Right so the topic of uh low charge one of my least favorite ones to to talk about because it is the thing that a lot of techs. Do you add a little freon go to the customer's house. The number one thing that they suggest that the system needs is you know just add a little freon um. We've got you know emily's made some pretty good memes about that and so i always i'm kind of careful to even talk about it because i don't want you to have excuses to just add refrigerant to the system.

But i do want to talk about kind of what's going on because i think it might make it a little more clear and then talk about what are the actual signs or actual symptoms for troubleshooting low charge. Let's talk about charge in the first place. So i do need to i do need to change my illustration. So we're going to we're going to start with the compressor on top.

This is the jim bergman way so we go to compressor and then we go to condenser over on this side. Here. Because the condenser is fed in the top and then as it goes down. Then it comes out as a liquid and then we go to our metering device.

I always like to just you know we always do this basic stuff again because it helps us and then in our evaporator coil. We feed our evaporator coil into the bottom. And then it comes out of vapor. Then it goes back to our compressor on the suction line.

So compressor pressure increaser condenser's heat rejector. Which turns into condensing changes it from a vapor to a liquid goes in all vapor. Desuper heats. Then it begins condensing through most of the condenser then at the end.

It has this kind of liquid seal down at the bottom. So liquid stacks from the bottom up. Then it goes to our metering device. Fully liquid metering device is our pressure dropper.

Generally a txv or electronic expansion. Valve. Nowadays comes out of that as a flash gas feeds. Our evaporator coil fills it mostly with boiling refrigerant.

Which is where most of that work is done that's the latent heat of vaporization that change from liquid to vapor and then at the very end it's fully vapor. Which is what we call superheat and that goes back to the compressor. So a couple things that we've talked about separately that i just want to make sure that we're anchoring here and remembering when we're looking at sub cooling sub cooling is telling us how full in the condenser. How full the condenser is of liquid.

Some of you have pointed out that sub cooling as a number doesn't tell us that and that of course is true. But a higher sub cooling. Number on the same system. Means.

That there is more liquid stacking in the condenser. A lower sub cooling. Number means. There's less liquid stacking in the condenser.

We need to make sure that we feed our metering device with liquid. So we need to have a high enough sub cooling. Number that we're that we ensure that even with the pressure drops in the liquid line before it makes it to our metering device that we have a full line of liquid entering our metering device. Okay so higher sub cool means more liquid stacking in the condenser.

Which we need to have a high enough number that it ensures that we feed our metering device. But we don't want higher sub cooling than we need unless we're doing mechanical sub cooling. Which is a totally different subject because higher sub cooling than we need means that we're taking up more of our condenser space with liquid. Which means that our head pressure is going to be driven up.

Therefore our compression ratio is going to be driven up means. Our compressor is not going to move as much refrigerant and that results in lower capacity. So we don't want to stack more liquid in the condenser than we need to but we want to make sure that we have enough liquid to provide a full line of liquid to our metering device evaporator coil higher superheat or should say let's start with lower superheat lower the lower the superheat. The more refrigerant is filling the evaporator coil with boiling refrigerant the higher the superheat the sooner in the coil.

It's boiling off so we want sufficient superheat to ensure that we don't get liquid back to our compressor. We want only vapor back to the compressor. So we want to make sure that we have sufficient superheat for that at the compressor that number is generally around 20 for most air conditioning systems. That's what copeland kind of publishes as their standard.

They'd like to see 20 degrees of superheat. But really anything at the compressor between 10 and 20 is going to be what we would typically expect leaving the evaporator coil uh. It's generally going to be you know anywhere from six at the lowest um to about 14 at the highest leaving our evaporator coil. So we're going to pick up a little bit of heat in the suction line before it goes back to the compressor those are some basic things that we look at and so we're in terms of fullness of our condenser and again a lot of people a lot of you will dispute that because again this is this conflict between a quantitative.

Which is a number which sub cooling is it's a quantitative measurement versus qualitative and when i say fullness fullness is a qualitative measurement how full is it with liquid. But i'm saying you know same system under similar conditions um. If it's more full that's going to be a higher sub cool. So more full equals higher sub cool and in terms of our evaporator coil fullness again that's a qualitative measurement.

But that's going to be a lower superheat. So that's one of the factors that we're looking at in terms of charge. Do we need more charge well with modern systems because we typically have rather than having fixed metering devices. We have actually actively controlled metering devices electronic expansion.

Valves and txv's and their job is to help control the superheat so our primary job in terms of setting. The charge is to make sure that we have a full line of liquid feeding the metering device. Now those of you who work in refrigeration. I was going to kind of put an homage out there if you've got a receiver in the system.

Well then we have a sight glass in our liquid line and the sight glass actually tells us whether we're full or not you're not going to see as much subcool. So if you have a receiver. You're not really counting on subcool as a number you're usually looking at a clear sight glass meaning. You're you're visually looking to see do i have a full line of liquid entering my metering device.

But that really is the point that's why we're doing this we're measuring subcool to make sure that we've got enough liquid and that it's going to make up for the fact that we've got this liquid line because when we're taking our measurements. When we're checking your high side pressure. And your low side pressure on a typical air conditioner. You're connecting your high side to the liquid line at the condenser and you're connecting your low side to the suction line at the compressor.

So you've got these lines in between we don't necessarily know what the pressures are at the inside. Because the inside unit is this stuff right here. A metering device in our evaporator coil. So we'll call that our air handler or coil.

So whether it's a gas furnace and you've got a case coil or an uncased coil or whether it's a fan coil or air handler. These two are inside and we've got our line set. Which is our suction and liquid line. So we'll write that here.

This is our liquid line and this is our suction line. This makes up our line set at least in cooling mode right or we call this this is a heat pump. We call this the vapor line. So because we have this differences in measurement.

We're measuring out here versus in here. We got to make sure that we've got enough. We've got the right number of superheat coming out and we've got the right number of superheat going back in and we get the right number of sub cooling. Coming out and sub cooling.

Is the first thing that we look at so. When we're checking for charge before we even get to that phase. Though i want to make sure that we're doing our visual inspections so look for the obvious right now we'll point. Many people will point out and rightly so that even if you have airflow problems.

You're still going to be able to rely on subcool. So subcool is a pretty reliable number. But i still especially for junior techs. I want you doing a thorough visual inspection first air filter walk the house make sure there's nothing dumb going on they don't have a couch pushed up against the return you know we see this a lot in rental houses.

Where people will pull down a non filter grill return and they'll jam a filter up in it so take your flashlight look up into into those return air grills to make sure nothing's jammed in them just obvious stuff like that look at your evaporator coil. Take a mirror or take your uh your cell phone. If you want and look underneath to make sure that you don't have a completely plugged evaporator coil. Take a look at your blower wheel and make sure it's not all gummed up make sure that your settings are proper on your air handler.

So that make sure they put the y wire on that y y2. Instead of y1. So it's ramping up to high speed again depending on the type of equipment you work on these things might vary. But you want to confirm these obvious things related to indoor airflow and then go outside take a look at your condenser is it is the thing just gummed up is it completely dirty those are things you want to address before you start trying to dial and charge.

But regardless we can rely pretty heavily on looking at our sub cool. Number. If our sub cool is zero. And we know it's zero.

Because we know that our our gauges are working our probes make sure you're zeroing stuff out all that kind of thing first you have to have reliable tools before you can measure this stuff make sure that they're depressing the schraders on the equipment. You see this so often where people are getting weird readings. And the reason is is because their actual schrader on their device isn't depressing uh the the ports. So we can rely heavily on this sub cool number.

And so i would say for modern systems sub. Cool is our first and foremost indicator. Next in line is we would call our suction pressure or our suction saturation. And this is where that kind of rule of thumb comes.

In that is a very reliable rule of thumb for our industry and that is that our evaporator td. Don't don't confuse evaporator td. With delta t not the same thing we take our return temperature and so let's say we have a return temperature of 80 degrees. Because we're assuming this unit's not working very well right we subtract 35 degrees from that and so if we take 35 degrees off of that then that indicates what our expected suction saturation temperature would be so that leaves us with 45 degrees right so if we use something like the refrigerant slider app here to use something like the ref tools refrigerant slider app from dan foss.

Which is kind of my favorite. If you guys don't have this on your on your phone. Then i would suggest you do that we got this on 410a and you can see that the uh at 45 degrees. That's 130 degree suction saturation.

So this 45 degree represents our evaporator temperature return temperature minus 35. This is what we would call our evaporator. Temperature and that relates to a 1301. We'll just say 130 psig suction.

Make sure that when you're using the app. That you're not using absolute pressure otherwise that's going to throw it off. So if you see absolute pressure is. 144 that's that 147.

That we add on for atmospheric. So it's 130 psig 130 pounds per square inch gauge pressure on our suction side equals a 45 degree evaporator temperature this is what we're going to expect to see now the problem here and this is one of the biggest things that people do wrong when they start adding charge to a system is that this could easily be too low. So this is what we would expect it to be let's but let's say that instead we've got a 30 degree. So this is what we would call a dtd a design temperature difference meaning.

We would expect based on the design that if the return is 80 minus 35. That we'd be running this pressure. But let's say we're running something different we're running 30 degrees. And so if we just want to do this just for the sake of it we'll take our refrigerant slider app.

Again and now that's 97 psig so let's say we're running a 97 psig suction. We walk up to this system. The typical knee jerk reaction is going to be to say hey. This is too low my design temperature difference is 45.

My actual is 30. We need to add refrigerant to it. But what we have to establish first is is this caused by an airflow problem. And that's where we go through and we check.

A couple things first we check that sub cool. If our sub cool is where it's supposed to be don't add refrigerant. If our sub cool is zero. Okay.

Add a refrigerant at that point. If you've got a zero sub cool. And you've got this you can go ahead and add now i do want you checking your super heat as well though if your system is operating with and again the txv controls the superheat. But if your system is operating within the expected superheat range that would be like i said typical you know.

14 degrees 10 to 14 degrees inside 10 to 20 degrees outside if it's within that range. I still want you to pause because we need to make sure because again at 30 degrees. We could actually start to get a little ice on that evaporator coil more likely that it would get down into the 20s before we start to build ice. But i need you to do that visual inspection.

Make sure. There is zero ice on your evaporator coil. Make sure. There are zero obvious air flow problems before you start adding charge.

Don't just look at this low. Suction pressure. You can also look at your condensing temperature over ambient number. Now the ctoa varies more depending on the efficiency of the system.

But your kind of modern ctoa that we would expect is you know 15 to 20 degrees over that outdoor temperature so whereas evap td 35. Degrees is pretty typical. I mean you will see some systems especially in humid environments with large evaporator coils. Where you may get down as low as 30 degrees.

Here rather than 35 and all that would do is just actually raise your expected evaporator temperature. Which would mean we were even further off with what we have here. But let's look at ctoa quickly and again. I know this is drinking from a fire hose.

I know the number one complaint. I'm going to get from you in the class and in the video is that i'm talking too fast. But what i'm trying to get you to do is slow down before you start adding refrigerant to the system. We've already talked about when you do add refrigerant to the system use a scale.

So that you're not just adding refrigerant unnecessarily. But i want you to be really clear so. That you can know for a fact when a system does need refrigerant. So let's look at ctoa ctoa.

Is condensing temperature over ambient and simply what that means is is that if you take the outdoor temperature. Let's say it's 90 degrees your kind of typical adder for modern high efficiency equipment is going to be 15 degrees. So this is what we would call our a design ctoa so dectoa. This is what we would expect that would be 105 degrees condensing temperature.

Which is you know how we get condensing temperature again if you go to your. Refrigerant slider. App here 105. Degrees is 3408 psig.

So we'll just say 341 341 psig. That's what we expect right and so if you have a system that's in this range. And you have sub cool meaning. 5.

10. Degrees of sub cool. Then you're really not low on charge. Adding.

More charge isn't going to solve this problem. Now. If you walk up to it and you're running 250. Psig or something like that and again that that wouldn't even be realistic it would be you would get closer to your outdoor temperature.

So let's just say it's 91 degrees. So like 278 that would equal a 1 degree. Ctoa condensing temperature over ambient now that would be an indication of low charge when coupled with a low or zero sub cooling. Number and a low evaporator temperature evaporative temperature is lower than what you would expect and again.

The language that i use might be confusing. But when i am measuring high side pressure. I'm converting that in my head to condensing temperature. So this 105 degrees.

Because that works better with that sort of expected rule of thumb that we call our design condensing temperature over ambient same thing with the evaporator temperature 75 degrees inside the easiest thing to remember here is if it's 75 degrees inside. I expect about a 40 degree evaporator temperature. If it's 90 degrees outside. I expect about 105 degree condensing temperature and the reason why those are the ones.

I remember is because in florida those are going to be pretty common in the summer. When i'm testing equipment as this outdoor temperature number goes down. So this is our outdoor temperature as this goes down. Then our condensing temperature is also going to go down proportionally with it point being that you can't just say hey the pressures seem low.

So i'm going to add some freon to it that's like the worst version of adding refrigerant. We add refrigerant when we see again modern system we see that we have a low or zero sub cooling. Number we see that we have a low evaporator temperature meaning a colder evaporator temperature than we would expect we're going to see a higher superheat than we would expect again that tells us that our evaporator coil is less full and we're going to see a lower condensing temperature than we would expect those are our indications when we measure our return temperature and our supply temperature. That's when you're going to start to see your delta t drop.

As well now that's not a reliable one it's not the one i want you to rely on mostly. But it is kind of a quick and dirty um estimation. It varies with humidity and airflow and a lot of things. Which is why i don't love it.

But what you're going to see is a low condensing temperature. Again. When you're low on charge. A low evaporator temperature.

Key indicator low sub cool lower sub cool. Than expected higher super heat. Because again lower sub cool means less full higher superheat means less full and when you're low on refrigerant that's what you expect an evaporative coil. That's less full a condenser that's less full a liquid line pressure.

Which equals a condensing temperature that's lower a evaporator pressure that equals a lower evaporator temperature. That's lower than expected that tells us it's time to add a refrigerant after you've done a full visual inspection. And even then don't add refrigerant till. The system's been running a little bit unless.

It's very very low and you can see that obviously in your pressures. Make sure that you're depressing those schrader cores. They're getting pushed in by your probes or your hoses or your stubs or whatever you're using. I see a lot of cases people see crazy pressures literally because they have their hose backwards.

Where the side that doesn't have a core depressor is being connected so visual inspection. Obvious things use a scale make sure that you're not you know that you're paying attention to what the factory charge is and what your line length is generally that you can kind of estimate how much refrigerant should be in it so that when you're adding refrigerant and things aren't changing the way you would expect you stop and you look at airflow or you look at refrigerant restrictions. If you have things like obvious line dryers that sort of thing in the system. Go ahead and take a temperature difference across that liquid line dryer make sure that the line dryer isn't restricted look for obvious issues.

Kinked liquid lines you know things like that uh. Before you make that decision to go ahead and start adding charge to the system. So that's a lot i know. But that's it consider all this stuff before you add refrigerant.

I had somebody post a super like emo uh comment on one of the videos. Saying that like this is way too hard and confusing and that he's just gonna deliver pizzas uh yeah. It's it isn't that bad actually once you practice it. But you can't just add.

Refrigerant and see what happens um for many reasons first off like it's not good for the equipment. It wastes time. But also refrigerant's really expensive now so adding refrigerant that you don't need to add waste a lot of money and it's going to be hard to stay in business. If you do that so doing all your visual inspections being really thorough before you say okay i know putting this refrigerant in the system is the right decision.

That's the that's the way to think about adding refrigerant to a system that has the symptoms of low charge 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 hvacrschoolcom.

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