All right so this morning i want to talk about a technical topic, the topic of sub cooling and what it is. So we just happen to have this little trainer here, which is which is nice. It's this guy right here, yeah, okay, so this is our evaporator. Wait, i'm confused i'm very confused.

Oh no! This is our condenser all right, i'm a little confused about how this one is piped, but regardless all right, so we're gon na talk a little bit about sub cooling. So, first off, how do we measure sub cooling everybody's feeling talkative this morning? I appreciate that britain, how do we measure sub cooling? Sorry, excuse me: you're excused uh pressure wise on the liquid line with the temperature. Yes, so you compare the pressure and saturation on the liquid line and then you convert that to what what do you have to do in order to convert your pressure to saturation on the liquid line reference table, you have to reference a table right and in most Cases that table nowadays is within our digital manifold. You just set the refrigerant and it automatically gives you your liquid line, saturation temperature right or condensing temperature.

Well, that's what i prefer to call it. I prefer to call it condensing temperature, because that's really what it is. It's the temperature at which the refrigerant is i'm we're going to do this as a group just to make sure that you're all alive, your condensing temperature is the temperature at which the refrigerant is condensing. Let's try this again.

Condensing temperature is the temperature at which the refrigerant is condensing. Thank you, condensing, condensing great, nothing like a morning meeting um. So when we're looking at our saturated liquid temperature, our saturated condensing temperature, our condensing temperature, what we're looking at is the temperature at which the refrigerant is condensing inside the condenser. So if i could pick a tube in the center of the condenser, this is our.

This is our condenser here if i could pick a tube in the center of the condenser, and i could attach a thermometer to that tube in the center of the condenser. I should see the very same temperature that i see when i measure my liquid line pressure and convert it to a condensing temperature or saturation temperature right. So i'm taking a pressure, i'm converting it to a temperature, but that represents a real life thing, and this is what i always want to get get through to text. When we talk about saturation temperature.

Is that i'm not it's? Not just some ambiguous number, that's meaningless! It actually means something when we're doing it on the suction side. That's our evaporator temperature. It is the temperature at which the refrigerant is evaporating in the evaporator and when i looking at condensing temperature or liquid saturation, it's the temperature at which it's condensing right exactly, and why does that matter to us? It matters because it gives us a reference point about several different things. It gives us a reference point about how well the refrigerant is being compressed.

It gives us a reference point, especially with that when compared to our outdoor temperature, that's what we call approach method right. It also gives us uh, that's not first method. That's ctoa approach method is liquid line temperature. Sorry, i have to back up there, so our condensing temperature compared to our outdoor temperatures.

What we call condensing temperature over ambient or ctoa is what we typically call it and then, if we just compare our liquid line temperature to outdoor temperature, that's what we call approach. But all of those are giving us some reference for how this system is doing. In terms of both compression mass flow rate, how much refrigerant is moving through the system and charge, so it tells us a lot. Sub cool tells us a lot, because it's a compound measurement.

So, starting with what britain said, we figure out what our liquid line saturation temperature is. That tells us our condensing temperature. Now we compare that to what our actual liquid line temperature is all right, so condensing temperature minus our actual liquid line temperature equals sub cooling right. How much temperature did it drop so in a single pass, condenser which this is not? But if it was a single pass condenser where you had your discharge, gas going in it would go in at the top and it would travel through until it became fully liquid.

At the bottom, okay, so you create a liquid seal at the bottom of your condenser. You need to because our systems don't have receivers in them. The bulk of the equipment we work on and air conditioning doesn't have a receiver in it. So the liquid is stored in the liquid line and in the bottom of the condenser that acts as basically our liquid storage tank for the system.

When it's running you guys following me so far, so if i don't have enough refrigerant in the system, what happens eventually, i lose that liquid seal has anybody ever seen a sight glass and a liquid line where you can actually look and see in, and you see The bubbles in the sight, glass, the bubbles in the sight glass, are an indication that we don't have a liquid seal down at the bottom, because you still have vapor and liquid mixed another name, for that is zero sub cooling. Another name for that is still at saturation, meaning it has not fully condensed. It is not 100 condensed, it doesn't mean it hasn't been condensing because it has because it's mostly liquid still, i mean it's very rare - that you're going to find a site glass. That's mostly vapor, it's still mostly liquid, but it hasn't fully changed to have a complete liquid seal in the bottom of this condenser okay.

So if we're measuring a lower sub cool than what our target is, let's back up, first off, where do you find on the equipment that you install we'll talk to installers here, equipment, you install jordan, you look like you're prying, your eyes open, so i'm going to Talk to you and the equipment that you install. Where do you find your target? Sub cool, the condenser, the condenser data tag just says it right there. It makes it so easy and even easier than that, it's almost always around 10. for most of the equipment.

We work on other than micro channel condensers, it's so close to 10. As to you can pretty much just guess: it's 10 in most cases and okay, maybe 8. It may be 14.. I'm not telling you not to follow what the data tag says, but i'm just saying that it's it's always close to 10..

So if we have less than that number so say the target is 10.. We but we're measuring the differential between our condensing temperature and our actual liquid line. Temperature is only three. What do we do? What do we do? Just coils check, coils, okay check head pressure.

What is it what if it's a brand new system? Second, refrigerant make sure it's charged yeah, so you would probably so if you did a new piece of equipment, most people they're going to charge to sub cool right. So what you do is you start jacking refrigerant in it until you get more sub cool you do it slowly. You don't jack it in right. You do it slowly! That's right and you use a scale and you do it only after only after what you've realized.

That's over on your feet, you do it only well, that's the line set is abnormally long, correct and then you're weighing it in based on your estimated length of line set and all that sort of thing. These are all the right answers to change the correct form. Right you're give you're giving you're giving a lot of good answers here. It's run a while you're weighing it in you're doing that, based on estimated line set length because all of them have information on how to do that within the installation instructions.

So you're rtfmming, you just you just eat it in there. No, no! You eat it in there right right, because eating is yeah right. Yeah just eat that refrigerant in there a good stopping manual about piping length. So you just have to do it till you get some cool, it doesn't say: hey.

I had this much refrigerant on carrier, sure uh. So i think so you bring up a good point, it's possible, it's been a while, so i will rtfm and uh and we will add it into the specific carrier chart. It may not be in the installation manual, it may be in the product data or it may be in the actual carrier, long line guideline because there's a separate guideline specifically for online applications. So we'll look at that and i'll get that out to you.

So you've got that because - and in fact that's actually that's actually universal um. The amount of refrigerant takes to fill a certain diameter of line um, that's it. This isn't like a manufacturer, specific thing and people would expect me to know these things off. The top of my head, i actually had somebody who's, a very well-known industry.

Trainer for a very large manufacturer asked me. So what number do you use? Do you use da da da da, or do you use dd d and i'm like i don't remember numbers. I look it up every time. I don't remember my own phone number basically and i used that exact voice and then he hung up on me and said.

I'm never watching your videos again, and i said that's fine. You said that after you're on the board yeah, he did he. He texted me. He sent me a snapchat he snapped and then he chatted yeah huh it's under product data, it's under product data it charged for 15 feet and then it had to be a whole formula to put in yeah a whole laundry list, and that is actually so.

We just i'm going to add that to the app that's a good one, to add to the app put in your line, size and then yeah, yeah i'll, add that to the app okay, huh yeah you'll get all the commissions. Oh, that's good, yeah, exactly yeah good times, good times, okay! So yes, the point is, is that we we do we charge by sub cool, and we do that because that's a good thing to do it's what the manufacturers tell you to do, but you got to do all this other stuff. First, right and matt made some good points, you're going to a piece of equipment. That's that you haven't that isn't brand new.

You need to go through all the other stuff first now. The thing with sub cool, though, is that those other factors actually don't change it. As much as you would think, subcool is a fairly stable number in comparison with other numbers. You know: super heat can move all around, especially with a fixed orifice, but sub cool is pretty fixed.

Even if you have low indoor airflow you're. Still, your sub cool is going to be pretty close. Generally speaking, it's not going to change a lot, so sub cool makes it it's a nice number because of that it's a fairly trustworthy guide to follow, but here's what i want. Here's where i kind of want to peg this is imagine to yourself when you have a higher sub cool number, that you're filling that condenser coil with more liquid right, you're stacking, more liquid in the condenser coil.

Now, what happens as you stack more and more liquid in that condenser coil? What happens to the effective space that condensing can occur? It becomes smaller right so because, when you have a liquid column, liquid doesn't compress - i mean it, it does compress, but so little that it doesn't matter so you've got this really solid column of liquid. That's filling up your condenser, so the more you pack liquid in here the smaller the area, and this is why you'll notice you'll hit a point. As you start charging a system that that head pressure will start going up pretty darn quick now, when you're dealing with ductless systems, you're going to hit a safety state where now it actually starts limiting its compression, and so that tricks you because your head pressure doesn't Just keep going up so that was, you know, we've run into that before, which is again why you have to use scales and you have to weigh it in and all that kind of stuff. Okay, yeah! It's good! That's good! Hey, hey education costs! You one way or another: that's what we always say.

I don't know who we is, and i don't think we always say it. But but, as you stack more liquid in that condenser, you reduce the amount of condensing area and that drives your head pressure up. Because now the area that's available for heat rejection is decreased because we have a limit to how cold that liquid can get right. There is a limit, there's a stopping point that liquid can only get so cold in the condenser and what is that stopping point? Nope? It's not freezing.

What is your condenser rejecting its heat too outdoor air right? So can your condenser by itself? Can it get the liquid colder than the outdoor air? The answer is no. So, in order for your sub cool to keep going up and we've seen this - you can get 20 25 30 degrees of sub cool right. Well, how do you get that because, as you add more charge, once you get to near outdoor air temperature, once you kind of hit that number that number can't drop anymore, so it doesn't keep cooling in order to get more sub cool? What happens your condensing temperature is rising because there's two pieces to that pie right your liquid. In order to get more sub cooling, your liquid line temperature can get cooler or your head pressure and condensing temperature can get higher because it's a differential number that makes sense.

So once we get to the point that we've stacked enough liquid in here, because the goal, the reason why we have sub cool is to ensure that we have good liquid quality when it enters that metering device inside. We want to have liquid and the condenser great. But we want to also make sure that we have enough liquid, that it can go through the pressure drop and any increase in temperature any lift on that liquid line, so that when it makes it to the metering device on the inside whether it's a piston txv Electronic expansion valve doesn't matter that we have solid column of liquid there. So that's why we have to have more sub cool but getting more sub cool by driving up our condensing temperature.

Does that do us any good? No, the point being once we approach once our liquid temperature approaches, the outdoor temperature there's no sense in adding more any more. You add, regardless of what your sub cool number is at. That point is only going to drive up your head pressure and your condensing temperature, which increases your compression ratio. We've talked about.

It makes your compressor work harder and, in turn, doesn't move as much refrigerant. So you reduce the efficiency when you do that, and this is where that term approach comes in, it's not not a measurement. You guys take a lot if you've used measure quick, you'll, see it on there. But approach is just comparing your liquid line temperature to your outdoor temperature.

That's all it is now approach is tricky because it's on a curve, you can hit approach two different ways and that's why we don't just charge to approach. I'm not telling you to just charge to approach, but once you know you have sub cooling, so you know: sub cooling exists whether it's 5 or 10 degrees, whatever 15 degrees. If you look at your approach, getting your approach number lower than 4 meaning getting your liquid line closer to the outdoor temperature than four degrees becomes a pointless exercise, because in order to do that, all you're doing is just jacking up your head pressure on. For no reason, i'm going to say that again because i think that's probably like i i i made a couple quantum leaps there.

But what we're doing is we're stacking liquid in the condenser. We know that to a point. We can stack liquid in the condenser and it doesn't really affect our condensing temperature because we have enough volume, but when we get to a point that we start stacking more in you start to see your head pressure go up, that's not what we want. We don't want to artificially drive up our head pressure in order to get additional sub cool, because all that does is increase compression ratio, a lot of people talk about and you'll even read it in books.

You can get additional efficiency by getting additional sub cool. Yes, so long as you're, not increasing your compression ratio, because if you're doing that, then you're hurting it on the other side of the equation. What we're really trying to do is have the lowest temperature solid column of liquid at that metering device that we can possibly get that's our goal so long as we're not driving up our head pressure. That's the balance we're trying to reach now.

People will say that about sub cooling, because there's such a thing as a mechanical sub cooler, where you actually intentionally cool the liquid lower by using additional cooling capacity, so they'll actually use heat, exchangers and refrigeration, where they'll use a whole separate, coil, basically and cool the Liquid colder, in order to get more capacity out of that liquid when it goes out into a grocery store, for example, and that does lead to additional efficiency of the evaporator coils. But in but in what we have. We don't have receivers. We don't have mechanical sub coolers.

What we got is a condenser and we've got to do both of these things. Well, meaning give ourselves plenty of room for condensing, so we're not driving up our head pressure and have a good, solid liquid seal at the bottom and the combination of knowing our outdoor temperature. So this drives diagnosticians crazy. When a new guy calls you man, this head pressure, seems high.

What's your outdoor temperature? Well, it's been a while, since i checked it or i think it's probably about 85., it's like give us a good, correct temperature, measured in the shade, where it's not affected by the coil temperature or the sunlight, or anything like that, so that we have something to Compare to because we need that, then we need your sub cooling and then we also want your liquid temperature, because we're going to compare the liquid temperature to the outdoor temperature and again, the outdoor temperature is the bottom number that that liquid can possibly get to. So, if your liquid temperature, if your outdoor temperature is 85 and your liquid temperature is 89, that means they're already within four degrees of each other. You have four degrees of approach and if you tell me that at that point, you've got 10 degrees of sub cool. That condenser is doing what it's supposed to do.

Any any addition that you add to that is just going to drive up our head pressure, which is going to decrease the amount of work that piece of equipment can do now again, when i throw around rules of thumb, you take them with a grain of salt And when, in comparison with manufacturer specifications, jessica's really good at looking at manufacture specifications. So if you want to see somebody who's really good at looking at the panel and seeing what the manufacturer says - she's really good at that - and that is a really uh important trait. And it's also good to then take that and compare that to some good general rules of thumb, knowledge to help with diagnostics, because it's not all about just charging kind of like to what to what matt and aaron both said. It's not all just charging.

You start with solid visual inspection. Looking at your coils, you know just making sure that everything's clean and in good shape before you even begin uh with adding charge, especially to an older unit. Does that make sense, though? Do you understand what sub cooling is it's telling us whether we've got liquid and the bigger? That number is the more liquid we're stacking in the condenser. Any questions - an interesting thing, i think, is: if you look on the inside of the cover for the panel, it actually shows different subcool based on ambient temperature on carrier systems, now, okay, which is interesting for me, because as we're trying to fill up that condenser right, The temperature that we're doing it changes on the outside consistently, so our sub cool is not always going to be 10, so say we set it to 10 right on startup, based on the temperature.

That number is going to change all right, so i think it's interesting to kind of look at how it correlates with our temperature and indoor air temperature. If i'm not mistaken - and i could be because i have seen this um - that, depending on the type of coil design, that number doesn't change a lot under the normal operating conditions um. But if it's a micro channel, then it changes a lot more because micro channel has a much smaller internal volume. Have you seen that it's highly variable in the charts you're looking at anywhere from six to twelve okay, and that sounds about right, um so again follow the manufacturer guidelines.

We are seeing more and more of that, where it's not just a single fixed number, but also do recognize to jesse's point if somebody sets it under different load conditions to one number and you show up and it's two degrees different than that. Don't do the old thing that one of the guys i used to ride with used to do all the time? I said. Oh man, this guy's, an idiot. He didn't charge his system right, and that was back when we were charging by super heat, and i mean super heats are all over the place depending on load conditions and you'd, be like.

Oh, this guy didn't charge this unit right, and it's just like. No, it's it's! It's a moving target to some degree and that's why it's kind of important to have that in mind. Even when you think about this, so you know six degrees of sub cool. You might be stacking this much liquid 14 degrees or sub cool.

You might be stacking that much liquid either way you're stacking liquid and either way so long as you end up with a solid column of liquid, that's as low a temperature as you can get it without sacrificing your condensing area inside then you're good to go cool. All right great have a wonderful week, 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.

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