matter (mostly air and nitrogen) from inside the system so that the pressure inside drops below atmospheric pressure.

We use a vacuum pump specifically designed for this, and we measure the vacuum in the microns of mercury pressure scale (micron) using a vacuum gauge.

The lower the pressure in the system in microns, the deeper the vacuum below atmospheric pressure.



Evacuation accomplishes two important tasks.

For starters, it removes air and any other vapor from inside the system. This is referred to as the “degassing stage.”

Then, during the dehydration stage, it boils off any liquid water from inside the system by decreasing the pressure to below the vapor pressure of water at the ambient temperature.

The initial degassing stage generally happens quickly and easily. It’s the dehydration portion that requires a tight system and a deep vacuum, and it can be hard to accomplish without good practices.



A charging manifold — typically small-diameter, ¼-inch charging hoses — and valve cores all impose restrictions to flow.

When pressurizing a system, we can achieve a high flow rate to bring the pressure up quickly by raising the pressure at the source well above the destination to overcome these restrictions. For instance, if we use a nitrogen regulator set to 500 psi for a 300 psi pressure test, we start pressurizing with a 500 psi pressure differential and finish pressurizing with a 200 psi differential.

Here’s why evacuation is different, and why removing restrictions and using large hoses is important. The goal is to reduce the system pressure from 14.7 psia (760,000 microns) to as close to zero as possible — but we have to accomplish that using a pump that’s limited to 14.7 psi difference at most.

An absolute pressure of zero or negative can’t be achieved, so the pressure at the pump intake is higher than zero no matter how big the pump is. A bigger pump won’t result in a large pressure differential to overcome the restrictions like the 500 psi regulator did when we positively pressurized the system. The largest pressure differential during evacuation between the pump and the system we’re connecting to is 14.7 psi when we start evacuating, and as the pump pulls the system down, this pressure differential falls quickly at first and eventually reaches almost zero.

Any restrictions in the path between the pump and the system will greatly reduce flow rates during the evacuation process, resulting in significantly longer evacuation time.

The physics of evacuation are quite different from charging and pressure testing.

An evacuation rig includes dedicated large-diameter vacuum hoses, core removal and depressor tools, and a vacuum gauge, and it is much more effective for evacuation than the tools we use for charging and pressure testing.



The greater challenges that prevent proper vacuum are:

  • Leaks, even very tiny ones. Common culprits are hoses, manifolds, joints, or flare fittings.
  • Vacuum restrictions. Because vacuum occurs at a maximum pressure differential of 14.7 psi, all restrictions can slow down the process. These include small hoses, longer hoses than necessary, valve cores, hose core depressors, manifolds, ¼-inch pump ports, and anything else that inhibits maximum flow.
  • Improper micron gauge placement.
  • Hose contamination or off-gassing. If the vacuum hoses have been used with refrigerant, they can contain contaminants or moisture. Some new hoses begin to off-gas when placed under deep vacuum.
  • Refrigerant sensor interference, which can occur if the system previously had refrigerant in it. The refrigerant can be pulled out of the system as well as released from the oil during evacuation. This can interfere with the micron gauge readings.
  • Technicians who use the “30-minute vacuum, and let it rip” approach instead of proper measurement.
  • A pump that isn’t working properly and will not easily pull below 50 microns when isolated.



The easiest way to test your gear is to start with the micron gauge connected to the pump only and see if it pulls down to under 50 microns in a few minutes. If it does not, make sure your pump ballast is closed, ensure all caps and fittings are tight, change your pump oil, and then try again.

If the condition of the oil was extremely poor, more than one oil change may be needed to restore proper performance, but you should see an improvement after each oil change.

If you suspect your micron gauge isn’t working properly, try cleaning it. Use an eye dropper and put a few drops of denatured alcohol into the gauge port. Allow it to sit a few seconds, then gently tip the gauge up and down a few times before dumping out the alcohol. Do this a few times, then retest by comparing with another gauge if possible.

Next, check the deepest vacuum the pump can pull at the end of the evacuation hose rig to see how comparable it is to the pump alone. A major discrepancy can indicate leaks in the hose or hose fittings. Replace the hose seals if necessary, or choose better hoses.

Bluetooth app for a Micron gauge. - The ACHR News

APPS MAKE IT EASY: Some micron gauges can now be used with Blueooth apps to track vacuum levels and decay over time.



Small leaks around hose connections in your vacuum rig can reduce your vacuum speed and have you chasing ghosts.

I use a little Nylog sealant on all of my vacuum connection points. It can make a big difference at the deep vacuum level because Nylog will be pulled into the small voids and seal the vacuum leaks.

Keep in mind, when using an assembly lubricant in this way, you will need to be extra vigilant to keep dirt away from your connections.

Reduce the number of connections as much as possible. Eliminating the gauge manifold when vacuuming increases the speed and the tightness because most manifolds leak a small amount at the deep vacuum level.

Anything that’s being pulled into the system through leaks is adding to the system contamination rather than reducing it. This is why you eliminate even the small ones before evacuation, and never vacuum a system with known leaks.



Most valve cores can be removed using a core removal tool (CRT) prior to evacuation. This greatly decreases evacuation time because cores restrict the flow from the system toward the negative pressure provided by the vacuum pump.

The CRT also acts as a handy place to connect a vacuum gauge, and as an easy method to shut off and isolate the system and micron gauge from the pump and hoses when testing.

In some cases, the cores may not be possible or practical to remove, or the system may be fitted with high-flow cores. In these cases, you can use a special core depressor tool that depresses the cores to their maximum level with minimum restriction.

In either case, using a hose with no core depressor in the connector helps to increase vacuum flow.



Many techs mistakenly think that using a larger pump is the key to a fast vacuum.

The pump size only matters when the pump cfm capacity is less than the cfm capacity of the vacuum rig (hoses and fittings). This condition is likely to occur only briefly during the initial stage of evacuation of most residential or light commercial systems.

In most cases, techs are already using pumps that are much higher capacity than their rig will support. Pulling through any ¼-inch hoses reduces the speed of vacuum so significantly that the size of your pump becomes irrelevant.

This is why using the largest possible evacuation hoses of the shortest practical length is one of the best things you can do to reduce evacuation times.



Using dedicated vacuum hoses like TruBlu from Accutools will help consistently achieve deep vacuum quickly. Vacuum hoses are designed specifically to hold under deep vacuum and to keep moisture and contaminants from bonding to the inside.

A typical refrigerant hose used for recovery and charging has been exposed to refrigerant, oil, moisture, and other contaminants.



The micron gauge should be located as close to the system as possible for an accurate reading. When you place a micron gauge at the pump, it is measuring the pressure of the pump, which can be drastically different than the vacuum level at the system itself — especially at the furthest point in the system from where the pump is connected.

We find that the best place to locate the vacuum (micron) gauge is either on the side port of the suction CRT when vacuum hoses are connected to both service ports, or directly on the liquid line port when using a single hose on the suction port.



After degassing and dehydrating the system as deeply and as quickly as possible, isolate the system and micron gauge from the pump and hoses and confirm that it will hold the vacuum.

This is easily done with CRTs by closing the valves after achieving the target vacuum level. Sometimes, air can get trapped in the ball valves of the CRTs and cause a significant rise when closing them. Slowly close and open the CRTs a couple of times before starting the decay test.

In general, on a newly installed residential split system, you should be able to pull to a target vacuum of 300 microns or less. Then, close the CRT valves. The pressure in the system will rise slowly; this is called “vacuum decay.” After at least 10 minutes of isolation, the system pressure should remain under 500 microns. When servicing an existing system, especially when the entire system, including the compressor, is being evacuated, a target vacuum level of 500 microns and decay to under 1000 microns after a minimum of 10 minutes of isolation is more realistic.

If there are any leaks, trapped refrigerant, or moisture, then these targets will be impossible to achieve with typical residential or light commercial equipment up to 5 tons in capacity.



Some manufacturers advise pulling down to certain levels, then breaking the vacuum with nitrogen to bring the system up to atmospheric pressure (0 psig/14.7 psia,) then repeating the process. This certainly isn’t a bad practice, especially when contamination from moisture or interference from refrigerant is suspected.

If moisture or other vapor contaminants are suspected, a “nitrogen sweep” can be performed between evacuations. This consists of bringing the system pressure up slowly from a vacuum to 0 psig/14.7 psia by injecting nitrogen into one port until the system is at or slightly above atmospheric pressure, then venting the other port to the atmosphere while continuing the nitrogen flow. This allows nitrogen to carry unwanted vapors out of the system by displacement.

Some techs think that nitrogen “absorbs” moisture, but it does not. Nitrogen flowing through the system can carry moisture out or help to release refrigerant from the oil through entrainment, but it doesn’t absorb anything. It does help to displace the air and the moisture in the air, which is worthwhile in some instances.



You may hear that pulling a vacuum too quickly results in water freezing in the system. While it is true that water can freeze under vacuum, this only happens in real life when significant liquid water is in the system and when ambient temperatures are already near or below freezing. In low ambient conditions, it is advised to use a heat gun to warm the accumulator and evaporator and turn on the crankcase heater to help drive out the moisture. Pulling a deep and fast vacuum is always a good idea, and in a very wet system, using a heat gun and periodically sweeping with nitrogen will also help.

Some techs say that pulling a vacuum below 250 microns can damage the compressor oil. I’ve done extensive research and spoken to many experts and have seen no evidence that evacuation below 250 microns causes any issues with POE or mineral oil.

One excuse I often hear is that the ports on the system are ¼ inch, so larger hoses don’t matter. This simply isn’t true; larger, shorter hoses with no manifold can easily reduce vacuum time by 10 times or more, and this has been demonstrated time and time again. Even with a ¼-inch “choke point” in a few spots, the size of the hoses still matters significantly.



  • Use proper assembly practices to keep everything clean, dry, and tight.
  • Remove or fully depress cores.
  • Use dedicated large-diameter vacuum hoses and keep them as short as possible.
  • Keep clean oil in your pump and test the pump regularly.
  • Nix the manifold when evacuating and connect straight from the pump to CRTs.
  • Isolate and test after the desired vacuum level has been achieved to ensure there is no moisture or leaks.

Using this method, you will find that newly installed residential equipment can be pulled down to under 500 microns in under five minutes with another 10 minutes for the decay test. This is a combination of time savings and best practices for a win-win result for you and the customer.

Publication date: 2/25/2019

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