Co-founder Bryan Orr explains how you can understand, check and set refrigerant charge. If you’re a fan of ours, be sure to share this video to show your friends and family how they too can experience “Simply Great Service”!

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Links and Items Mentioned In This Video
  • How to read an HVAC Manifold
  • How to read and troubleshoot refrigerant pressures
  • The difference between refrigerants (R-22 and R-410a)

All right so today, we’re gonna be talking about basics of the refrigerant circuit, charging, checking charges, setting charges.  It’s like most classes that I give – what you want me to tell you is not gonna be probably what I tell you because what I most often get questions about are things like what should my super heat be or what should my suction pressure be and I’m not gonna answer that question.  I’m going to answer the bigger question which is how can you know what it should be.  How can you put together a lot of different information and actually come up with a conclusion about what is correct given the conditions that you’re experiencing and there is no one simple answer to that question because very rarely are we actually working on brand new systems unless you’re doing installation.  In most cases, we’re working on systems that are not brand new.  Therefore, the way they’re going to run is not going to be exactly the same as they did when they came out of box

So we’re gonna be kind of talking about how to understand the different things that are going on in a system that dictate how a charge is set and how a refrigerant charge is checked.

So to start with, I’m gonna do a quick review of the concept of pressure-temperature relationship.  I’ll talk about how temperature is average molecular velocity within a matter.  So the higher the molecular velocity, the faster the molecules are moving, the more pressure they exert on the outside, whatever space they’re contained by.  So when we’re talking about a refrigeration circuit, we’re talking about a closed system, meaning that the refrigerant is completely encapsulated inside this system.  So depending on the type of refrigerant that you have, depending on the temperature conditions, depending on the pressure, all those things affect the ways, one affects the other.  So whenever you have an increase of temperature in a closed vessel, you also have an increase of pressure in that vessel.  If we were to take this room and we’re gonna seal it off completely tight and we would just start to heat this room that you would actually increase the pressure inside this room because the molecules are moving faster, therefore they’re exerting more force on the outside walls.   You’re with me?

So the temperature surrounding this room and the temperature in the room affect the pressure that would be in a sealed room.  We’re not used to dealing with sealed containers when we’re typically working with them.  If we think about in regular life, we think about boiling water, boiling 212, well like we talked about, it boils at 212 because it’s atmospheric pressure, but if we were able to affect the pressure that surrounded that vessel of water, we could also affect the temperature at which it boil.

So when we’re looking a set of refrigerants, I’m gonna jump, that’s the basic.  I’m not gonna connect all the dots here.  I’m just kind of jumping from one to the next just so we can make sure we’re on the same page.  So when we’re talking about refrigerant gauges, what we’re looking is we’re looking at the pressure that exists inside the sealed container and we have essentially two constants.  The constant one is we know what refrigerant is.  We’re making that assumption.  Now, could you figure out what refrigerant you had based on the pressure that you saw inside the system?

Audience:  Yeah.  You would be if you would have.

Right.  You could have a pretty good guess and how do you come up with that pretty good guess?

Audience:  Well, if you go to a unit and your pressures are 75 psi and your unit is frozen, there’s a very good chance it would be 410.

That’s a very practical example.

Audience:  A larger principle would be comparing the pressure to the temperature of the lines.

Well, what we tend to do is…

Audience:  Or looking at data points.

We had a case in the day, I don’t know who it was or I knew who it was, who went to the system that someone had like R422DS.

Audience:  R38.

Yeah, some other type of alternate refrigerant.  Well then at that point, even if they have very similar pressure temperature characteristics to R22 say, but how can you know that’s R22 unless someone tells you?  Well, you can’t know exactly, but you can have a pretty good idea and Heather shares how we do it.  As long as that refrigerant is a mixture of liquid and vapor, that’s what we call at saturation, it’s a mixture of the two, it will have certain pressure characteristics at a certain temperature.  So what you do is when you walk up to a system and it’s off, that’s what we call static pressure meaning that you don’t have anything affecting motion of the refrigerant in the system.  So you hook up to a system that’s off, you call that static pressure.  When you take that static pressure assuming that there is a mixture of liquid and vapor in that system which there should be unless it’s almost completely flat, you can then take a temperature around that system in a condenser.  Generally speaking that’s where most refrigerant is held.  Take a temperature around there and then you can look on your saturation scale in here, in this case R22 is our green scale here and you can go up to say, 75 degrees outside. You can go up to 75 degrees and you can see that that’s gonna be right about 135 psi.  So if you hook up to a system and it says 135 psi static pressure, then you can have a pretty good indication that’s R22 because it’s in that saturation phase.

Now that’s another assumption that we’re making.  How could you know that?  Well, there is a couple of different ways and we’ll go into that but generally speaking when you walk up to a system, you would assume that it would have liquid and vapor.

Now, why can we make that sort of leap of faith?  Why can we say that it is R22?  Well it’s experience.  You worked with the system, you’re in their place, those types of things.

We really only have two different types of refrigerants that are generally used in residential HVAC systems.  So we’re really just kind of comparing the two.  R410A runs much higher pressures even in static.  If it were an R410A system with 75 degrees, then we would be running closer to about 225 psi when it’s 75 degrees temperature around it.

So what we’re essentially doing in this whole exercise here is not establishing how to figure out what refrigerants are, but establishing the idea that there are these constants that exist that temperature does affect in a constant way the pressures that are exerted by molecules inside of a system as long as we know what the molecules are.  So if we have that constant knowing what the molecules are, then by knowing either the pressure or the temperature, we can then get an idea of what’s going on in a system whether or not we’re in the saturated state, whether we’re in the vapor state or whether we’re in the liquid state and that’s what we’re trying to establish because when we talk about refrigerant being in a mixed state, we’re saying that both vapor and liquid are existing here.  Well if it were to be colder than that temperature that was indicated, if the temperature of the line say or the refrigerant line or the refrigerant in that system, what’s colder than the pressure-temperature relationship desaturation then we could say confirmably it’s a liquid because in order for it to get colder than the temperature of saturation meaning the mixed state, we think of it like boiling water.  Boiling water is 212 in atmospheric pressure that’s what we call saturation.  It’s at that point where it’s completely saturated with heat at this given atmospheric pressure and now it’s having to change state because it’s completely saturated.  So if water is colder than 212, we can say at atmospheric pressure, it is liquid.  If water is hotter at atmospheric pressure than 212, inequivocally, it is vapor.  If we say that it is 212 at atmospheric pressure, we can say that it is…

Audience:  At saturation.

It’s at saturation.  It’s boiling because we know what substance it is and we know the pressure.  You know what I’m saying?  So by knowing those pieces of information, we can know something about the system. Now if we could say for example, let’s say we can observe the boiling, that’s what we’re trying around in terms of the vessel of water. We’re essentially using logic here.  You look at a vessel of water sitting at sea level and you see it’s boiling, you can say that water is 212 degrees right.  Now what if I were to see a vessel that had a glass over it and I could see a thermometer in that and I could see that that thermometer that that water is boiling and that thermometer read 300 degrees?  What then could I assume?

Audience:  Was that there is a higher pressure inside of that seal.

Inside of that sealed vessel, correct.  Here is what I’m establishing; we’re using logic based on what we know in order to know what’s going with what we’re dealing now.

Now inside of a sealed vessel, we don’t see what’s going on so we don’t know for a fact.  For example, I haven’t walked up to an air conditioner and without some data, I can’t say it’s boiling there, it’s liquid there, it’s vapor there.  I can’t do that unless I’ve taken the pressure and temperature readings and then I can tell that.

So let’s jump right into what’s supposed to happen inside of an air conditioner.  For those of who may not know, what are these two knobs here do?  What’s the purpose of it?  What’s actually happening inside here when I open and close these valves?

Audience:  [Cross talk] … one with the other into the Manifold.

You’re allowing it to flow inside the Manifold body.  A lot of people who look at a gauge like this will think that what you’re doing here is somehow affecting what you’re gonna engage, but it’s not at all.  These hoses still travel through the gauge regardless of whether or not these handles are open.  All these handles are doing is allowing it to mix in between these.  So if I were to open this mouth here, what two lines am I mixing together?

Audience:  Center and blue.

Yellow and blue.  When I shut this one off and I open up this one, which two lines am I mixing together?

Audience:  Yellow and red.

Now I’m opening up here.  If I open up both, now what am I doing?

Audience:  Yellow, red and blue.

Now I’m mixing all of them together.   Does it make sense?  I mean the connection between all of them in both of these reels.  Does it make sense?

Audience:  Yep.

We’re clear here?  Okay.  It got nothing to do with what reels I’m engaged.  So if I take this, if I take my suction gauge off here, let me first tell you that this is the compressor, condenser, metering device, evaporator.  Where am I hooking this gauge to?  Can someone describe to me where is this gauge hooking to?

Audience:  Right before it goes into the compressor.

Right before it goes into the compressor so I’m hooking here in a typical residential system.  And with this one here, where am I hooking to?

Audience:  As it comes out of the condenser.

As it comes out of the condenser so right here okay.  So am I hooking up right before it goes in and out of the compressor?  No I’m not. I’m hooking up before it goes into the compressor and I’m hooking up after it comes out of the condenser.   So for those of you who don’t know which you probably do, but let’s go to the different states of what occurs inside the system.  So we’ve got our compressor which is a giant pump.  It actually compresses the refrigerant.  So what state is the refrigerant in right here?

Audience:  High temperature vapor.

High temperature vapor.  High pressure, high temperature vapor.  What state is the refrigerant in here?

Audience:  That builds in liquid.

We got a direction here.  We know the circumferential.

Audience:  [Cross talk] Low temperature, low pressure vapor.

Low temperature, low pressure vapor.   Okay so it goes into the compressor the low temperature, low pressure vapor, comes out of the compressor, high temperature, high pressure vapor.  Why doesn’t it just become liquid as soon as it’s compressed?

Audience:  As you increase pressure, it’s also increasing the energy per volume so you’re making it hotter as you compress that.

Correct.  As the same amount of heat it had before.  You’re not actually increasing the amount of heat, but you’re increasing the temperature.  Remember how we talked about that.  Because by reducing the amount of volume that it has to operate and you’re exciting the molecules so the molecules are moving faster so it’s directly proportional meaning that the more you compress it, the hotter it gets.  You can never turn something into a liquid.  As you compress it, it just gets hotter and hotter and hotter.  So you got to dissipate that heat in order to condense it.  In order to turn it into a liquid, you got to allow that heat to dissipate.  So we reveal heat, we expose it by compressing it and driving the temperature up.  And now by compressing it and driving the temperature up, now it flows to the condenser coil.  So now what is the state of the refrigerant right here.

Audience:   Liquid.

The state it was here, now it’s a liquid because in a condenser, the condenser condenses.  You have high temperature, high pressure vapor going into it.  It condenses and then it comes out as a liquid.  Now what is the name of this line?

Audience:  The liquid line.

The liquid line.  That’s a tricky one right?  Liquid line because it’s liquid.  What’s the name of this line?

Audience:  Discharge.

Okay.  How are they different?

Audience:  At your discharge line, the pressures are a lot higher and thus high-pressure vapor.

They’re actually not that much higher.  The only difference…

Audience:  Higher as well.

It is higher.   The only difference in pressure though is actually the pressure drop through the condenser.  It’s actually just pressure drop.  It’s the same pressure because you’re forcing it together.  It’s at a changed state because now you’re dissipating heat over it and allowing it to condense now, settle down into a liquid state fully.  Does that make sense?

So high pressure, high temperature vapor here, goes to the condenser, condenses, comes out as a liquid, heats this, what is this?  First and common metering devices?

Audience:  [Cross talk] TXV.

Systems determine if you use actuator orifice.  Back in the day, capillary tubes are still commonly used in refrigerators and ice machines which is just a little tiny tube of copper.   So what’s the purpose of this?

Audience:  To back up the liquid refrigerant, allow rapid expansion.

It acts as a center point between the two houses of the system.  Really you can think of it in two different ways.  You can describe it in two ways and then both of this is correct.  You can describe it the way that Nathan Bishop just did which it backs up the liquid which is true, because without your metering device, you don’t have adequate pressure over here.  In order to have pressure, you have to be building pressure up against something.

Audience:  And that’s what causing the pressure difference right?

Correct.  So over here, we have the high side of our system and over here, we have the low side of our system.  So your compressor and your metering device split those two halves of your system.

Audience:  And so your difference takes place.

If you guys are using the system, where is the point in which you start to see temperature change from warm to cold?

Audience:  [Cross talk] After the metering device.

Directly after the metering device, but even more so than that because we tend to think of an expansion valve as the whole part.  We always like to think of things as their parts, but it’s the exact point at which the pressure drop occurs is where you see the delay

Audience:  Right in the evaporator.

Right where that orifice is, right where it goes from big to small, right where it restricts, that’s where the pressure drop occurs and that’s where you see the difference in pressure.  So what starts to happen now as the pressure is reduced?

Audience:  It starts to boil.

It starts to boil because now you’ve reduced the bounds on those molecules and they started to rapidly expand.

Audience:  So it’s boiling even though there is at this point no energy difference, there is no increase in temperature.

Well but there is.  No because you’re still absorbing heat from the space around.  You can’t have boiling without heat being absorbed, but heat is being absorbed because at that instantaneous moment because the pressure has dropped so quickly, what you’re essentially doing even though in terms of water is you’ve taken this water and you decreased the atmospheric pressure on it until it’s boiling at like 40 degrees Fahrenheit.  Well the space around is warmer than 40 degrees Fahrenheit so now the space around starts to give its heat up to that refrigerant that’s in the evaporator as its boiling and changing state.

So this is the point where you transfer from high side to low side.  What is this line called between the metering device and the evaporator coil?  This is called the expansion line.  Now, where do you see an expansion line?  There is a place that you see an expansion line, but you just don’t think of it very often as an expansion line because in a regular home air conditioner, you don’t have an expansion line.  The expansion valve is pretty much right on that evaporator coil that we just get really used to and are kind of married into one almost.

Audience:  Why isn’t the evap coil, your expansion line because you’re expanding through that evaporator coil?

It is.  I mean, this really just becomes an extension of the evaporator coil really because in the state of refrigerant and this is what we call as flash gas – meaning it’s in the process of boiling up.

Audience:  It goes pretty much from there straight through that evap.

Correct, pretty much yeah, yeah.  Why do we insulate the liquid line on a ductless system?  Because it’s not a liquid line, because it’s actually an expansion line, because the metering device is in a condenser in a ductless system, all these are in one place and then you have a line that travels through this so this is the cold line.  So your small line is the cold line and then it’s because it’s actually not a liquid line at all.  It’s actually an expansion line.  It’s a low pressure flash gas line which is why it will sweat.  Now they’re not necessarily all that way but that’s how a lot of them are.  So anyway, it is the real thing.  I’m just using that to demonstrate expansion line is actually the real thing.  I mean all this kind on how these are positioned, you would say in classes, I could build an air conditioner, I could take a condenser and put it over there, I could take a compressor and put it on the other side of the building and wrap it over here, expansion line and a metering device over there and just hook all them together and it would still work.  We tend to think of air conditioning system in terms of an air handler and a condenser which are standardized grouping that we use in a split system residential applications, but that isn’t how it has to be.  That’s just how they do it in a production air conditioning, but thinking of an air conditioner like this in terms of its four components and its four lines in the different states of the refrigerant in lines is much more accurate and it will help you think much more clearly about what it is that you’re accomplishing.   Because let’s give an example, let’ say you’re in a commercial rooftop packaging and a lot of times, where will the force be in a commercial rooftop packaging?  At which two lines?

Audience:  Discharge

You’ll have it before here and another four here.  What happens if you try and take your super cool here?

Audience:  You’re not gonna get a true reading?

You’re not gonna get any reading entirely at all because you’re not getting any sub cool because it’s gonna be the same temperature or actually a lot higher temperature because it’s actually still in a vapor state which means that the temperature of that line is gonna be much higher than saturation.  When any matters in the vapor state, it’s going to be higher than saturation and there’s no real way of knowing how much higher it’s going to be.  That’s like saying well, how can I tell how hot this air is gonna be based on the temperature around it or based on the pressure?  We can’t know because it’s not in saturation.  This air isn’t in the process of boiling so I can’t really tell you anything about what a temperature is going to be.

Audience:  There is no reference point.  If it’s in the process of boiling, it can’t be much higher than the saturation as the window of reference.

It can’t be higher than saturation at all as long as you know what the pressure is and you know exactly what the matter is that you’re dealing with.  If you know what the pressure is and you know what the matter is and it’s in the process of boiling, it will be this temperature in the same way, say water will be 212 at atmospheric pressure at sea level.

So when we’re starting to talk specifically about how we test an air conditioning system with gauges and what we’re hooking up to and what we’re reading, you have to understand this.  Let me be very clear on what everything is supposed to be in each sector and what’s actually going on throughout the system.

Audience:  If I may add real quick, now if you just get out of this room and go to a pool to a heat pump and you’re looking, it’s the same way though right because a lot of times, your high side is actually the discharge line on a pool pump depending upon whether it’s before or after.

Correct.  So practically speaking, here is what happens; you’re gonna be taking your pressure here and your temperature here.  So that will affect it because you are going to have pressure drop.  So your pressure here maybe, you say it’s 250 psi.  Well you can’t use 250 psi as your number in order to factor in your sub cool at this point because it’s gonna have dropped.  Now, how much would have it dropped?  You’re gonna have to kind of guess at that point.  It’s not a perfect science, but you can get a good indication.  Generally speaking, I’ll say, all right on a pool heated, I figure it’ll probably drop 10 psi or 20 psi and that will give you a general idea, but yeah, it’s not exactly that point, it can’t be.  In order to take a reading, in order to figure out the state of the refrigerant, you have to be taking your temperature and pressure readings at the exact same point in order to be sure.

Audience:  Which you can’t do.

Which you can’t always do.  Let’s give another example; people will say something like, I’m jumping ahead of myself, but for the technicians here, someone would say, hey this TXV is supposed to maintain a 6-degree super heat.  Well where is that TXV maintaining super heat at?

Audience:  At the evaporator.

At the evaporator because where is the valve mounted?

Audience:  On the evaporator.

On the suction line so it’s maybe in here and so we’re taking a super heat where?

Audience:  Down side.

Down here.  Well, does that TXV had any idea whether or not that suction line is insulated, how long the line set is, any of that stuff where it got a 44 foot rise on a condo?  No, it’s got no clue.  It’s trying to do its job at the outlet or evaporator.  It doesn’t know what’s going on here.  A lot of guys will say, oh that’s where you need to bring it forward here and the others will say, well let’s give it a hot wash.  It’s not how it works.  It’s not real life.  Real life is you measure it here and by taking some more readings, you can go inside and take the temperature reading there and you can get a good indication based on how long the lines there is and everything else.  You just have to use common sense which is kind of what I’m saying.  So that’s it Dave.  That’s all I’m gonna say about this.

If there’s anything that you can memorize in the industry that will help you as far as it pertains to what I’m gonna talk about next, it’s to stand there and write over and over again and say over and over again; compressor, condenser, metering device evaporator.  Compressor, condenser, metering device… I’m not kidding.  Discharge line, liquid line, expansion line, suction line.  Discharge line, liquid line, expansion line, suction line.  When I went into AC school the first day, my instructor introduced me to different hang out.  This is the room.  Here’s the tool room, that and that and that.  Okay here’s the board.  All right I’m gonna write these four.  All right here is what it is; compressor, condenser, metering device, evaporator.  Okay I’m gonna erase it.  Do that for the next two hours, come back to me when you’re done until you’re able to memorize them.  Okay I’m done.  All right now here’s the next thing, discharge line, liquid line, expansion line, suction line.   All right, now we’re right in the states.  Now switch here.  Now split it in half based on the state of the refrigerant.  Now switch it in half based on high side and low side because you do it the other way here too.  So this is low side, this is high side and then this is the vapor side.  This is the liquid side.  It’s actually saturated liquid as it comes out the other side here, but you just start to kind of build on that.

And then he thought us the feel from the system with our hands.  All right, this is what a suction line should feel like.  This is what a liquid line should feel like.  Ouch, that’s what a discharge line should feel like.  No seriously, because then it gives you an indication, especially when you’re newer to the trade, you start to be able to tell, ouch, that’s not the liquid line.  You know, that’s the discharge line and certainly, that is what you end up doing.  I’m not gonna necessarily tell you to grab a discharge line here but this is key to this because if you don’t ever get this, then you won’t ever get what I’m about to talk about next.

So now we’re gonna talk about checking and setting a refrigerant charge.  So I talked about the five pillars.  I came up with that before I knew anything about the truth of your real five pillars.  So you have suction pressure, you have head pressure, you have sub cool, you have super heat, you have air temperature as well.  We’re talking generally about our regular day in and day out residential system.  This is something in place when I talk about chillers here, about the splits and standard basic stuff.

So what is the most common thing in the industry when your average technician says, hey that system is low?  What is he usually saying when he says that?

Audience:  The suction pressure is low.

He’s saying the suction pressure is low yeah.  That’s generally what he’s saying.  I mean he has hooked his suction gauge up on an R22 system and he saw it that it read 40 psi.  Now, if you did go up to the system and you hooked up your suction gauge and it read 40 psi, what does that actually tell you?

Audience:  Nothing.

It doesn’t tell you nothing, but what you could actually say is it tells me the saturation temperature that exists within that evaporator or generally, it’s an evaporator but in the suction line.

Audience:  You’re not completely through the start of the coil to the end of the coil boiling off.

Well, you don’t know that because what it didn’t tell you is it was reading 40 psi but it’s frozen to a solid block of ice so it doesn’t tell you that.  You have to have a temperature-pressure relationship in order to know whether or not that’s the case because what’s the other common cause of the unit running 40 psi suction?

Audience:  Low air flow.

It could be really air flow, you’re right.  That thing could be frozen solid and trust me, I’ve seen a lot of guys who walked up to a system that’s literally frozen, actually still has ice on it and starting to put refrigerant in it and there’s a guy I rode with at one point in time back when I worked at the old, old country and he did that.  He went to a system that was totally iced up like it’s right on the evening and just said, oh the temperature is low.  I’ll just gas it up and so he added up 4 to 5 pounds to it and he said, we’ll come back tomorrow and check it again.  It was the air flow totally so I think the guy just created a filter in it or something.  He didn’t even check anything.  So really by itself, it tells you very little.

What does an air temperature split tell you?  Let’s say you take a 20-degree split.  You got ice that’s 85 degrees inside and I got 65 degrees coming on top of that unit.  What does that tell you?

Audience:  Again, it doesn’t tell you that your coil is dirty or…

It doesn’t tell you much of anything.

Audience:  It doesn’t tell you if you’re low on in charge and you have the dirty filtering as well.

Sure you need to be low and try to clean a dirty filler.  It could be running 355 psi head and the compressor is about to blow sky high.  You got no idea.

Audience:  Or the wheel is completely filthy.

It could be any of those things.  So by themselves, it doesn’t tell you much.  Now, when you start to get into actually checking the charge accurately, you have to be a little more detailed about what you’re supposed to be accomplishing.  So let’s say that I’m supposed to have liquid here, but I don’t have a liquid.  I have a mixture of liquid and vapor.  How am I gonna know that?  Practically speaking, knowing what we know now, you’re going to give me any specific numbers, but I’m supposed to have liquid here because this is the liquid line, but I don’t have liquid in that liquid line.  How am I gonna know?

Audience:  When they make this noise, swish, swish, swish, swish.

Well that’s an experience worth knowing.

Audience:  Saturation temperature.

Because if I read the pressure here and I read the temperature here and they’re match at saturation so if the saturation temperature is 98 degrees at that pressure and the temperature is 98 degrees then that tells me just in the same way if you measure 212 degree boiling water at atmospheric pressure, it’s telling me that it’s still a mixture of liquid and vapor.

Audience:  Suction line, we find that to.

Maybe not, but I’m just specifically saying do I have a solid line of liquid here or not?  If you had a side glass, you could tell right? But we don’t so we gotta use our temperatures and our pressures in order to tell us whether or not we have full liquid here.

Audience:  How about if we standardize to putting on side glass on everything?


Because they really don’t have any point honestly.  The only point of  side glass is for people who don’t know how to read sub cool because if you’re reading sub cool, if you have one degree of sub cool meaning that the saturation temperature is ringing 98 and it’s 97, what does that tell you?  You have one degree of sub cool and the state of the refrigerant here is liquid.

Audience:  It’s all liquid.

Here’s the problem, your gauges aren’t that good because you could be reading 97 now, oh yeah, I got one 1 degree sub cool, you may actually have zero sub cool.  You may be reading 5 degrees of sub cool and have zero sub cool a lot of times.

Audience:  So is that why as a tech, you’ll say, you wanna make sure you have at least a couple degrees of sub cool?

It’s more than that.

Audience:  At least a couple of gauges.

It’s more than that because there are more things at play here than just sub cool.  I used to even teach this back when I was stupid.  I used to say one degree of sub cool is actually the ideal sub cool to have because that way you’re running your lowest potential head pressure and lower head pressure is good because that means less amperage.  Wrong because we’re dealing with matter here and the matter that we got is refrigerant and the higher the sub cool, the more density you have overall.  Meaning we’re actually moving more refrigerant overall. The more refrigerant you can tuck into a system, the more cooling capacity you’re going to have because we’re talking about cooling capacity per pound or ounce whatever total weight of refrigerant and so the more refrigerant you get into a system, the more cooling capacity you’re actually going to have.  Meaning that… I’m sorry?

Audience:  And even saturate at the condenser.

Well it saturate at the condenser is what we mean.

Audience:  What kind of your vapor if you got it to saturate be possibly?

What you’re saying is you’re going to overheat the evaporator coil as well.  Right.  There are other things to factor in there.  What I’m saying is and I used to dumbly say, oh well yeah, one degree of liquid is actually ideal but you have to add more because you don’t how accurate your equipment is.  Actually, that’s not true.  You know what the perfect amount of refrigerant to have in a system is?

Audience:  I believe that depends.

The exact amount that it’s supposed to have in it meaning the exact amount it was designed for because if you add more than what it was designed for, then it’s gonna cause other problems meaning like Leslie said, you’re gonna overfeed the evaporator coil potentially.  You’re gonna go on higher head pressure which makes larger compressor.  You’re on higher amperage.  If you under charge your system, then you’re gonna have less refrigerant density, you run the risk of it freezing up.  It has to have the amount of refrigerant that it’s designed to have and then it will be running ideally, but the problem is that we’re dealing with systems that aren’t brand new and aren’t perfect so we’re having to try to figure out that kind of happy medium.

So what we wanna know here as it pertains to the liquid line is, what we wanna know when we’re taking readings on a liquid line is we wanna make sure that we’ve got liquid in our liquid line.  That’s step one.  Second step is to try to take the readings based on what the manufacture suggests in order to tell us do we have correct sub cool.   And this is like what I was saying, it’s super important that you make sure that your equipment is as well calibrated as it possibly can be recognizing that realistically, a 2 to 3 psi variance here, a 1 to 2 degree temperature variance there is not unrealistic.  You’re probably not gonna do much better than not.  Not only is our equipment not perfect but we’re also not actually reading the temperature of the liquid in the line.  We’re reading the temperature of the copper around the liquid in the line.  So that also makes it a little bit of a difference.  So once again, we’re not functioning in a perfect world.

So let’s say that we are working on a ductless system and we can actually read the pressure and temperature on the expansion line?  What do we wanna actually see going on in this expansion line?

Audience:  We wanna see a mix of them.

Exactly because you wanna see that flash gas that’s in the process of changing.  We call that boiling practically in our world when we say boiling.  You can call it flash gas, you can call it mixed vapor and liquid or whatever you wanna call it, but that means that if we can measure temperature here, we want that to be matching what’s on our PT or pressure-temperature correlation scale on our gauges.   We want that to be matching saturation because we want to be able to prove that the refrigerant here is in that change of state.

Now, we know that it’s flash gas here meaning it’s in the process of changing.  Now this is our evaporator coil here and it’s here and here and here and here.  It’s got all these tubes feeding it and then it hits the suction line.  How far through this evaporator coil do we wanna see it changing state from liquid to vapor?

Audience:  Throughout the whole coil.

Well if your whole coil was change of state then where would your super heat be right here?

Audience:   At 2/3 of it.  I mean you don’t want it to come out the other side, but we want it to follow the change state probably three quarter of the length through the coil.

Right.  That’s the balance that you’re making because you do want to use out as much of the coil as you can as it’s reasonable to use them but you don’t want boiling liquid change of state, refrigerant travelling down the suction line towards the compressor.  Why?

Audience:  It will slug the compressor.

It will slug the compressor which means what?  Looking back on the compressor which moves the oil and causes oil migration and all kinds of different things and stuff.

Audience:  On the evap coil, is it the amount of refrigerant that’s telling you at what point it changes state right there?

Well, it isn’t just that.  You’ve got two different factors here.  You’ve got the volume of mixed refrigerant that’s traveling through it and you have the amount of heat absorption going on around it which is the reason why you can take an evaporator coil and you throw what you can on it, what does it do with the pressure?  You guys know because you’re working on it every day.  You throw a cat hair on a coil, what does it do with the evaporator coil?

Audience:  Drops some pressure.

Why is that?

Audience:  It’s slowing the air flow across it and it’s absorbing more heat.

It’s absorbing less heat.  You’re exposing this coil to less heat than you would normally have.  Now why does that affect anything?  Well, because now you’ve got this liquid.  It’s gotta be boiled somehow right?  What you’ve just done is you’ve turned down the heat on the stove.  So think of it like stove.  I know it’s kind of two different stoves, hot and it’s boiling, but it is.  This is a big boiling pot of water and we wanna boil off this water.  If you throw cat hair on it, you turn down the stove because you’re now exposing that to less heat than you would have in the first place because remember, all that air that’s coming across that coil, that’s giving it heat.   That’s the flame underneath the boiling pot of water.  You reduce that, now you’re not exposing to as much.  Because remember, in a pressure-temperature relationship once again, if you don’t expose it to heat that you normally would have, now it’s not expanding as much.

Audience:  It’s just that it’s not doing what’s not.

Because the amount of refrigerant remains constant.  You’re still flowing that same amount of refrigerant through there and that coil just gets colder and colder and colder because it’s not exposing to the heat or pressure.  It keeps dropping lower and lower and lower.  It’s 32-degree saturation, ice and you’re done.  If it wasn’t for the fact that coils formed ice, we would be running as cold as we could.  Ice is really the only thing that keeps us in direct expansion air conditioning which is what we work on.  That’s the only thing that causes us trouble.  If it wasn’t for ice, we would run coils at -10 degrees, but the problem is we build ice, it blocks the coil and now you’re done, not getting air through anymore.

So next line, we got our suction line here.  What’s the state supposed to be in a suction line?

Audience:  Fully vapor.

Fully vapor.  Compressor only pumps vapor.  How can we prove that it’s vapor in the suction line?

Audience:  By super heat.

We prove it by super heat which is what?  What does super heat mean?

Audience:  Difference in temperature between the center and the coil and the suction line.  Well it’s the difference between temperature between what it’s actually running at the lines and the saturation temperature would be for that refrigerant.

Correct.  You have to describe it in an interesting way.  What Jeff said is that it’s the increase in temperature from the middle of the coil to here.  That’s a way of saying it which is really saying a difference in temperature, an increase in temperature from the saturation, meaning from the temperature that it was in which it was boiling to the temperature that it now is that is fully vapor because just like that 212 degree pot of water, you can only heat it more until once it’s fully vapor and then you can continue to heat it and then it will get hotter.  In the same way, until this boiling pot of water completely boils and completely becomes vapor, it cannot get warmer than the saturation temperature and for us saturation temperature we find on the gauge because the pressure fluctuates, therefore the saturation pressure fluctuates.  If you look at that gauge and it tells you, that refrigerant, we know what the refrigerant is so we can see that, okay, that’s our saturation temperature.  If the actual physical temperature of that line is higher, then we know that it’s fully vapor.  If the physical temperature of the line is colder than the saturation then we know that it’s fully liquid.  If it’s the same as saturation, then we know it’s a mixed state.  It’s not necessarily flash gas with flash gas when we’re talking in this side of the system, but in the condenser coil, it’s also saturation.

If you’re gonna take a thermometer and stick it, get a good reading right in the middle of your condenser coil, you would be reading the same saturation as you can read on your gauge as long as you can get them close to each other because at that point, you’re reading a changing state but it’s the other direction.  It’s changing state from a vapor to a liquid.  So actually at that point, it is condensing.  Here’s the important thing, I had a guy who I worked with, good technician, who would write up on his invoices, negative superheat.  He would write down superheat -2.  Do you see the problem with that?  Because negative superheat is what?

Audience:  Sub cool.

It’s sub cool.  So what does sub cool tell you?

Audience:  That it’s liquid.

It’s fully liquid.  So what he’s saying is the suction line is 100% liquid.  Is that possible?

Audience:  Yes if not working.

It’s theoretically possible, but there is no running air conditioner that exists in that state so what does that tell you?

Audience:  That his tools are miscalibrating.

Correct.  It tells you this tool is not working properly.  So probably what he had was zero super heat, but he wrote -2 so if you ever have a -2 super heat or a -2 sub cool, that’s not telling you that you actually have negative super heat or sub cool because that can’t happen with a running system, but what it is telling you that, oh shoot, I probably have a zero sub cool and my gauges aren’t right or my thermometer is not right.  Because again, you’re talking about two different devices unless you’re using it less which are trying to give you a comparison so that way you can decipher whether or not it’s liquid or vapor.

Audience:  Or a changing state.

So let’s get down to the nitty-gritty now – the five pillars.  We’re measuring air temperature split.  We’re measuring super heat.  We’re measuring sub cool.  We’re measuring suction pressure.  We’re measuring head pressure.  The only way you can get any really good information is to have at least 4/5 of those.  Now let’s say you got a TXV system, you walked up to a new running 40 suction on the system and you hooked up your high side and you’re reading 150 and the suction line when you come out is warm, what is that gonna tell you?  Experienced guys here?  Fill those out.

Audience:  Low in charge.

Because I didn’t even have to really check the actual super heat on the suction line.  I’m writing 45 psi on an R22 system which on this is gonna be about 20 degrees and if I grab that line it feels just barely cool, well that right there is telling me about a 40-degree super heat.  I don’t have to use a thermometer to tell me that.

Audience:  You also need to use your hand over your condenser fan motor.

That can also give you an indication because that’s telling you how much heat you’ve dissipated.  So you can use your senses in order to tell you some of that.  I don’t have to know whether the super heat is 30 or 45 to know that a super heat is high so that’s what I’m doing with my hand there.  Now that doesn’t tell me the super heat and that’s not enough, but when I’m in the process of diagnosis, I can tell you right there, I can tell you within 15 seconds of hooking my equipment of what’s going on with the system and I tell guys this all the time because those are two that get misdiagnosed the most often, the difference between the system that has low air flow and low in refrigerant.  That’s the most misdiagnosed refrigerant condition that exists, but I can tell you whether it’s one or the other instantaneously as long as the system is not frozen.  By walking up to a system that’s not frozen, I hook up my suction gauge and it reads low and I see that the suction line is sweating and I grab it and it’s cold, it’s not low in refrigerant if I know it’s not iced over so I’ll double check that as well.  But if I walk up to it, the suction line is warm or just barely cool and not putting up much heat at the top, right there it said my suction pressure is low then I know it’s low on charge at that point.

Audience:  Along with your head pressure.

Along with my head pressure exactly because there could be restriction in there, correct.  But that’s only because I understand the five pillars.  If I didn’t understand the five pillars, if I didn’t understand what was supposed to be happening here, I can easily get confused about it because what is a cold suction line or a warm suction line telling me?  It’s really just an inaccurate measuring of super heat.  By actually using a thermometer, I can tell you exactly what the number is.

So speaking specifically, it’s a little hard in the class to go over all of the different things that can happen inside of a system, but that’s what I’m essentially giving you there in that list is that I’ve given it to you in both directions.  I’ve given it to you saying, here is some common problems and here is the symptoms that you’ll see.  Here are the symptoms and here are the common problems there could be.  You could compare them.  Use process of elimination to compare the different symptoms and issues.  But the best gift that I can give you is getting to the place where you start to interpret using logical process of elimination to use the five different readings that you can take in order to tell you things about the system.

Remember the basics here.  Get back to the basics about what causes the different changes and condition of the refrigerant, what affects the pressures inside the system.

So right outside of your box of thinking, air handler and condenser.    That’s how we tend to think of in the trade.  It’s not an air handler and condenser.  It’s compressor, condenser, metering device, evaporator and it can give you any different kind of configuration and will accomplish those that we’re trying to accomplish.

Visit or call (352) 243-7088 for more information!

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