How to Evacuate an AC system, Full Vacuum Procedure

[Music] As I start talking about vacuum pumps, I want to pause real quick and just let you know that I've teamed up with my buddy Paul over at the Engineering Mindset channel for a video where he's gonna run you through the inner workings of a vacuum pump and how it actually works, as well as some animated models so you can visualize and build your understanding of the pump itself. So please check that out down at the links below and subscribe to his channel, Engineering Mindset. Hey, thanks for watching this complete procedure video for evacuation or vacuum for air conditioning refrigeration systems.

First off, why do we need to pull a vacuum? Well, the goal is to get everything out of the system—that's the copper lines, evaporator, condenser, compressor, all the components of that system. Everything that isn't refrigerant or oil needs to be out of there, and air has some things in it that we really don't want in there, namely water vapor and oxygen.

But even the primary constituent of air, which is nitrogen, we don't want in the system because it's what we call a non-condensable gas. It can't be condensed; it doesn't change state, and so it results in poor system performance and capacity. But especially oxygen and water vapor can result in corrosion and reactions inside the system that can be both harmful to the system and ultimately even dangerous.

So, first, let's establish that there really isn't any such thing as vacuum. Vacuum is just the absence of molecules. When we have our atmospheric pressure at sea level at 14.

7 PSI (pounds per square inch absolute), that is the pressure that our atmosphere exerts on us in all directions when we're near sea level. If you go up into the mountains, that pressure is a little bit less, but either way, we always have this pressure, and that pressure is exerted by the stuff that makes up our atmosphere, namely our air, which, like I said, is primarily nitrogen but also contains some other components such as oxygen, argon, CO2, and water vapor as its primary constituent parts. With a bunch of other stuff in there, we do not want that inside the system.

So, what we do is we create a negative pressure so that the molecules from inside the system move out, and we use a vacuum pump in order to create that negative pressure, causing those molecules to push their way out. So, while we think of a vacuum pump as sucking, really what it's doing is creating low pressure, and then those molecules are actually forcing their way towards that lower pressure. So, very simply, rather than thinking of it in terms of being sucked out, think of it as they want to come out; those molecules want to come out of the system when there's a lower pressure outside of the system, and the lowest pressure that we can create is at the vacuum pump itself.

So, our goal is, in everything that we connect to that system, to make it as easy as possible for those air molecules to get out of that system, out of that copper, out of that tubing, and into our vacuum pump. Now, in vacuum, we measure in very, very small units of measure, and those are called microns. A micron just means one millionth of a meter of mercury column; we're using mercury column as the measurement in this case.

So, a micron is literally a tiny, tiny, tiny measurement of pressure. At atmospheric pressure, we have 760 mm; microns equal atmospheric pressure. So, we have to go from 760 mm microns down to something much lower, and industry standard is typically about 500 microns.

So, you can see we have to create a pretty significant pressure differential in order to get from atmospheric pressure—14. 7 PSI or 760 mm microns—down to 500 microns or even lower. And so, we've got to have a properly functioning vacuum pump, first off, in order to do that.

Now, before we even get to the vacuum pump, in order to pull a deep vacuum and get those molecules out, we need to have a clean, dry, and tight system, which means that before we even get to the stage of hooking up that vacuum pump and hooking up our hoses, we want to make sure that, A, we don't let unnecessary water get into the system, we don't leave the lines open any more than we need to, and we keep any solid contaminants out of the system. Because a vacuum pump is not going to remove solid contamination; it can boil off liquid water, but it's time-consuming, and it's not something that you want your vacuum pump doing. You primarily want your vacuum pump removing that air and water vapor.

So, if you're working in rain, if you're working in a situation where your copper is going to be open for a really long time, you want to make sure that it stays completely sealed and that while you're working, you are purging and then flowing dry nitrogen in order to make sure that you're not getting any unnecessary moisture or contaminants into the system. So, let's talk quickly about purge, then flow, and all that means is that when you're working with copper tubing—which is primarily what we work with, though it might be aluminum or some other material, but generally we work in copper—you want to first purge your lines with dry nitrogen to fill those lines with nitrogen, and then you want to flow nitrogen while you're brazing. This is going to help prevent the build-up of cupric oxide or copper oxide inside your lines, which are basically like little copper flakes that come off of the copper when heated in the presence of oxygen, which brings.

. . Us back to this whole subject of oxygen: we really want to keep oxygen and water vapor out.

So, while we're working and we can't quite have a vacuum yet because we're in the process of brazing or fitting our tubing together, assembling our system, we want the system to be full of nitrogen rather than air that contains that oxygen and water vapor. So, step one is to make sure that the system is clean, dry, and tight. Step two is to make sure that the system is leak-free before connecting the vacuum pump.

Your vacuum pump and your vacuum are not there to help you find leaks. Now, we do a process to make sure that you aren't leaking under vacuum, but before you get to that stage, you want to have purged nitrogen, then flowed nitrogen while working, and then you want to pressurize the system to ensure that you have no leaks. So, the best procedure there is to pressurize the system to whatever the system design test pressure is, then do a bubble test on any filled joints that you've made to ensure that you have no leaks.

Let that stand, depending on the application, anywhere from 20 minutes up to a couple of days, depending on how mission-critical it is, to make sure that you have no leaks. But the result is to ensure that you have no leaks. So, now we're pressure testing using the Java link probes from Fieldpiece and the Measure Quick app to make sure that we're not dropping pressure, even after visually inspecting all of the joints.

Pressure testing to make sure that the system is completely leak-free is critical before you attempt to pull a good vacuum, because this is a system change out; we're only pulling on the evaporator coil in the line set. So, we're going to pressurize to the low side test pressure, which is 250 psi or thereabouts. One thing you'll notice whenever you're pressurizing a residential split system is that as you add pressure to the high side, eventually it will stop, and the suction pressure, the suction set will stop going up.

That's when you have a hard shut-off TXV. So, it'll often list as HSO or non-bleed. What occurs is that because the external equalizer is the closing force for the TXV, eventually that will overcome the opening force of the bulb and won't allow any more flow through.

At this point, we've set it right in at just under 250 psi, and we're going to monitor it to make sure that there are no leaks. I do have to give a caveat here: when we say leak-free, no system is leak-free; every system leaks tiny, tiny, tiny amounts, even if it's just at the molecular level. So, anytime you have any mechanical fittings, threaded fittings, things like flares where you have a press fitting between copper and brass, there are going to be microscopic leaks at those points, and you're going to find that when you get into deep vacuum and you start to do a decay test.

So, we're not looking for perfection, but we don't want to have any clear mistakes that lead to leaks, such as a braze joint or something like that. You are going to have those molecular leaks, and that's something that we just have to kind of live with. Now, let's get together the tools required for a proper vacuum or evacuation.

You need a vacuum pump, a good modern two-stage vacuum pump that can pull down to 50 microns or below when isolated with a micron gauge. You need a set of hoses; now we recommend using large gauge hoses and preferably hoses that you only use for vacuum versus for refrigerant delivery because when you use hoses, especially for things like recovery, they tend to become contaminated with refrigerant and refrigerant oil, making them more difficult to pull a deep vacuum with. When we say large gauge, that means larger than a quarter inch—3/8, half inch, or 3/4 inch—the bigger, the better when it comes to hoses, because we want to make it as easy as possible for those molecules to come out of the system and go into the vacuum pump.

We also are going to need a core remover tool, and the reason for that is that many systems, especially residential or smaller systems, have Schrader valves in them. A Schrader valve has a very, very small opening; it's like a valve on your tire. When you depress that Schrader, it only opens a little, tiny port that doesn't allow much flow through, and that's a huge restriction limiting the speed and the depth of your vacuum.

So, you want to use a core remover tool to get those cores out, which is going to greatly increase the speed of vacuum. Then finally, you need a vacuum gauge, a good quality vacuum gauge that reads down into the micron scale. It has to be a good modern vacuum gauge; you cannot use something like a compound refrigerant gauge.

The blue gauge doesn't have near the accuracy that you need. The example that Jim Bergman always gives is that trying to measure vacuum with a traditional gauge is sort of like trying to measure inches on your odometer in your car. While your odometer does measure distances, it's designed for larger distances.

In the case of your refrigerant gauges, they're designed to measure PSI at a time, not a micron at a time. A micron is a tiny, tiny measurement of pressure—one millionth of a meter of mercury column, like we discussed before. So, whenever measuring a vacuum, you have to use a vacuum gauge, often called a micron gauge.

So this is a core remover tool, and the core remover inside grabs that Schrader core, and these jaws hold it out using the system. Pressure, but also it kind of just grabs the head of the core. When you connect your vacuum, you want to connect here when you're sealing off for isolation purposes, either with refrigerant or when you're under vacuum doing your decay test.

That's when you shut off this valve, but you can only do that with this or remover out when it's in. It can be shut. So, the way you remove a core is as follows: back out the plunger, install it onto the service valve, place it over the core, and make sure that this handle is all the way open as you do this.

All the way, then you pull it out, and there is our Schrader core. In typical applications, the Schrader core is depressed with a core depressor inside the hose, a quarter-inch hose, and the internal volume is greatly restricted. Okay, so now we're ready to do our evacuation.

The first thing that you want to do is confirm that your vacuum pump is working properly. Before you even connect it to power, check that oil in the vacuum pump. If you haven't changed it recently, you need to change your oil in your vacuum pump, which is very easy to do.

The first thing you do is warm it up; you run the pump, isolate it for just a little bit so it warms the oil up, shut it off, and drain the oil. As you're draining the oil, you want to fill it with a little bit of clean oil while the dirty oil is coming out, just to make sure that you get all of that dirty oil out, and then fill it up to the proper fill level on the sight glass of your vacuum pump. You always want to visually inspect your oil as well to make sure that it doesn't show any signs of cloudiness or contamination, which can be an indication that that oil has moisture in it from a previous evacuation or from being left open.

You always want to store your vacuum pump sealed up so that way water vapor from the atmosphere can't make it into your vacuum pump. Once you know that it has nice clean oil in it, then you want to connect your vacuum pump just to a micron gauge, and you want to ensure that that vacuum pump pulls down to below 50 microns, fully isolated. Now, some manufacturers will say a hundred; you can look at what the ultimate pulldown rating of your particular vacuum pump is, but I like to use vacuum pumps that pull well below 50 microns in a matter of a few seconds.

If you connect it and it's only pulling down to five hundred microns or so, then that shows you that you're not going to be able to pull the system down that low. If you think of vacuum like a hill, those molecules have to fall into the pump from higher pressure to lower pressure. So, if your pump can't pull down to a very, very low micron level by itself, then you're not going to be able to get those molecules out of the system, and you're going to have a really tough time pulling a deep vacuum.

Another feature of your vacuum pump that I want you to understand is your gas ballast. A lot of manufacturers or technicians will advocate leaving the gas ballast open until you pull down into that deep vacuum stage and then closing it off. In practice, you can generally leave the gas ballast closed as long as the system that you're pulling on is known to be dry.

If you do have moisture in the system, though, it's best to pull down and remove that water vapor with the gas ballast open in order to keep from contaminating your oil. Also, another tip: if you know that you're pulling on a wet system, you can actually leave the gas ballast open once the oil has become moisture and contaminated, and that will help to dry that oil back out. I've seen several cases where you have creamy contaminated oil, and by running it with the gas ballast open, it will actually dry its own oil out.

Now, keep in mind that a vacuum pump works to remove those molecules out, but if you do have liquid water in the system, which does occur in some cases, that vacuum pump is pulling down to this deep vacuum, which is then boiling the water off. You're actually dropping the pressure to the point that water's going to boil because now the boiling temperature of that water is below your atmospheric temperature, so it's going to enter the system and boil that water off. Now, a quick note: if you have a system that's known to have water in it, it's helpful to heat the system with a heat gun, especially if you know where the moisture may be, and that will help drive some of that out and speed up the process, especially when you're working in low temperatures.

If you're in a low ambient condition, just where you live is cold basically, or if you're working on refrigeration equipment, using a heat gun is a really nice tip to help speed up that boiling of that water out of the system. So now you know your system is clean, dry, and tight. You've done your pressure test; you know that your vacuum pump is working properly as well as your micron gauge because now you've connected your micron gauge directly to the pump and confirmed that it's working properly.

You've inspected your oil, you understand how to use the gas ballast, and now it's time to go ahead and connect to the system. Step number one is going to be to make sure that there is no pressure on the system before you start to pull a vacuum. You do not pull a vacuum.

On a pressurized system, you use your core remover tools to remove the cores from the system wherever you're going to be drawing from. Now, in this case, we're going to primarily show a one-hose setup, and this is because, for typical residential applications, the one-hose setup for a change-out situation—which is a very common case where we need to pull a vacuum—is one of the easiest and best ways to pull a vacuum for a standard air conditioning technician or installer. You can also do a two-hose setup, and in that case, you would have to remove cores from both sides.

If you look at typical service valves in an air conditioner, you have your suction line, which is the big line, and you have your liquid line, which is the smaller line. Your suction line is going towards the compressor from the evaporator coil, and your liquid line is going towards the metering device from the condenser coil. So these are the two access points you have.

If you're going to be pulling on the entire system—such as the compressor, condenser coil, and evaporator all at once—then we suggest a two-hose setup. If you're only going to be pulling on the evaporator coil and line set, then the one-hose setup is generally going to be appropriate. So, you remove the core from whatever side you're pulling from with your core remover tool; you remove the end, and that's where you connect your hose going directly to your vacuum pump.

You'll notice here that we do not recommend using quarter-inch hoses, and we do not recommend using a manifold when pulling your vacuum. The reason for this is that quarter-inch hoses and manifolds tend to be both leaky and restrictive. Therefore, we're looking to eliminate leak points that can cause problems with your vacuum, and we're also looking to remove restrictions because we want to be able to get those molecules out the easiest way possible.

Now, a quick note: A lot of technicians will say that because the ports on the unit are quarter-inch, it doesn't matter that the hoses are larger going to the pump, and that’s just not true. The easiest way to think of this is in terms of a tollbooth on a highway. Tollbooths are very narrow, but they are only a short distance.

While they do act as a restriction to the flow of vehicles, you still have that entire highway, which is much wider, and that results in more cars being able to be moved. Again, our goal is to reduce the resistance of those molecules as they're traveling down the line, bumping up against the walls, and that sort of thing. We want to get them out as easily as possible.

We do want to remove the Schrader cores, and we do want to use large-gauge hoses for the fastest possible evacuation. We've done test after test, showing that both removing the cores and using large-gauge hoses that are used specifically for evacuation is the fastest possible way to pull a good vacuum. So now we’re connecting straight back to the vacuum pump, and we're using a micron gauge on the system itself.

Now, what we’ve found to be the easiest way is just to leave the Schrader in on the liquid line if you're just pulling on the line set and the evaporator coil and use a brass adapter going to your micron gauge with a corded pressure on it and attach it to the liquid line. The reason why we do this is that now it’s on the furthest point away from the system. Your micron gauge is now far, far away from where you're pulling from, which means that when we show 500 microns or less at that point, it proves that you have that deep vacuum all the way through the system.

One of the biggest mistakes that we see technicians make—and where they don't get great vacuum results—is when they connect their micron gauge at the vacuum pump. Now remember, your lowest pressure is at the pump, so you may see 500 microns at the vacuum pump and still have in the thousands of microns on the other side of the system; which is why, when you connect your micron gauge, you want to connect it the furthest possible away from the pump as you can in the system itself. If you're working on a large commercial system, sometimes you can even find another port on the other side that's going to give you a better indication of whether or not the vacuum has reached that distance.

You also don’t want to use pull-through ports on your micron gauge, as that acts as another restriction. It just increases the time of evacuation, and this is a significant difference in speed. I want to be clear here: A lot of technicians may see this and think this is overkill, but what we see is that on clean, dry, and tight systems, we're generally pulling our vacuums on change-outs in under 5 minutes, while many other technicians who are using manifolds, quarter-inch hoses, and installing their micron gauges in the wrong place are not getting a proper vacuum in the first place because they're measuring it at the pump—where it's at its lowest pressure—and, in some cases, it’s taking hours rather than minutes.

So, it’s a big time-saver, and you can ensure that you have a deep vacuum when that micron gauge is connected further away from the system and not pulling through the gauge. For typical residential air conditioning equipment, we want to pull down below 500 microns; we want to then isolate by shutting off our core tool going to our pump and make sure that our system does not decay. The term "decay" refers to a rise in pressure.

Remember, a lower micron reading means lower pressure, which is what we're looking for. Some carrier installation guides say to pull to 500 microns, isolate for 10 minutes, and ensure that it doesn't rise above a thousand. Now, our procedure is a little more thorough than that.

The procedure that we use at Kalos is to pull to below 500 microns, generally 250 or 300 microns, and ensure that it doesn't rise above 500 microns in 10 minutes. Now, the reason why you do this—why it would rise at all—is because, like we said, every system leaks a little bit, even if it's just around the seals of your core remover tool, at your flare fittings, or at any shaft lift connections. Those sorts of things are going to have tiny molecular leaks that aren't going to result in any significant refrigerant loss over the life of that system but do show up when you're using a micron gauge after you've pulled a deep vacuum.

Because again, remember, a micron is a tiny, tiny measurement of pressure. So when you're doing this decay test, you can be sure that you don't have significant leaks by watching that decay rate. Now, if you do have a decay rate that continues to go up, that means that you either have moisture contamination or leaks.

There are more thorough videos done by QED Tools and Blue Vac that will show you some of those procedures, but just know that if you have a really quick rise where you are rising more than 500 microns in a 10-minute period of time, you need to address that further. Now we're going to use our large aged hose, and we're going to use a half-inch connector to the pump—a dedicated vacuum hose with a very smooth bore, low permeability surface on the inside, which means that it doesn't have any refrigerant oil because it's never been used to transmit refrigerant. Therefore, it's going to pull a quicker vacuum.

So we have the vacuum pump, a 1/2 inch connector to the large seal clamp that attaches it to the 3/4 inch hose tying into the suction line. I'm going to put a cap on that just to make sure that we don't have any leakage whatsoever. It’s going to pull from the suction line out of the evaporator coil until it gets all the way back to this point.

Then we're going to read our deepest vacuum right here at this spot. Ready, set, initially you pull out a lot of air and nitrogen, and then very quickly it's going to strip. You can see the rate at which it's coming down, and remember, this is the far side of the system, so this is the deepest—this is the furthest away.

You can see at the pump itself we're already making a much lower vacuum, and that's because the vacuum, by its very nature, has to be lower than the furthest side of the system, so that way the molecules move from the farthest part of the system down to the lower pressure. Many technicians would think that they're already even a vacuum, but simply it's not the case. We can see that the hose out of the system is still… and see, this is a three and a half ton carrier system.

What system do you see here? One hundred and fifteen microns, and that's correct, but we're still taking a little while to get them all out. Remember, the lower the micron, the lower the vacuum level here, which vacuum level is just a measurement really of how many molecules are present.

So there are still more molecules in the curve inside the system than there are all the way back here. This is why you never want to connect your micron gauge at the pump, or if you can help it, even at the side that you’re drawing. You want to connect your micron gauge as far away from the pump and the connecting hoses as possible to show that you’re pulling that deep vacuum all the way down on the other side of this.

Alright, so we just hit 500 microns on the far side of the system, and you can see at the pump, okay, so now we’re down to 377 microns on the far side. We’re going to go ahead and valve off the core remover tool, and that way it separates the pump from the system. Now, we're going to make sure that this does not rise, and in fact, initially, it may actually go down just a little bit—we’re really holding 397, so there's a little bit of curve to the decay rate as well within the allowable range.

Now, with a good quality micron gauge, it's okay to actually open up the refrigerant circuit with the micron gauge still attached. You have to check the pressure rating of your gauge; it is good to leave it tipped upwards when you do this, so that way oil doesn’t have any chance to run down into it. If you're worried about this, what you can do instead of connecting it direct is actually do it on the side of a shutoff tool, like a quarter press or something of that nature, in order to prevent this from hitting the micron gauge.

What would be three bad gauges? Another thing to keep in mind when you're pulling on a system that previously had refrigerant in it—say, for example, you're replacing a compressor, or you're placing a condenser coil, or a reversing valve—something in a system that previously was operational and now you're going to pull a vacuum on it. Keep in mind that a micron gauge is actually a thermal sensor, and it's calibrated to air or nitrogen.

It is not calibrated to the thermal mass of refrigerant, so when that sensor comes in contact with little bursts of refrigerant, whether. . .

That refrigerant is in the system or entrained in the oil of the compressor; it's going to cause your gauge to go crazy for a little bit. So just keep in mind, when you're pulling on a system that previously had a refrigerant in it, and some of that refrigerant may be coming out of the oil, initially you may see some jumping readings on your micron gauge, and that's normal. Just continue pulling a deep vacuum until those numbers begin to stabilize.

There are a couple of really big myths that technicians will think when they watch this video, or whenever I talk about this. One is that you'll freeze moisture in the system by pulling a vacuum too fast, and the second is that you'll damage the oil by pulling too deep. We'll start with the second one: you're not gonna damage the oil by pulling too deep.

Typical system oil, like POE, in fact, requires a very, very deep vacuum if you are gonna pull on the oil. Unless you are pulling below 1 micron, which you're not, you're not going to damage the oil; that is not a concern. The second thing that a lot of technicians will say is that you're gonna freeze moisture in the system if you pull too fast.

It is possible to freeze moisture in a vacuum; there are many demonstrations that have been done on this, but generally, they're done in glass vessels where you don't have a lot of heat coming in to melt that ice as you're pulling down. Of course, it is possible to freeze ice inside of a system when you are near freezing conditions. So, obviously, if it's below 32 degrees outside and there’s water in the system, it's very easy to freeze ice because, of course, water freezes at 32 degrees.

Keep in mind that air conditioning and refrigeration equipment is designed to bring heat in and reject heat out of the system. So, whenever you are pulling down, yes, it is possible for that water temperature to drop, but because these systems are made of copper and aluminum, they're going to conduct heat in very quickly, which is going to help melt that ice. Even if you have ice in the system, you can still remove that ice via sublimation through deep vacuum.

It can be time-consuming, but the best bet there is, if you know that you have liquid water in the system, use a heat gun in conjunction with a deep vacuum in order to get the best result. I do not suggest trying to pull your vacuum slower because, again, time is money, and deep vacuum is the best way to ensure that you do not have those extra molecules in the system, resulting in system contamination. So, deep vacuum is the way to go.

If you're really concerned, you can break the vacuum with nitrogen periodically. That's often called a triple evacuation. A lot of people misunderstand and think that the nitrogen somehow carries the water; it can carry a little bit of moisture through entrainment, but in general, what you're doing when you see a really good result after breaking with nitrogen is that it's recalibrating the sensor on your micron gauge.

Because the micron gauge sensor was being affected by refrigerant, in a lot of cases where technicians are seeing issues and then they break with nitrogen, and then it works properly, it's more about the sensor and the micron gauge now working properly and less about actual liquid moisture being the issue. I want to finish up by showing the two-hose process. If you are gonna pull those two hoses, now what we recommend is still connecting the micron gauge.

At this point, you connect it on the side port of the core remover tool, either on the liquid line or the suction line, whichever you prefer. Now you just have to recognize that when you do your isolation test, it's almost certainly gonna jump up a little bit as it equalizes because when you're pulling from the same side that the micron gauge is attached to, it's being affected by that lower pressure at the pump. So once you isolate off, those micron numbers will jump up a little bit, which means that when you're using the two-hose setup and you're connecting with that micron gauge closer to the pump, now you're just gonna have to pull it down a little bit deeper and recognize that as soon as you shut off those ball valves on your core remover tools, that pressure is going to jump up just a little bit initially.

So that is the complete procedure for pulling a proper vacuum. Recognize that vacuum is good for removing moisture from the system and air from the system. The goal is to get all those molecules out, so that way the only thing you have in the system is refrigerant and oil, which is what the manufacturer wants.

It's going to result in a much longer life of the system; you're gonna have lower fail rates on things like expansion valves and compressors when you follow this procedure to ensure that your system is clean, dry, and tight. Thanks again for watching this. As I mentioned before, go over to the Engineering Mindset YouTube channel and check out Paul's video specifically about the internal workings of the vacuum pump.

My video focuses more on the application in the field, and his focus is more on specifically how a vacuum pump works, so do check that out. The links are down below. Hey, thanks for watching!