All right, so first thing i want to talk about this morning - is something that came up again. It's come up many times and will continue to come up because that's how life works. You know nothing new is under the sun. Isn't that right, absolutely nothing's new under the sun and that is accidentally overcharging a system.

So why do you think a technician? Let's just use yourself as an example. Why do you think you would accidentally overcharge a system? What would be the reason that you would do that? I think the txb is messed up, so i'm gon na add some. You think the txv's four pounds, you're gon na add, like four pounds, see what happens: yeah yeah um, true story. You've done it before.

Okay, all right, so you think the txv, but why would adding charge help you decide if the txt is messed up? You eliminate if it's low, okay, interesting all right, all right, okay, fair enough! So what are some other reasons? What do you think? The most common reason somebody adds charge when charge is not what's needed, burnt. Sorry, okay, low airflow, but we're jumping right to the cause. But what are they seeing? What are they seeing? That's making them think. Ah, this sucker needs some freon all right hold on we're.

Gon na write it on the board needs a lil oh hold on. I spelled lil wrong, lil freon all right there we go low, suction pressure, low suction pressure. So that's what i want to talk about is low suction pressure, because sometimes what i realize i do far too often i no i do that. Yes, i do, and actually last night was an example of it.

We had christmas at the clearabouts and i was standing there. All the sisters were talking and i was like way over in the corner and i let a silent one rip and nobody could hear and then all like, like three of them, prick up that. Sometimes, who did that was that you, but i was far enough away that that i didn't get blamed, so that was good. That was good.

Hopefully they don't watch this video anyway, good one good one, my wife's like what do you do for a living, all right, low, suction pressure, and so what i realize sometimes, is that i talk about this and that and the other thing - and i all your eyes - Glaze over and you don't get anything out of the class. So what i want you to get out of the class today is why low suction pressure happens. I want you to come away, knowing why that happens without a doubt. In your mind, no doubt in my mind why low suction pressure happens.

Okay, so let's get some. Let's get some theories. What are some theories of why a system might have low suction pressure? Okay, we're gon na, say low, we'll, say low air flow but low airflow, where let's be very specific low airflow wear over the evaporator coil over the evaporator. But let's see if we can think of a better way of saying that, because is low air flow.

Actually, the problem are you: just trying to sam is whispering in your ear. These whispering sweet nothings, so i don't so low load is actually what i i like to say: low load. That's a phrase i like to use and like folks who do refrigeration like to use that, but i want you to come up with a better way of thinking about the term low load and what it means. So, hey zeus, i was talking to jesus over here.

Hey zeus, how would you what's another way you can think of to say low load, not enough air going through the coil, okay, so low airflow over the evaporator coil? I think that's close enough to that one. But are there other things? What is what is actually happening when air goes over the evaporator coil? What is happening to the refrigerant inside the coil? It's changing state that is true, but what's causing it to change state the air, but what about the air is causing it to change state? It's the it's the what it's the heat. Okay, it's heat comes down to heat, so not enough heat entering the coil transfer, not enough heat entering the coil right. Ooh! Wait! No! You just said it right.

What did you? What did you just say? I want you to say it again: i liked it enthalpy enthalpy what is enthalpy anybody? Anybody want to try to tackle enthalpy total heat, yep, total heat content, and so our goal is to that's why i gave out some of those super secret hats that nobody knows about our goal is delta h. Our goal is to change total enthalpy. That's what we're trying to do simply stated we're trying to move heat around is that is that better we're trying to move heat around, and so in order to get heat out of the house. We have to put it where in where in the evaporative coil correct.

So we're taking heat, we're putting it into the evaporator coil if we're not putting enough heat into the evaporator coil, how does that show up in our symptom in our system shows up as low suction pressure, but more specifically, what's another thing: we can call low suction Pressure there's another name that i want to call low suction pressure, who said saturation eli saturation, but either there's even a so saturation is kind of a fancy term. Who here would like to tackle what saturation means exactly so as much as i talk about saturation being that uh being the point at which liquid and vapor are at the same place at the same time and the temperature that corresponds to that at a given pressure? That's not something that's easy for you to hold in your head right. So, let's make it simpler, we can call it suction pressure. We can call it suction saturation, but what's another thing we can call it there's another term.

They use it in refrigeration. All the time, low pressure that you can call the low pressure, you can call it evaporator temperature. Now. Why can we call it evaporator temperature, because it isn't actually evaporative temperature right, we're measuring we're generally on a split system, we're measuring our suction pressure outside at the condensing unit on the suction line? How on earth can we call that coil temperature? Anybody know saturation because when you take that temperature, when you take that pressure and you convert it to the saturation temperature, that is our coil temperature, so long as there's, not some significant pressure difference in between the two, which generally there's not going to be, i mean, Like 99.99 of the time, you're not going to have a significant pressure difference between your suction line and the pressure inside the evaporator coil.

That makes sense and when we say coil temperature, we're talking about a specific point in the coil. What point in the coil are we talking about i'm gon na? Let you think about it till you get it. What part of the coil are we talking about evaporative coil? So all right, let's start here! This is a. This is a crudely drawn triangle that is supposed to represent an evaporative coil.

Where do we feed the liquid refrigerant into the evaporator coil feed it into the bottom right and that's kind of a misnomer to call it liquid refrigerant? It's actually flash gas right because it goes in there's a pressure drop at our metering device right. Our meter device is the pressure dropper and it goes into our evaporator coil and then there's a you know, generally multiple rows and it goes through and then it comes out the top and the top is what we call. What what do we call the line coming out of the top of the evaporator coil suction line, right cool? Call it the suction line now, which part of the evaporator coil is going to be the same as your suction saturation temperature? No, no, the first 80, or so it's probably more than that. Generally speaking, but we you know, we say that so this part here and it's the part, that's full of boiling refrigerant right makes sense.

So that's what we can call this. We can say when we see low suction pressure. Here's how i want you to think of it. I don't want you to think of just some ambiguous pressure that you're supposed to hit, because somebody told you you're supposed to hit it now.

We know that it needs to be above 32 degrees in air conditioning. Why? Because it'll freeze otherwise, but the answer is not when you're, when your suction pressure is too low to add a refrigerant to it i mean it may be the end solution, but that's not what our brain should jump to. Here's. What i want you to think when you see low suction pressure that relates to a temperature i want you to think is that evaporator temperature correct? That's what i want you to think and when you look at an evaporator temperature that is more than 35 degrees lower than the return temperature.

That's when you can begin to think my evaporator temperature is too low, but i don't want you to jump to it's. Probably refrigerant, i want you to jump to it's probably airflow. I want you to just. I want your initial impulse relating to low coil temperature, low evaporator coil temperature to be that it's probably airflow, not enough heat entering the coil right.

That's even better way of thinking of it, and there are multiple reasons why not enough heat could be entering the coil, but it could be airflow right. In many cases it is airflow. For example, could you have enough air moving over the coil, but the coil has all of its fins missing? Could you have an evaporative coil that has no fins on it or very or the fins are all messed up? Absolutely if you work in a really salt environment, a corrosive environment, you can get a case where the fins actually fall off. This is a good way to imagine this.

So could you have perfectly good airflow, meaning the airflow that it's designed to have and still not get enough heat into the refrigerant? Absolutely that's what would happen if you had an evaporator coil with no fins on it. So would your suction pressure or your coil temperature be low or high? In that circumstance, it would be low right because in order to have the proper suction pressure, ie coil temperature - you have to be getting heat into that coil, because if you're, not that pressure and temperature is just going to keep diving the practice of adding a little Refrigerant to see what it does is not something that i'm going to eradicate from the field, because in some cases it actually is a good practice, and i'm not going to argue with all the super techs out there who want to tell me it's not. But here's. The problem the problem is, is that some of you are doing it without using a scale, if you add a little refrigerant, say: half a pound with a scale.

I'm fine with that you're not going to hurt anything on most equipment that we work on by adding half a pound with a scale. What you are going to cause trouble with is when you're not using a scale, and you just keep jacking it in what was only half a pound. Oh, i think it was only a half a pound. I think it was only about a half a pound right right.

That's what the text says. That's what you say in your mind. I think it was only a half a pound. Now i say this a lot, but if this is a practice that i can get you to change, it will make a really big difference for you, which is, whenever you put refrigerant in use a scale if you're working on complicated equipment and you're really starting to Get concerned about what's going on with it, especially when you have variable speed compression and it's acting weird, as my good friend larry dion, used to always say doing something: weird love larry, what yeah absolutely absolutely um the the challenge is, is that you run into this Situation and you think to yourself: well, i'm just going to add a little i'm just going to take a little out.

The best bet at that point is to go ahead and pull it essentially all out and weigh it out. We have the advantage nowadays that and i'm not telling you to waste refrigerant by any means. Um but 410a is not nearly as expensive as r22 once was, and the vast majority of what we work on is 410a and so to take a system, especially under warranty, especially if we installed it and pull the charge out just to see. What's in it, how much weight is in, it makes a lot of sense all right that should be a that should be one of the first things that we do under those circumstances, when you've got something really weird going on, especially with variable speed equipment where the Compressors can vary, condenser fan can vary, blower can vary, it can do a lot of things and it can trick you.

It can fool you so two sides of this. Primarily i'm wanting to say, weigh in refrigerant. If you are going to add a little bit but then secondarily, if you really are feel like you're chasing your tail, go ahead and pull it out, you don't have to pull it all out, don't pull it out to atmosphere, just pull it out till you get Down to you know 15 20 psi so that you can see how much you have in it. Now the question of whether you're going to take that refrigerant, you just pulled out and put it right back and again really depends on the condition of your tank.

Was it completely evacuated brand new tank before if it was something that you've been using for a while? You wouldn't want to put that back in another thing that i'm wanting to get us to do, and i know this is maybe a couple steps away is whenever you are recovering use a nice big, 3, 8 flare, filter, dryer and then charge back through a filter. Dryer as well out of the tank just to ensure you're not putting contaminated refrigerant, even if it's a fresh, even if you know the tank was evacuated and brand new, and you just pulled the refrigerant out, because those tanks can have sludge down in the bottom of Them because they've been processed many many times right, you buying what i'm selling sex. Sometimes you start putting it back. Yes, yeah.

That also happens so just just a few things to think about there, but go ahead so on this, because thinking to where i would have struggled with this in the field it most time it tended to be with two-stage equipment that i may not have realized, was Two-Stage equipment, right or variable, speed equipment that i may or may not have realized was variable speed, equipment or just variable speed equipment that i didn't see because i saw a ton of single stage equipment because i worked on a ton of property managers. So i got really good at single stage, but every once in a while. I would come across something that i didn't quite understand right and they do something weird narratively speaking right, um now it's as you become well-versed with them that it starts to make sense. But the biggest thing with the two stage, with the variable speed, is: how do we test those right so right, and what we're getting at here is that you have to be in high stage.

You have to be in high stage. You have to be in the appropriate cooling airflow in order to properly test and charge a multi-stage piece of equipment and that's a common mistake. If it's in low stage now you're trying to charge it and you might be adding refrigerant to it and wondering what's going on or maybe your airflow is ramped down, maybe you have a dehumidification mode on the equipment and so airflow is ramped down and now you're Trying to charge, but the main thing i'm wanting to cover and just make sure that you get anchored in your head, is that when you see low suction pressure, stop thinking low suction pressure, as if there's just some number you're supposed to hit instead think that's my Evaporator temperature, now heat entering the coil, affects your evaporator temperature. How much heat is entering that coil, but then also the amount of refrigerant also does right.

So if you are starving your evaporator coil, if your metering device is under feeding or if you are significantly undercharged, that will also cause low evaporator temperature. But i don't want you to start there. I always want you to start by thinking. Am i getting enough heat into this evaporator coil now, if you follow like our cheat chart with you know the colorful cheat chart that shows you you're going to know pretty quick if you're also looking at suction temperature you're.

Also, looking at super heat you're going to know pretty quick, whether or not it is an airflow problem or whether it's an underfeeding of the evaporator problem, so again, just to just to nail this down low airflow low load, not enough heat entering the coil. That's one category and then under feeding is the other category and back to what grant was saying he added a little refrigerant to it, a little refrigerant to it, because he was wanting to see if maybe it was low on charge, or maybe it was the metering Device but again keep in mind a txv's job is to maintain the superheat and the majority of what we work on nowadays are txvs. That's the majority of the meteor devices we work on and so we're not charging too superheat we're letting the txv do its job. We're just charging to sub cooling, so in terms of how we charge it's gotten a lot simpler if we know that we're in proper conditions like today, obviously it's cold enough outside that.

If you ran systems and cooling, you couldn't charge them properly. Unless you use a charging jacket or something else, but as long as you have proper conditions for running the system in cool mode and as long as the system is on high stage, meaning it's ramped all the way up, then you charge by sub cool, pretty simple Right but you do it with what a scale, because if you got a brand new piece of equipment that just went in and it comes with a factory charge of 14 pounds or whatever it is, and you are and you've got a 40-foot line set and you're. Putting more than a pound in you got to start thinking, something else is going on and primarily you want to start with low load airflow, not enough heat entering the evaporator coil. That's where you want to start on new installs, a really big thing that we need to be doing.

We've talked about it before is checking static pressure. I know we slide away from doing that. It's a very easy test to do, but, even more importantly, the more common cause of airflow problems on new installs are actually how we set up the system. So if it's an infinity system, how are we programming the control? If it's a you know, it's kind of your standard fv we're talking carrier model numbers here, but it's the same for pretty much every brand.

You have your communicating super high end equipment that you program from the controller, and then you have some versions that you program on the pins and the air handler. That's a common cause of problems or it's wired incorrectly. You've got you're landing your y wire on the wrong terminal or you're, not getting a call on your g terminal like we talked about recently. If you don't get a call on your g terminal, your blower is not going to ramp up on a lot of variable speed equipment, your dh terminal isn't wired properly or your d or d hume or whatever, whatever brand you're working on.

So those are all things that when, if i'm saying this and you're like - oh, i don't know what he's talking about that's okay, right, but now next time you go to one you need to know before you leave. You can't just assume you can't just say like we often do, like texts often do well. The air flow is fine. When you say the airflow was fine ronnie.

Have you ever said that before never? Yes, he has, if you say the airflow's fine. What do you think regular? What do you think an average technician means by the airflow was fine. The fan was running and they looked at stuff. They looked at the return, they looked at the filters, they looked at the evaporative coil right and that's good.

That's a really good place to start. In fact i advocate for that. I am not somebody who thinks that you need to pull all the fanciest tools and start doing that right away. It's just mostly going to be a waste of time for the bulk of what we do.

But after you do your visual inspection, the answer is not the airflow is fine. The answer is, i did a visual inspection, the evaporator coil looked clean, the blower wheel looked clean, the filter was clean, but there's a lot more, you don't know, did you inspect all the duct work inch by inch? No, of course you didn't. Did you check the static pressure? Well, i hope so, but statistically speaking the odds, are you probably didn't so that's the next step? Did you check all of the settings on the control board? Did you check the settings on the thermostat? Did you make sure all the wires were landed properly and they were getting the proper calls for the mode? Were you ramped all the way up and my point being that this is where you start like, i'm not being hyperbolic. I'm not exaggerating here.

This is where you start before you say: well, the airflow is fine, because very often we're adjusting charges adding a little taken a little out when that's just a waste of time. It's just an airflow problem. It was just a heat load problem, there's just not enough heat entering the evaporator coil problem and in terms of like you, don't have to be a super tech who knows the entire rack manual front and back in order to know this start with your visual inspections. If you're a good visual inspecting cleaning technician, that's a great start.

That's where you do start if you've gotten thrown out there and stuck right because a lot of us that's how we learn the trade. But the next step is now don't just go to adding and removing refrigerant go to understanding heat load in the evaporator coil, specifically airflow. And then the next step is, if you are going to add or take out a little charge, because there are cases that that's going to happen. Just use a scale and use some common sense about how much you're putting in and taken out and my standard rule.

Would be if you're going to do the add a little see what happens, add, half a pound and see what happens, don't add a pound, don't add two pounds, don't add three pounds or four pounds or six pounds or 23 pounds or just keep loading tanks in It boss, i ran out of freon on my truck. How much did you start with? Well, i only had three jugs 22. just need one more. I mean we've all seen it and we've probably all done it.

So i'm not making fun of i mean i am kind of making fun of people, but i'm making fun of myself too, because we've all done it. Um a zoning system, yeah one of the danvers closed or a bypass damper's open yep. I did. I did one where i instead of uh, because back in the day we used to have all these zoning systems, all these old honeywell troll attempt systems and they would have bypass dampers in them, and this one.

I thought that i tied the bypass damper closed, but i actually tied it open and it had a real low, suction pressure. I had to keep putting freon in that sucker and she just wasn't coming up. So, just remember that the one of the primary factors that drive your coil temperature is how much heat is going into that coil and the primary factor that drives how much heat is going into that coil is how much airflow is moving over the coil. Now, before you go too far, the other way - because this is another mistake - i did - you - can have too much airflow like there - it's not like you just keep increasing airflow, and that makes the system work better.

All right, that's not what we want. We want around a 40 degree, evaporator coil at 75 degree, indoor temperature, that's a typical operating condition, a standard operating condition in our trade and as soon as i say, stuff like that they're like whoa, i always get it well, that's not true of all equipment. I understand that i'm talking about residential typical equipment in our market, that is generally what we want to see a 40 degree: suction saturation, a 40 degree evaporator temperature at 75 degree, indoor temperature and that tracks right, indoor, temperature's, higher suction saturation's, going to be higher coil Evaporative coil temperature is going to be higher if it's lower, but if your air flows off, then those two aren't going to match. If your air flow is too high, then your coil temperature is going to be higher than it should be, and you're not going to remove as much condensation as you should you're not going to dehumidify as well as you should coil temperature's too low system's not going To work as well as it should it's not going to have the capacity it's going to be prone to freezing right makes sense.

Does everybody get that what i'm trying to hammer here, the other side of it is you know we're going to talk about this in a future class? Is thinking about condensing temperature rather than head pressure, same sort of thing, but on the other side. But if you start using a scale, a lot of these problems will go away. You're going to catch yourself, uh you'll check yourself for your wreck yourself. Sorry, i had to say it.

You know, that's how it goes. Um any heat across the questions this time of year with installs and service because of load conditions right: okay, so yeah, that's a good point. If you're gon na go out and you're gon na start a piece of equipment today, um you need to check it. In heat mode, pretty much every piece of equipment is going to have a heat mode check guideline and if you're, installing a new piece of equipment, you need to follow that guideline.

I have a whole set of rules of thumb that are great for service technicians. Working on older equipment, but for newer equipment. Look at the heat check guideline the the manufacturer that you're installing what it says. I can see some eyes there like.

Well, i don't think there is one whatever check it confirm start by weighing it in the length of your line set. You know all that, and mostly we don't have to make. These are not significant changes on a brand new piece of equipment, you're going to check it and cool just enough to make sure that it runs in cool you're, not going to run it for 20 minutes you're going to run it for 5 minutes and just make Sure it runs and cool and then you're going to run it in heat and use the heat check chart and that's when it's 65 degrees, outdoor ambient or lower that's pretty standard. Some brands are slightly different, but they're all on the panel yep.

That's yes! So follow what the panel says. That's that does that answer the question! Yep great, that's good! You do have a lot of people adding charge this time of year, because they're, just not used to seeing these low pressures yep and again in terms of things that you should have a backup of a scale, is a pretty big. One like a scale is something i don't want you to go without a scale is something i don't want you to say: hey where's your scale, oh well. I loaned it to somebody when, oh, i don't know a couple months ago, whatever right, it's like a micron gauge in that regard, you remember how much of this we went through with micron gauges when i was really when i really first started making sure you were Using them just like well, it's somewhere in here, someplace, you know kind of a thing i want you to have a scale.

Keep extra batteries, maybe get an analog scale, just as a backup in case you're. At about you know, i don't care just come up with a system, so that way, you always have a scale that you can rely on and again scales. You know, depending on the ground, that you're on and how level it is. It's not like you're going to be always perfect to the ounce.

I'm not worried about that for most of what we do. I'm worried that you're not putting four pounds in of a piece of equipment that doesn't need four pounds added to it. That's what i'm concerned about it's a waste. I mean the the fact is we're gon na make it right to the customer if we ever catch it, but it's just very wasteful and you're making equipment not work.

Well, so don't do that all right! That's it! Thanks for watching our video, if you enjoyed it and got something out of it, if you wouldn't mind hitting the thumbs up button to like the video subscribe to the channel and click, the notifications bell to be notified when new videos come out, hvac school is far More than a youtube channel, you can find out more by going to hvacrschool.com, which is our website and hub for all of our content, including tech tips, videos, podcasts and so much more. You can also subscribe to the podcast on any podcast app of your choosing. You can also join our facebook group if you want to weigh in on the conversation yourself thanks again for watching you.