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In today's live stream, Bryan talks about charging practices and common mistakes. He primarily discusses residential equipment, but many of the principles apply to commercial equipment as well.
The goal of the refrigeration circuit is to move heat around; it absorbs heat from a space and rejects it somewhere else via the refrigerant. We maximize the amount of heat absorbed by allowing the refrigerant to undergo phase changes (liquid and vapor). Many technicians add or remove refrigerant whenever the system does not work as it should, but adjusting the charge is not always the answer.
If a system has too little refrigerant, the refrigerant could boil off too quickly and at a lower temperature; the insufficient amount of molecules don't exert enough force to keep the pressure high enough to keep the boiling temperature up. So, the evaporator coil could freeze up. Your compressor could also heat up because some of the refrigerant cools the compressor, and you may have an insufficient amount. Systems with low charge also have low capacity because they feed less liquid into the evaporator.
If a system has too much refrigerant, it will run high head pressure. Too much liquid refrigerant in the condenser decreases the amount of space where condensing may occur. As such, the refrigerant will still condense, but it will occur at a higher temperature and pressure. The compression ratio is affected and negatively impacts system performance (poor efficiency, low mass flow rate of refrigerant). Systems with receivers can help control the charge by holding excess refrigerant charge, so long as the receiver doesn't exceed 80% capacity.
Before adjusting the charge, check the evaporator and condenser airflow. Look at air filters, blower wheels, and coil cleanliness. See what type of metering device you have; a dynamic metering device (TXV, EEV) will generally be charged by subcool, and a fixed orifice (piston, cap tube, header crimp) typically gets charged by superheat. (Check BOTH subcooling and superheat regardless of the metering device!) Assess the blower technology; make sure you are at full airflow and system capacity. Also, check if the variable-speed motor has been set up properly. Know your return and indoor air temperature, and check the outdoor temperature in the shade. Finally, you'll want to weigh in on the line length and look at the manufacturer's charging guidelines before adding or removing any charge.
We charge TXV systems by subcooling because they maintain a constant superheat. We charge fixed orifice systems based on superheat because we don't want to raise the head pressure and overfeed the evaporator by charging via subcooling. Superheat indicates evaporator feeding, and subcooling indicates condenser stacking. In residential HVAC, the superheat can be as low as 5 degrees (F) at the evaporator outlet, and it may approach 25 degrees near the compressor. On the other hand, a solid subcooling range is 5-20 degrees (F).
To charge via superheat, take the indoor wet-bulb temperature in the return and outdoor dry-bulb in the shade. Then, use the charging chart to determine your target superheat. Add refrigerant to decrease the superheat; recover refrigerant to increase the superheat.
When adding refrigerant, you must consider if the system is running or not running. If a system is not running, you can add the charge to the liquid line so that you don't flood the compressor upon startup. If a system is running, SLOWLY add refrigerant to the suction line.
Common charging errors include mixing refrigerants, loss on the disconnect, and using the wrong charging method. Some techs also fail to purge their hoses, depress Schrader cores, or weigh the charge with a scale. Techs may also make incorrect measurements by taking the temperatures at incorrect locations, failing to zero out their devices, using analog probes instead of digital ones, and taking temperatures readings in full sunlight. Some techs may also charge too quickly, which can flood the compressor.
Bryan also responds to some viewers' questions and comments about:
Profitability, pricing, and best practices
Things to check regularly
Setting the charge in winter
Delta T
The recent efficiency-longevity tradeoff
Charging with refrigerant blends (glide)
Read all the tech tips, take the quizzes, and find our handy calculators at https://www.hvacrschool.com/.


Yep we're live on youtube now. We've got uh a handful of apprentices here with us and uh. We may have some guests stop in not sure, but either way i am prepared to go solo and i'm counting on you youtube. I'm gon na pull you up so that way.

I can read your comments too, counting on you to comment, ask questions and generally be difficult. The way that you always are, but that's why we love you. You know you wouldn't be youtube if you weren't a little difficult. Okay, let me just make sure that i got that open all right, good, there's already 48 of you there.

So, as you enter on youtube, just say, hey say say what you're working on today, where you're from that kind of thing and uh and we'll get rolling. So today we're going to be talking about charging practices and common mistakes um. You never can't talk too much about charging right. I mean it's something that everybody always wants to learn more about.

Some of you may think that you know everything there is to know about it, but i can assure you you do not um. Obviously, when i talk about anything, i'm usually focusing on air conditioning and more residential air conditioning, but we can talk any direction you want. So if you've got questions, we can talk about larger equipment. If you like, a lot of these principles are going to.

Most of them actually are going to hold true for most equipment, at least to some degree, and i always like to back it up and and talk about the principles more than just the specifics. So i mean we got a lot of people coming in here: matt and calvin and arturo and more calvin and franco and jeff from sandusky little sandusky. You know that is a good. It's a good name for a town, just got ta, say jared's here so yeah.

All kinds of good stuff pedro, so uh yeah, come on in tell your friends, uh post it on facebook. We're gon na, be here the next two hours, so you can pop in and out as you like. But if you want to go on some of the groups and let people know that we're live uh with the link, that would also be awesome, because i did not prepare anybody for this in usual fashion, but i think i think you're going to enjoy it. I think there's going to be some good stuff, so here we go charging practices and common mistakes uh once again uh.

This is a live class uh with our hvac apprentices uh from lake technical college, uh registered apprenticeship program program here in the state of florida and the city, a beautiful eustis and uh historic eustis florida named after general eustis, actually, which regrettably, uh is one of the Things he's best known for is burning down a uh, a native american uh seminole indian uh encampment, very close to where i'm sitting right now, uh. Strangely enough, so uh it all comes full circle. I'm not sure what comes full circle, but something anyway. So thanks for joining here we go first things first, so let's talk through um because again you know we could.

We could just jump straight into all right. You know you add refrigerant in order to get your sub cooling up and that's it, but i want to talk through kind of what charge really even does and how to think about charge differently than you probably have in the past. That's going to help you make sense of maybe more complicated problems, so, first off, what's the goal of the refrigeration circuit? What are we trying to do in the refrigeration circuit? You know air conditioning, simultaneous control of you know: airflow humidity, temperature, uh, noise, filtration. All those things we can add in a few more that even willis carrier probably didn't think about.


Initially that's air conditioning, but the refrigeration circuit has a very specific goal and the specific goal is to move heat around now. A lot of people focus on the latent phase, change the change from a liquid to a vapor or a vapor to a liquid, and they like to focus on that. As what causes you know, what really the refrigeration circuit is, but it isn't necessarily in fact - and i've mentioned this before in other live streams and videos and podcasts that some of the first refrigeration circuits were actually single phase, meaning that all they did was compress and Decompress vapor, usually air, and then they started figuring out hey. We can leverage this latent phase change in order to move a lot more heat around so uh.

It's kind of a kind of a cool thing, uh that we can do by changing from liquid to vapor. Vapor to liquid and you've probably seen the charts before i won't bore you with it, but you move a lot more btus when you're phase changing but regardless the goal is to get heat moved from one place to another so to absorb it and then reject it, And we're absorbing heat into the evaporator coil. That's where we're picking it up at and then we're rejecting it in the condenser. So that's the goal.

Refrigerant is the medium by which we do that. But it's not uh. It's not the whole story and a lot of times the adding and removal of refrigerant is what technicians rely on, because it's sort of the the thing you can do. It's the easy dial you can turn on the equipment in order to make a change in it.

But it's really uh often overused and often not really the problem, and so it becomes something that causes more problems than it solves when text start either removing refrigerant or adding refrigerant. Generally, adding you know, it seems to be that the answer is add a little freon to almost everything, and at least that's the customer's perspective. So let's talk about that a little bit. What does having more or less refrigerant do to the system, so i don't want to.

I want one of the students to pipe up, so i'm going to give you a second to get prepared on your mute button there. So what happens to the system? There's no right or wrong answers here: okay, there are right or wrong answers, but i'm not going to say i'm not going to yell wrong at you if you say them wrong what happens to the system if it has too little refrigerant in it? What happens to the system? Somebody want to take that one or do i need a call on you, uh start freezing up, so it could freeze up right and actually that's the uh. That's the slide that i have here. The image that i have here is an r9 panel on diagnosing a freezing evaporator, and i did that for a reason, because this focuses on all the other things that can cause freezing, but potentially it could cause the system to freeze up sure.


But why is that? Why does having less refrigerant? Why can that result in the system freezing up specifically the evaporative coil freezing up uh, because you're not boiling it off an evaporator as it goes through the coil, it's boiling off too quickly, okay, boiling off too quickly, yeah, that's true! But why does that result in freezing liquid refrigerant? It's cold? Okay, so it boils at a low temperature all right so, but why is it boiling so so we kind of nailed it down here we're chasing down the answer. It's boiling it too low of a temperature right and the reason it's boiling at too low of a temperature when you have an undercharged condition, is because you don't have enough stuff, you don't have enough molecules right. So when you don't have enough stuff, you don't have enough molecules that are changing. Then what tends to happen is it doesn't exert enough force inside the evaporator coil in order to keep that pressure up and control that boiling temperature where we want it to be, but in terms of a freezing, evaporator coil, that's actually not the first place.

You that's not the first thing you do right that can result low refrigerant can result in a frozen evaporator coil. I actually got an argument with someone about this is that it can never cause a frozen evaporator coil, and that is uh most certainly not true. Again. We're talking about air conditioning here, um, you know! If you, a lot of people will say you know like if they're working, refrigeration they'll be like that coil's freezing up it's like well, if it's a if it's a freezer, then that coil better freeze up um.

Otherwise, it's not going to uh it's not going to cool the thing that is attempting to cool, but in air conditioning we it's undesirable for an evaporative coil to freeze up and so not having enough stuff in the evaporative coil can result in not having enough pressure Because higher pressure equals remember, we have done this higher temperature right. Higher pressure equals higher temperature, lower pressure equals lower temperature right, and so, when you have, the pressure in your evaporator coil is too low that results in a lower temperature, evaporator coil, but there's another little trick there we're gon na we're gon na get to that. So what what happens in the entire system? So not just the evaporator coil, what happens in the entire system? What are some other things that happens when you don't have enough refrigerant in it uh? Well, some majority of the compressors are refrigerant cooled, so your compressor's gon na run warm it's gon na run hot yep. That's a good one! Hey cory! Why don't you unmute? What are some things that happen in the system when you have uh too little refrigerant in it? If you have no liquid or not enough liquid in your condenser, okay, not enough liquid in your condenser and what's the result, what's the negative result of not having enough liquid in your condenser, close up cool and low sub cool means? What what does low sub cool mean less liquid, getting to the evaporator sure it means that you're, not you don't uh, you could potentially actually not have a full line of liquid when it uh gets to your metering device.


So when that refrigerant gets there, you could actually already have refrigerant. That's boiled off already and now that's lost capacity right. So there's lots of things that happen in the system when you have low refrigerant what happens if you have too much refrigerant? What does that do to the system? You just throw some things out there high head pressure? Okay, and why do you have high head pressure? Because you have a oh, you have too much liquid in your system too much liquid, specifically in one component, you over feed the compressor yeah in your condenser yeah. So, specifically, you get high head pressure because you have too much refrigerant in your condenser and what that does is and we'll talk about this a little later.

I'm we're doing this exercise because i want you to get your minds primed. So it's not just me talking is that in your condenser, those bottom that bottom row or bottom couple rows are full of liquid. When you add more refrigerant that liquid starts stacking up inside that condenser by itself, that's not an issue until you actually get to the place where you have hydrostatic pressure and that's a serious problem, but as you're stacking liquid in there. What does that do to the effective area of the condenser? That's actually dedicated to condensing uh causes your blue side to go up.

It causes your blue side to go up. Oh geez right side here inside joke people um what happens? Go ahead? Green, oh god! There. You go um, i totally forgot. Oh um, yeah you're, not you're, not um, giving that enough time to the liquid enough time.

You basically just have a full condenser full of liquid, so correct, correct and when you feel a condenser full of liquid, what happens? Is you decrease the amount of space that condensing can occur and when that happens, that refrigerant is still going to condense, but it's going to happen at a higher temperature and a higher pressure. So you know i try not to talk too much about in terms of like suction pressure and head pressure, liquid pressure, because really what we care about in the case of the condenser is the condensing temperature right when the condensing temperature goes up, because now there's less Space that also impacts your compression ratio. Compression ratio is absolute head pressure, divided by absolute suction pressure. When that happens, your compressor doesn't perform as well, so it doesn't actually move as much refrigerant and that results in a lower efficiency.


Obviously your current is going to go up when you have higher head pressure and you're going to move less refrigerant. Your mass flow rate is going to go down, so it's just a lose-lose all the way around to have unnecessarily high head pressure. We never want unnecessarily high head pressure. We want to keep it as low as we can stand to keep it so having too much refrigerant in the system is no bueno.

Refrigerant is just the medium by which we're moving the heat around it's. Actually, the heat going in and out of the refrigerant at the proper rates that controls and modulates our suction pressure and our head pressure, ie our evaporator temperature and our condensing temperature, because we want our evaporator and temperature and our condensing temperature to be in certain ranges. We talk about sub cool superheat, all this, but really what we're saying is we want enough refrigerant in that system to fill up that bottom row or two of the condenser based on the design with liquid, to have our liquid line full of liquid and then to Have enough refrigerant boiling in our evaporator coil, so that we're filling most of that evaporator coil with boiling refrigerant anything else any additional. In addition to that, actually results in less efficiency.

It actually makes the system run worse. So we don't having a little extra, isn't helpful. Now, a couple of guys here, both chad and corey, working market, refrigeration, grocery refrigeration, when you have a receiver that receiver holds that additional liquid and so that kind of acts as your buffer tank. You know it kind of allows you to operate uh with a proper charge over a wider range of overall system uh charge weight as long as it's below 80.

What's that you don't want to have it above 80. No, you never want to have your receiver above. Above 80, and and really it from a design consideration, you want to be able to pump down your entire system. Your entire system charge should fit into that receiver without it being more than 80 full um.

Now, that's not always the case, but that's the way that they uh should be designed so good, sorry, um, but uh. When you put your hand in your fan motor and it's not rejecting the heat, you know it's just cool. You know ambient air temp uh. That's that's overcharged, though right would that be undercharged? No, that would it could be a lot of different things um, but what that means is that you have a low condensing temperature, so um that in and of itself isn't the problem.


But what it often means is that you don't you you're, not rejecting much heat, uh, meaning you're, not absorbing much heat. So if you're not feeling much heat coming off your condenser fan, that's all the heat from whatever you're pulling it out of so that means you're. Not picking up much heat, and that can be a myriad of different things it could be undercharged could be a poor compression. Um could be a lot of things uh.

You know it could be that all the fans are off. You know on the evaporator side and so you're, just not absorbing much heat that way, but that's when you, whenever you don't feel much heat projection on the outside less than would be normal and that's what that is on the flip side, if you feel very hot Air coming out of the top of the condenser. That means that you have a higher condensing temperature, i.e higher head pressure. So when your discharge air out of your condenser, it feels hotter than normal and again we're talking completely subjective here.

But you know, if you throw your hand up over enough condensers, you start to get a pretty good sense of what's normal and what's not that's that's what that would indicate would be high head pressure and so all the things that cause high head pressure would cause High condenser discharge air temperature cool. I like how i like how you all are engaging. That's good stuff, mario, you just unmuted, okay, no, i thought you wanted to say something. That's all right! I did earlier.

I guess i didn't mute it again. Okay, well feel free to butt in any time all right. So those are just a few things i wanted to cover um, so everything's all connected. But let's talk about things you need to know before ever adjusting the charge you need to know or do you always do these before you mess with charge, and i don't care what kind of unit you're working on um it.

You know these things aren't going to necessarily all be exactly the same, but the categories that they represent are going to be the same always check everything that has to do with evaporator and condenser airflow before you start messing with charge. Now, when i say everything, obviously you're not going to do do a full duct inspection every time uh probably, but you need to at least do these visual things, because it's going to save you so much heartache. So look at your air filters. Look at your evaporator coil condition, look at your blower wheel and your condenser cleanliness pay attention to other obvious things like does it look like somebody put a different motor in here, a non-factory motor? Does it look like maybe somebody put a new fan blade in on the condenser and maybe put it in a different position in the shroud than it was supposed to be? Maybe somebody put in a condenser fan, that's a different motor than it was originally designed for so always do a solid visual inspection for those.


Those of you guys who work in market refrigeration. That would be an example would be where you use the vein anemometer and you go through and you make sure that all your cases have proper air velocity coming out of them. You don't want to go adjusting valves or making changes to charge, or anything like that until you have ensured to the best of your ability to the best practical that you have proper system cleanliness and that you have proper air flow. Now, when we say measure air flow, that's largely a misnomer.

So, for example, when you use that vein, anemometer and you go through and you are testing the air coming out of those cases - you're not measuring air flow in terms of cfm you're measuring velocity. Only. But it's enough to compare one case to the other and see you know, you're kind of doing one of these things is not like the other. You go from case to case and all of a sudden you hit one that doesn't have nearly the same velocity now.

You're going to investigate and see what's causing that airflow problem, but that's the point you want to make sure just from a practical standpoint: you're always doing your visual inspections. First, before you start messing with adding or removing refrigerant, you need to know what type of metering device you have. Does anybody know why you need to know what type of metering device you have before you start adding refrigerant to a system, because if you have a fixed metering device, you're going to be that's going to determine what you're going to be charging by? So if you have a piston, for example, superheat expansion valve that's a constant superheat device, so you'd be looking more at yourself, correct, yeah, so you're going to um you're going to charge based on the type of metering device. You have the the the method you're going to use.

Now, even that's not always the case. I mean, for example, uh ductless systems. You know they're going to have electronic expansion valves in them, but the manufacturers are generally not even going to give you guidelines of how to charge based on measurements they're going to tell you weigh the charge out and weigh it back in. So the right answer is that we charge based on the way the manufacturer tells us, but for the majority of the uh air conditioning equipment that we work on, that rule holds true.

If you have a txv you charge by sub cooling, if you have a fixed metering device, something like a piston or maybe even the you know, the kind of the header crimp metering devices that we see on some of these rooftop units. In that case, you're going to charge by super heat, but what that doesn't mean is that you only check sub cooling on a txv and you only check superheat on a piston system. Does anybody know why? That is why? Why is that a misnomer? Why don't we only check sub cooling on txv systems? Why do we also check super heat? Anybody know because you're super heat still gon na well there's a lot of different reasons but um. You know if you have an expansion valve you're, still gon na wan na make sure that that big about you know that the power head is doing its job and you don't have too high super heat too low is super heat same thing with airflow.


If you have a lot, if you don't have enough airflow across that coil, obviously that's going to throw off a bunch of your reading um if you don't have a full pump liquid go with that extension valve. That's all, and it's only going to meter properly but yeah exactly you broke up there a little bit at the end, but yeah you have to look at more than just charging because again, like we're saying we like to adjust things based on adjusting charge, but often The problem with the system isn't charged so we're check we're measuring super heat and sub cooling and watching both of them, because we're not just looking at getting our charge right. We're looking at is the entire system working properly and super heat and sub cool together. Tell us the full story and we're going to talk more about what that full story is as we move forward, but very simply, superheat tells us about evaporator, filling and sub cooling tells us about condenser filling now, some of you.

That may be the first time you've ever heard that, but i want to say it again and i want you to think about it, especially those of you who are a little more experienced and tell me if i'm wrong superheat is about evaporator filling sub cooling is About condenser filling we'll we'll we'll cover that more as we go forward, but just think on that one all right. So again we have to know what type of metering device we have. We have to know what type of blower technology we have now why on earth? Before we charge a system, would we need to know what type of blower technology we have? Anybody want to take a stab at that one? Why would we even care what type of blower we have in place? The reason we care i i figured nobody would want to take that one the reason. Well, i talked about the motor or this, the actual, the blower itself, not the wheel but yeah the type of motor.

Yes, the type of motor. Why? Why do we care? What type of motor we have in place before we start um, adding charge to a system? I mean you've got variable, speed, single speed. Um, you know. Is it gon na ramp up higher? When you know it's really trying to cool down - or you know once it's close within a degree or two of you know what you have it set to it's gon na it can slow down that blower and that's when your numbers didn't get thrown off a little Bit yeah, so a couple things here.

One is and some somebody's saying that in the comments too, is that we need to make sure that we're at full airflow and full system capacity for charging purposes. That's one, but also, if you have a variable, speed, setup and again we're talking about air conditioning here, mostly if you have a variable speed motor setup, there's a very good likelihood that it was not set up properly to begin with, meaning that you know if it's An x13 it might not be tapped properly if it's an ecm, it's possible that it's set for the wrong tonnage or maybe you're not getting a g call which can cause it to ramp down. There's a lot of things that people do wrong when they set up variable speed motors. Even you know the dehumidification.


If that's not set up properly, the thing could be running into humidification mode all the time we want to make sure that before we adjust charge with that we're at full blower speed. Obviously, if it's a multi-stage compressor variable, speed, compressor or whatever, we also want to make sure that's at high stage. But we need to know - and i probably should have added that here - you know what is it a single stage system or not, but we need to know what type of system we've got. We need to know what type of blower we have, because if we do have those more advanced technologies, there's a very good chance that the issue that we're seeing is due to that being set up improperly, so you've got to start there all right.

Next, you need to know your return, indoor air temperature. Now, if you're going to set the charge by super heat, you need to also know your wet bulb temperature in order to you know, charge a piston or a txv system um, but at minimum you need to know the actual return indoor air temperature. That's just something you got to know anyway. I say indoor temperature, slash return, it's really great.

If you know both it's great, if you know what the thermostat's saying and then also what it is in the return and that's handy uh, because it can tell you a little something about duct leakage, return leakage if you're drawing in air from your attic, for example, Then your air temperature by your unit is going to be higher than it is at the thermostat. So it's nice, if you can kind of pay attention to both of those. Now again, when you have a thermostat and then a you know, a probe thermometer they're, not necessarily always going to read the same thing um, so you got to take into account, for you know, differences in calibration. Obviously, then, you also have to know your outdoor temperature and i specify in the shade you got to know what your outdoor temperature is in the shade um.

So these are things you always gon na know before you before. You start messing with charge. Don't mess with charge until you know these things till you check these things until you've resolved these issues and that alone i could stop here, and you would, if you follow this, uh you'd be a better technician, because most of the issues that occur with charging problems Is you know, people overcharging, a system or maybe dumping charge out because they have high head pressure and they're, not paying attention to other things can be resolved by just very thorough inspection and then taking some basic measurements having what you need. The next thing, though, is weighing in on line length.


I want to talk about that quickly because it's important that, if you are charging a system to begin with, i don't want you just focusing on super heat and subcool. I want you to also have paid attention to at least an estimation of line length and looked at the manufacturer's information as far as what the original factory charge was and then that's going to give you a sense of as you're, adding or removing charge you're going To get a much better idea again, of course, using a scale, that's the biggest thing if you're ever recovering or charging, you need to always have a scale under that tank and look i'm the chief of sinners. When i first started off in the trade, nobody was using scales under their tanks and it was all a guess, guessing game. But that's not a good business plan uh.

It can be unethical, uh and also you're gon na run into more trouble, because if you got a system for example, where the total factory charge is, you know six pounds we'll say six pounds of 410a and your line length is, you know, maybe 40 feet or Something like that and then you look at the manufacturer. Uh you know chart and you say: okay, that's gon na end up with a few more ounces of charge, and now you start adding in a pound two pounds and you're not seeing a significant change. You really need to stop because, as a percentage you've already added uh, you know, like thirty 30 of the total system charge and that's you should have seen more change than that and that's where using a scale helps you do that. This is where it kind of prevents the tech from you know: jacking 8, 10 15 pounds in a system before they find out oops.

It's an airflow problem right, so paying attention to that line length. If you are going to charge a system you're going to commission, it initially weigh it in appropriately. You know: look at your line length. Look at the manufacturer's guidelines, weigh that in and then look at your super heat, sub cool and all your other measurements.

So that's! That's big all right next thing: um, i, like the term fixed versus dynamic metering device, a dynamic metering device would be your txv tev, exv um. Whatever you want to call them, i mean everybody gets worked up about. You know if you call them the wrong thing, but in the case of a dynamic metering device, there's something that moves inside of it and that's all dynamic means. It just means that it moves and so with the dynamic metering device.

That's where we're going to primarily use sub cooling as our charging factor and with the fixed metering device. That's where we're going to primarily use superheat as our charging factor, the most common fixed metering devices that we're going to see are pistons. Some of the brands call it an acurator for a while. I think it might be a carrier term, capillary tubes and smaller pieces of equipment.


You know small self-contained fridges, that sort of thing and then on package units, sometimes you'll see the header crimp and the header crimp is just simply um and i don't, i think, there's actually a technical name for that, and i don't remember. I never remember what it is, but um, where you're gon na see a liquid line, and it looks like it just goes straight from the liquid line right into the evaporator coil. You know that can't be the case. There has to be a pressure drop in order for there to be boiling in the evaporative coil, and so, if you pay attention, you'll often see these little spots and that's where it's metered and it's in multiple places.

I know corey you've had some uh you've had you've seen some systems with issues with those header crimps before right, yeah, all the time um, but the header crimp ones to my understanding are called accurators. Oh they're called i could be wrong, but that's what i've always called them. Okay, yeah. I know there was at least one manufacturer who called a piston inactivator um.

This goes all the way back to like mid-90s, something like that, but um, but yeah. That's that that is possible um, but either way it's it's just a it's just a pressure drop um and the problem that they have generally. My understanding is that they get plugged up with wax um right. That's what that's the that's the issue with them: yep yeah um yeah, especially with when you run high head pressure.

You know dirty condenser, coils and stuff like that: it'll um, like kind of cook, that oil and clog and they actually sell a conversion kit to convert all that into a an expansion valve yep, yep. So good thing to know. If you ever see those um, i should have put a picture here of those, but it's something to watch out for uh. So i want to talk quickly.

Why do you think why don't we charge a txv, tev exv? Why don't we charge those by super heat? What's the reason we use super heat on a fixed metering device and we use sub cooling when we're doing a tx vtev go ahead. Any of you take a stab at it. Why? What's the reason i'm waiting, i'm just going to wait! Anybody go ahead. Uh chad cory um go ahead.

Chad! You just repeat that my bad, it's okay! Why do we use ouch? Why don't we use superheat when we're charging uh as the as the primary charging factor when we're charging a tx vtev electronic expansion valve? Oh because it's okay, you can take a pass. You get a screaming kid go ahead. Corey you can take it um, so the expansion valve is going to open and close based on the load. Well, suction line temperature in particular um.

So, depending on how hot it is, uh and the space that you're trying to cool, you know that thing's gon na be wide open. It's gon na it's basically gon na meter. It's gon na try to maintain a the superheat that that needs as far as a fixed metering device or orifice you're going to have to provide that flow through. So you need a you know the proper charge to go through that metering device for it to get the super heat.


So basically the expansion valve is going to do the superheat for you, provided you have the correct amount of charge going to it. So it'll fluctuate yeah full line of liquid so with it with it, except what i was going to say: that's exactly what you're gon na say, yeah and then your kid's screaming in your ear, and it just distracts you i get it i've been there. You know. I got 10 of them, they're all they're, all just outside.

I lock them all outside and make them play with the ferret now get a ferret. That's a tip that i have. If you have screaming kids uh, especially a bitey ferret, but then it makes them scream more, but for different, more different reasons anyway. So all you got to do in order to make that that txv do its job.

When i say all you've got to do it's not all, but it's the main factor is feed it with a full line of liquid - and we see this in this image here, i'm going to go ahead and get my pointer out, so you can see what i'm Pointing at here oops there we go, so all you got to do to make the valve work properly is to feed it with a full line of liquid, and then it uses a balance between your bulb that feeds to the power head, which is your p1 force And your external equalizer and then it balances based on superheat, so another name for a txv one that i prefer is a constant superheat valve the reason why you can't charge a system so to answer the question. The reason why you can't charge a txt system by superheat, just because that's the valve's job, the valve's job, is to set superheat. So we just need to make sure we feed it proper, liquid and sub cooling is basically a measurement that tells us whether or not we're feeding it. A full line of liquid, even one degree of sub cooling is a full line of liquid, but it's kind of you know it's on the edge right, so we give it more so that way it doesn't begin to flash by the time it makes it from the Outlet of the condenser, where we set the sub cooling to where it gets to the valve.

Now, if you work in a you, know: refrigeration, you know that sub cooling isn't really what they use. They use a clear sight, glass and then they have a receiver and the receiver holds the excess liquid. So either way you know we we can get obsessed with sub cooling, as if sub cooling in in and of itself is the is the main thing. The main thing is feeding the valve with the full line of liquid.

Now, if that liquid temperature can be lower, that improves the efficiency of the evaporator coil, so we can do more with that evaporator coil. If it's it's a lower temperature liquid going into it but uh, but that's actually a little more complicated than a lot of people realize um. Really. What we're trying to do is just make sure that we have a full line of liquid feeding it.


So when we have a fixed metering device, we have to set the charge based on superheat because, as we adjust the charge, we're going to be adjusting our head pressure, increasing and decreasing our high pressure. And when you have higher pressure going into a fixed metering device. Guess what happens to the pressure going out higher pressure in equals? Higher pressure out? I was hoping somebody would pipe up, and i didn't give you enough time. Higher pressure into a fixed metering device equals higher pressure out of a fixed metering device, and we need to make sure that we don't over feed that evaporator coil, because we don't want to run suction.

We don't want to run liquid down the suction line into the compressor. We will kill that compressor if we feed it with liquid, and so we got to prevent that from happening, and so we've got to get that super heat range set improperly. Also, we don't want the superheat to be too high if the superheat's too high, then our evaporator coil isn't being fully fed. So that's inefficient and also our compressor is going to run hot because the compressor relies on the suction gas.

So that's the issue with the fixed metering device, it's good because of its simplicity, but it's challenging because, depending on various load conditions, it could either be over feeding or under feeding the evaporator coil. And so we have to kind of set it in in kind of a happy medium. So that way we don't run into those problems, but it's less. I just wanted its efficiency.

I want to jump in real, quick and just say also that uh, it's very important, that when you are charging a system that you don't you you want to make sure you have. You know your proper outdoor. It's like you, don't want to be charging when it's pouring down rain or if it's you know, super super cold outside right, yep. That's another good point! So on air conditioning the general rule - and this varies a little bit, but what what different manufacturers will tell you? But the general rule is when you're charging and cooling and air conditioning you don't want to charge when it's below 65 degrees outside it kind of gets you outside of the scale out of the normal operating conditions.

So remember, these systems are designed their normal operating conditions. As far as how they were rated is 95 degree, outdoor temperature, 80 degree, uh indoor temperature, 80 degree return temperature. So as far as the manufacturers are concerned for efficiency ratings, that is the operating temperature. But you know more realistically you're going to be anywhere from, like you know, 85 to 95 outside and then anywhere from.

You know, 70 to 80 inside that's sort of your normal operating conditions. When you start getting outside of that, then the system isn't really operating uh. As designed but again, 65 degree outdoor temperature and you make a really good point on rain when it's raining outside, what's happening to the system, what happens to the system when it rains? Anybody know we're cooling that outdoor it's cooling, that condenser off scrolling condenser off, which results in what uh probably would be a higher say that corey lower saturation temperature, yes you're, going to have a lower condensing temperature and you're going to have lower head pressure. And when you decrease your head pressure, you're also going to tend to decrease your suction pressure too, so you potentially could get outside of the the proper operation range and run uh, lower suction pressure too.


Now, obviously, big systems like rack refrigeration. They account for all of that, but it is going that system is going to be operating in a condition that it wasn't designed for now. It's actually generally going to be operating more efficiently than it's designed for, because when you have a wet condenser coil, you have this evaporative cooling, the fancy term, for that is adiabatic cooling and that results in lower condensing temperature and therefore lower compression ratio. Lower compression ratio means the compressor moves more refrigerant, and so you actually get higher capacity out of the system.

If you ever want to do this just for fun, i i've done it. In the past uh, you hook up your psychrometers on the system hook up measure quick so that you can actually pay attention to your btus that the system's producing then go outside and wet your condenser with a hose and you'll watch. Your total system capacity uh rises. The actual total amount of cooling that the system produces rises and that's a that's kind of an old school trick.

They use it in grocery all the time if you've got a really really hot day or you've got. You know an issue with a with a condenser. That's starting to deteriorate, or even if you lose a fan, sometimes they'll run sprinklers on the condenser in order to get more efficiency out of it or to get by, and it's actually not a terrible idea if you're just doing it temporarily, you just don't want to Do it for an extended period of time, because it's going to rot out whatever that water's running on over time, but i'll tell people that if you're trying to get you know customer buy for a couple days, i'm like the hottest, you know say you have it On seasonably hot weather, nothing wrong with with having people run a sprinkler on it for a couple days, not gon na hurt anything unless they have like ridiculously high chlorine levels, um and so the i you know, i actually wrote a newsletter at one point: uh, when The temperatures were going to be over 100 degrees, and i said, look you know if you want to get more out of your air conditioner, just put it on mist and run it run it next to your condenser and let it you know, let it suck in That that water, um, probably the saint john's water management district, probably doesn't appreciate that much and uh in some places that would be complete heresy, but it does help with the system. Um does help with the system, efficiency, the amount of uh efficiency and capacity all right.


So that was a little bit of an assign, but a good thing for everybody to know superheat and sub cooling. Why do we use different charging methods for each metering device type? Well, we already talked about that all right. So, let's talk about some acceptable ranges because um there's going to be some variation depending on the system depending on the technology depending on the category, but the there's some pretty standard acceptable ranges where, when you're outside of these acceptable ranges, you know it's pretty much a Problem, so let's talk about that. What is the acceptable range of superheat? What is the acceptable range? So, what? Let's start here? What is the lowest superheat that you should measure now again we're not talking about a flooded system or anything like that? We're talking about a typical air conditioning refrigeration system.

What's the lowest superheat that you should measure residential go ahead? 0.1. 1.1. Okay, that's uh! So that's true right! You don't want to get down to zero. Zero means you're flooding back um, but that's that's actually outside of the acceptable range just because our thermometers aren't even that accurate and there's this go ahead.

Refrigeration. You know like our like our freezers and walk-ins. You know we. We have those like three to five, but right, i feel like residential, i think, like what 16 18 super heat is successful.

It is going to be, it is going to be higher in residential and often it's because of where we're measuring so in refrigeration, when you're setting the superheat on a case. Where are you measuring the superheat at the case at the case right so right at the outlet of that evaporator coil? If you measure the superheat right before it goes into the compressor, have you have you've done that before and seen what it is? I have not actually oh that's a thing to do, especially if you have a compressor feeling go ahead. What was it, what did you say, i said be around like 30, 40. it'll be high, it'll be high, and this is in grocery, specifically um, and so that means that, even if you set your superheat at five say at the outlet of a case by the Time it makes it up all the way back to the rack it could be significantly higher and in residential we measure our super heat outside all the time right, we're not measuring it right at the outlet evaporate coil we're measuring it outside, and so that's why we're Commonly going to see, you know 15, 16, 17.

18 in that range. We're going to see that quite often, if you measured it at the outlet of the evaporator coil it'd probably be more like 10 12 13 on average. So that's that's pretty typical for air conditioning. In refrigeration, where you're trying to get the most you can out of that coil and your risk of flood back is a lot less um.


I shouldn't say that it's not a lot less. It's it's! It's actually worse for different reasons, but you have to have about a five degree: sub superheat minimum for most valves, especially txvs, because there's something called a minimum, stable superheat, which means that you get to a point with that valve. If you adjust it any more than that, where it's feeding more, it can potentially lose control and overfeed. So there's just this point where, if you try to get a little closer than that, it'll just lose control and overfeed, and so for a lot of valves.

Minimum stable superheat is, is you know five six in that range for txvs electronic expansion valves can get a little tighter than that because obviously they're using electronics rather than you know, old-school um. You know hydraulic forces so uh with electronic expansion valves. You can get down to three, you know three four more consistently and that that alone can make a significant capacity difference, especially in a really critical environment like grocery or industrial industrial. So your typical ranges uh, depending or you know, regardless of the valve you're, using regardless of what you're working on, is going to be somewhere between three to four at the lowest your super heat's going to be, and then, if you're measuring it back at the compressor.

Your highest superheats are going to be 30 ish, sometimes more than that. You know in certain applications, but you know for typical units. That's going to be about your standard, and the reason is: is that copeland and a lot of manufacturers will publish that - and this is an air conditioning number, but they don't want return gas. They don't want suction gas.

Coming back to the compressor with a temperature. That's higher than 65 degrees fahrenheit, and anybody know why. That is why don't we want our return gas going to our compressor being more than 65 degrees, fahrenheit won't cool. The compressor won't cool the compressor, very simple right.

So if you're checking suction line temperature, you can actually tell a lot about how a system's running, just by checking suction line temperature, if you've already done all the other stuff, that we talked about. Evaporative coil air filter, indoor temperature. All that sort of thing right, because if the system is operating in under normal operating conditions, what's a normal temperature, a normal average temperature that people keep it inside with their air conditioning 72. I'll, say 75 is the average right.

But if you have that number, so you know if it's 72 or 75 or 80, you know what it is. Now you can kind of anchor what your expected suction line temperature is. It's actually pretty pretty cool thing to do, and so, if you're running a system and it's 75 degrees inside and you go outside and check your suction line temperature and it's 70 right off the bat. You know there's a problem now that doesn't mean that it's charge, it could be a restriction, it could be a metering device, that's not feeding properly, it could be charge could be that it's undercharged, but what that's telling you is is that your suction gas, going back To that compressor is warm enough that, given enough time, it's going to result in damage to that compressor.


Now that doesn't mean it's going to explode instantaneously. A lot of people think when i say it's going to damage the compressor. That means instantaneous failure. No, that's rarely.

The case it just means over time the oil is going to break down loss of lubrication. That compressor is going to fail, so we don't want to run at least in air conditioning. We don't want to run return gas temperatures over 65, which means that if you think of the average evaporator coil temperature, this may be beyond some of you. But i want you to think about this.

What is a typical evaporator coil temperature that we run a run in air conditioning 40 degrees 40 degrees right. So if your standard indoor temperature is 75 and your standard uh td design, temperature difference is 35 degrees, meaning that you have 35 degree lower evaporator temperature than inside, then that means a standard evaporator coil temperature, if it's, if they have it 75 degrees inside it's going To be about 40 degrees right, so, if 65 degrees is the maximum that suction gas should be, what is our maximum superheat that we should have at our compressor with a 40 degree? Coil 65 is the maximum number 40 degree. Coil superheat is uh temperature. That's added beyond boiling beyond that evaporator temperature.

So what's the maximum superheat, we should have 25 25. You got it. I was waiting for somebody to do it. I wasn't gon na.

I wasn't gon na bail, you out there. So 25 degrees is pretty much your maximum superheat if you have a 40 degree, evaporator now, obviously uh. If the conditions are different, then that may vary um. But that's you know kind of a kind of your your average maximum.

You wouldn't want to get below five, because five degrees is super heat, because that means you've lost control potentially of your valve, and you could flood back. You could uh run liquid down into your compressor, compressor's, not a liquid pump, and so that could damage it. So suction gas, too hot equals compressor damage suction gas, zero superheat equals compressor damage. So we got to keep it in that range right, sub cooling.

What's our acceptable range of sub cooling uh? Well, it depends on the manufacturer depends on the system. Right does depend on the system, but if we think of what you're typically going to see and like refrigeration, sub cooling is going to be low because you're using your receiver as your liquid storage tank right and you don't even really generally measure it. You just make sure you have a full line of liquid you're using a side glass generally, and so there it doesn't really matter. But you know for most systems under most conditions, even a piston you're going to have somewhere between five degrees of sub cooling and maybe on the high end 20..


You know like it's going to be in there there's going to be very rare circumstances, you're going to see outside of that range in most of the equipment we work on now. Obviously, in order to know what the sub cooling should be, you got to look at the manufacturer data plate or their information, but so now you've kind of got something to go on on both of these you got somewhere to you know you kind of have a Good sense of where you're supposed to be, even if you don't know much about the equipment, you know that you shouldn't have a zero sub cool. You know you shouldn't, have a zero superheat, you know 30 degrees of superheat is probably too high. You know, 30 degrees.

A sub cool is probably too high for this piece of equipment, so it gives you something to go off of all right. We talked about this before, but when you have a superheat that is too low. That means that you are over feeding your evaporator coil. Okay, superheat, that is too low, means you're feeding liquid too far.

So superheat is an indication of how well you are feeding your evaporator coil with refrigerant. I want you to think about that. Superheat is telling you how well you're feeding your evaporator coil with refrigerant. If you've got a low superheat, it means too low.

That means that you're feeding it too much if you've got a superheat, that's too high. It means you're feeding it too little, but that doesn't mean that it's charged that's doing it right. You could have a bunch of refrigerant, that's backed up in the condenser, and you have a liquid line. Filter dryer, that's restricted, and so it's not letting enough refrigerant to make make it into that evaporator coil, which could result in high superheat.

You could have a a thv that has a failed power head or has a screen. That's plugged that's also causing that evaporator coil to under feed, which would result in high superheat does not always charge, but it could be right if you're charging a system by superheat and you've already gone through everything else and you're saying: okay, it is charged. Well, then, you add more charge in order to decrease the superheat in order to feed more cold liquid through that evaporate more boiling refrigerant through that evaporator coil, whatever tert worm. Whatever word, you want to use more saturated refrigerant through the evaporator.


A lot of people don't like to think of it as boiling, because that confuses them, even though that really is what it is in the evaporative coil. We have boiling refrigerant that boils at a low temperature. So superheat tells us how well we're feeding that evaporator coil sub cooling tells us how much liquid we're stacking in the condenser. Now, if you have a this, is assuming you don't have a mechanical subcooler, because last time i talked about this stacking of of liquid refrigerant somebody's like well.

If you have a mechanical side cooler, that's not how that works. Okay, most systems that most of us work on don't have mechanical subcoolers. If you do, then actually the mechanical subcooler is cooling the liquid, but in most cases, when we see 10 degrees of sub cool that tells us that we're stacking the right amount. If that system is rated for 10 degrees, is that cool we're stacking the right amount of liquid down in the bottom rows of that condenser? If we add more refrigerant, then we're going to get a higher sub cool because we're stacking more liquid, we're we're taking up space in that condenser with more liquid, which is raising our head pressure unnecessarily by the way, which is then driving up that sub cool number.

Because sub cooling is the difference between our condensing temperature and our actual liquid temperature you're going to notice here that in this presentation, i'm not in this class, i'm not going back and teaching you how to measure sub goal and superheat like we're, making an assumption that You know how to get these numbers um, but we are talking about how to charge a system in a smart way so that you really know what's going on with the equipment. So when you see sub cool going up, it tells you that you're feeding more refrigerant, you're stacking, more refrigerant from the bottom up in that evaporator coil. If you see subcool go down, that means that the amount of liquid in the bottom of that condenser is getting to be less and less make sense. Awesome actually any questions from any of any of the students before i move on that, but your discharge line should be normal operating 30 above ambient correct 30 above ambient you're saying the discharge line.

Temperature will be 30 above ambient yeah uh, not necessarily i mean like it can be significantly higher than that um, that's not going to be a universal. That's not going to be a universal rule at all. Um discharge line temperatures so like if it was only three yeah. I mean that's a good example like if it was only 30 degrees over your outdoor temperature.

Your ambient temperature you'd never burn yourself on it right because you're not going to burn yourself on something.

17 thoughts on “Charging considerations and mistakes – class 7/6/2021”
  1. Avataaar/Circle Created with python_avatars EDRIS HVAC Ductor says:

    @hvacschool love the information you provided how can I get in touch with you I have a small business in the dmv area and would like to ask a few questions, if you have time. Thanks again for your time

  2. Avataaar/Circle Created with python_avatars Eric Sanders says:

    I want to know who are the two chuckle heads that gave you a thumbs down. Something’s wrong with them because Bryan is one of the best teachers on YouTube. Are you in Barrhaven ?

  3. Avataaar/Circle Created with python_avatars R C says:

    You are a FANTASTIC teacher! I can tell you live this and love it!

  4. Avataaar/Circle Created with python_avatars Jericosha says:

    Great reminders. I really appreciate your videos every week. Gotta keep beating this into my noggin despite thinking I know it.

  5. Avataaar/Circle Created with python_avatars Nicholus Edwards says:

    On a 410a system the right set point 129 on the gauge

  6. Avataaar/Circle Created with python_avatars Nicholus Edwards says:

    Am always learning when I watch your channel???? When you going to ave class on split system

  7. Avataaar/Circle Created with python_avatars tanyaradzwa mwoyosvi says:

    I'm a Zimbabwean based in South Africa that's where I'm following you from Service area Orleans??

  8. Avataaar/Circle Created with python_avatars Burnard Hayes says:

    I have a mvac unit it blow cold when driving when not driving and car ldiling it blow cold for 30minutes then it blows warm just vacuum the unit then check for leaks no leaks check to see if low refrigerant still to speck what could be the problem anything you could recommend I do

  9. Avataaar/Circle Created with python_avatars Jenny Leung says:

    thank you from california

  10. Avataaar/Circle Created with python_avatars Christopher Borner says:

    You can use probe style gauges so you don't have any loss.

  11. Avataaar/Circle Created with python_avatars The Jammer says:

    A txv is variable that is why you use some cooling the other is fixed that is why you use superheat

  12. Avataaar/Circle Created with python_avatars The Jammer says:

    Pressure is not consistent with temperature low charge means low pressure which means low boiling point instead of having the correct boiling point

  13. Avataaar/Circle Created with python_avatars The Jammer says:

    Hvac instructor still keeping up with you tonight. Can NEVER stop learning for my students so I can be A better Instructor.

  14. Avataaar/Circle Created with python_avatars Christopher Borner says:

    Chris from Milwaukee, WI. Thanks for your videos.

  15. Avataaar/Circle Created with python_avatars Jerry Filipek says:

    Is it illegal and wrong to charge with r407-c in an R22 rated system.

  16. Avataaar/Circle Created with python_avatars Robert Blando says:

    "I'm prepared to go solo", one relates! Are you in Kanata ?

  17. Avataaar/Circle Created with python_avatars Abdorakeeb Naji says:

    Interesting to follow ur chanal ..
    AA service tech. MI.

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