Bryan teaches a class about what superheat signifies. Superheat is the difference between a vapor’s actual temperature and its suction saturation temperature; it lets us know how much an HVAC system feeds its evaporator coil with boiling refrigerant.
Liquid refrigerant goes into the metering device, and there needs to be enough liquid going into the metering device to achieve the desired effects of air conditioning but not so much that it floods the evaporator coil.
We want to know the evaporation temperature (the temperature at which the refrigerant boils), which we can determine with P-T charts or apps like the Danfoss Ref Tools app. A cold evaporator coil is desirable for moisture removal, but an evaporator coil that gets too cold may freeze.
We also don’t want the coil to get too cold because it could negatively affect the compression ratio by dropping the suction pressure. A cooler, lower-pressure vapor is less dense than a warmer, higher-pressure vapor, and the compressor has to do more work to raise that vapor’s temperature and pressure with each stroke or oscillation. So, you’re moving less refrigerant.
As long as a substance is still boiling as a liquid-vapor mixture, it will maintain a constant temperature as heat continues to be added to it; the temperature won’t rise or fall until boiling or condensation has been completed. Refrigerant with a 45-degree evaporation temperature will be 45 degrees as it boils, but it will go higher than 45 degrees once it has completely vaporized. That additional heat is called the superheat.
According to those rules regarding latent heat, it would stand to reason that lower superheat makes for a more efficient evaporator coil; there would be more boiling refrigerant in there. However, low superheat would put a compressor at risk of flooding if the refrigerant were to condense in the suction line. TXVs also have a minimum stable superheat that must be met. So, the efficiency of a lower superheat comes at the expense of increased flooding risk (which can lead to costly failures).
TXVs can set the superheat, and they must be charged by subcooling. However, older piston systems would require the superheat to be set, and you would need to do that with the indoor wet-bulb temperature, outdoor dry-bulb, and a superheat calculator as you charge a system.
If the superheat is too low on a TXV system, that indicates that the TXV is overfeeding the evaporator coil. On the other hand, if the superheat is too high, the TXV is likely underfeeding the evaporator coil. To prevent a failed TXV misdiagnosis, you must check several other things than the superheat; look for temperature drops across the liquid line filter-drier, airflow problems, and improper subcooling. Even when charging a system by subcooling, it helps to be aware of those conditions, the evaporation temperature, and the superheat.
On residential TXV systems, a typical rule of thumb is that the superheat should be 10 +/-5 degrees. The readings can deviate from the rule of thumb depending on things like long line sets and the location of your data point. In some cases, up to 20 degrees of superheat is acceptable in those exceptional cases where we can’t do anything about the system design, even though that may not necessarily be good for the system over the long term.
Read all the tech tips, take the quizzes, and find our handy calculators at https://www.hvacrschool.com/.
Liquid refrigerant goes into the metering device, and there needs to be enough liquid going into the metering device to achieve the desired effects of air conditioning but not so much that it floods the evaporator coil.
We want to know the evaporation temperature (the temperature at which the refrigerant boils), which we can determine with P-T charts or apps like the Danfoss Ref Tools app. A cold evaporator coil is desirable for moisture removal, but an evaporator coil that gets too cold may freeze.
We also don’t want the coil to get too cold because it could negatively affect the compression ratio by dropping the suction pressure. A cooler, lower-pressure vapor is less dense than a warmer, higher-pressure vapor, and the compressor has to do more work to raise that vapor’s temperature and pressure with each stroke or oscillation. So, you’re moving less refrigerant.
As long as a substance is still boiling as a liquid-vapor mixture, it will maintain a constant temperature as heat continues to be added to it; the temperature won’t rise or fall until boiling or condensation has been completed. Refrigerant with a 45-degree evaporation temperature will be 45 degrees as it boils, but it will go higher than 45 degrees once it has completely vaporized. That additional heat is called the superheat.
According to those rules regarding latent heat, it would stand to reason that lower superheat makes for a more efficient evaporator coil; there would be more boiling refrigerant in there. However, low superheat would put a compressor at risk of flooding if the refrigerant were to condense in the suction line. TXVs also have a minimum stable superheat that must be met. So, the efficiency of a lower superheat comes at the expense of increased flooding risk (which can lead to costly failures).
TXVs can set the superheat, and they must be charged by subcooling. However, older piston systems would require the superheat to be set, and you would need to do that with the indoor wet-bulb temperature, outdoor dry-bulb, and a superheat calculator as you charge a system.
If the superheat is too low on a TXV system, that indicates that the TXV is overfeeding the evaporator coil. On the other hand, if the superheat is too high, the TXV is likely underfeeding the evaporator coil. To prevent a failed TXV misdiagnosis, you must check several other things than the superheat; look for temperature drops across the liquid line filter-drier, airflow problems, and improper subcooling. Even when charging a system by subcooling, it helps to be aware of those conditions, the evaporation temperature, and the superheat.
On residential TXV systems, a typical rule of thumb is that the superheat should be 10 +/-5 degrees. The readings can deviate from the rule of thumb depending on things like long line sets and the location of your data point. In some cases, up to 20 degrees of superheat is acceptable in those exceptional cases where we can’t do anything about the system design, even though that may not necessarily be good for the system over the long term.
Read all the tech tips, take the quizzes, and find our handy calculators at https://www.hvacrschool.com/.
Today, we're going to talk about superheat to start with what is what is superheat? So not. How do you measure it? Not? How do you do the math, because those are fairly easy learning how to do the math learning, how to measure superheat is very different than what it is, what it represents. So, let's focus on that. Does anybody know what superheat represents heat absorbed from the home uh? That is, that is one way to look at it.
It's above the boiling point, so you could have a system that was uh improperly set up and you could be absorbing a lot of heat and you could still have an improper superheat. So superheat itself tells us really one thing: how would we, how would we define that look at the image on the screen? It's well, we don't always have txvs, though so txv is one type of metering device option. So if we have a txv, it does tell us about the function of the tsv txt. That's one of the reasons we measure it, but i want to get even more kind of to the root of what it is.
What is superheat superheat is a measurement of how full the evaporator coil is with boiling refrigerant. When we say uh boiling boiling is another word for we kind of use it synonymously with evaporating they're, not exactly the same. We really should call it boiling rather than evaporating, but regardless it's a change of state from liquid to vapor right and when we feed an evaporator coil. What goes into that metering device? What type of refrigerant goes into the metering device liquid refrigerant right? We want it needs to be all liquid refrigerant, that's one of our primary goals.
That's why we measure sub cool when we measure sub cool, like we talked about last week, sub cooling we're looking at how much liquid refrigerant. Are we stacking in that condenser right? The more liquid refrigerant we stack the more likely we're going to deliver liquid refrigerant to the metering device right, but the more liquid we stack, the less effective area we give in our condenser, which drives our condensing temperature up, drives our head pressure up right, which increases Our compression ratio, which means our compressor, doesn't move as much refrigerant, so we've got all these. Everything that we do in the refrigerant cycle is a balancing act right. That's why not having enough refrigerant is a problem.
Having too much refrigerant is a problem having a metering device, that's not feeding enough refrigerant into the evaporative coil is a problem. Have a metering device, that's feeding too much refrigerant into the evaporator. Coil is a problem so when we think about an evaporative coil, there's two things that we really want to know in terms of what's going on inside that evaporative coil one is what is the temperature of the refrigerant boiling in the evaporator coil? What temperature is it? We call that evaporation, temperature or evaporating temperature, and how do we know what that is? How do we know what the evap temperature is on an evaporator, coil, suction saturation right you take that suction pressure. You look at the little scale that you know goes across for the refrigerant you're working on, and it's going to tell you the temperature right. So what's a normal um, what's a normal pressure that you would see on an r410a system on the suction side? What's that 130s? Okay, so somebody pull out a uh, a pt chart app. You can use the uh the ref tools, one! You can use the emerson one, whichever one you want, any of you who don't have an app like this get one um, the one that i prefer is called ref tools it's by dan foss and it has a an app within that called refrigerant slider. And so you set to r410a, you put your pressure in what did you say 130? Make sure that it's set on gauge pressure and not atmospheric pressure, because what do we measure? We measure gauge pressure? It already takes into account subtracts out that 14.7 atmospheric pressure. So somebody give me the answer: what is it 44.95? So if we have a suction pressure of 130 psi now again in order to know for sure we would have to measure it at the evaporative coil, but pressure drops in the suction line and a residential system are generally nominal.
I mean they're almost nothing. That means that our evaporator temperature is right about 45 degrees right, so at 130, psi suction. Our evaporator temperature is about 45 degrees which in florida with the systems that we work on day in and day out with our humidity and large evaporator coils. That is pretty standard, pretty standard on a 75 to 77 degree indoor temperature, if it's a system, that's in dehumidify mode or a little better, dehumidifying system, it's going to be closer to 40 degrees, and so, if we plug in 40 degrees, that's 118.4 psi and r410a.
That's evaporator temperature. What temperature is the evaporator coil make sense. It makes sense why we'd want to know that right. We want a cold evaporative coil, but we don't want it too cold.
Why don't we want it too? Cold could freeze up right. We also don't want it colder than it needs to be, because we talked about that compression ratio lower pressure gas going back to the compressor right, so imagine you're the you're in the suction line and you're going back to the compressor if it is lower pressure that Also means that it is lower density. That means it's lighter right if there's lower pressure, that means that suction gas is lighter, because we know, what's in the lines right, what should be the only thing? That's inside those lines refrigerant there shouldn't be any air shouldn't, be any moisture shouldn't, be any nitrogen shouldn't, be any other refrigerants right. So that's a big part of us being able to control for this things, get really crazy when we start having stuff in the system.
That's not the refrigerant that we think it is then all of a sudden all bets are off right. The pressures we think we should have. We don't have and nothing's working the way it should, but so long as there's all refrigerant in there. If we have less pressure, that means that the molecules are more separated right, we're pushing on them less. That means they're lighter right. So if we have lower suction pressure, that means lighter gas going back to the compressor you all buy, that lighter gas. Going back to the compressor means, with every stroke of that compressor or every oscillation of that scroll, it moves less because, while it's moving the same volume with every stroke, each volume has less mass. So it's less dense, therefore you're moving less refrigerant, lower suction pressure means you're moving less refrigerant.
That's why, when you have a system, that's running in dehumidify mode, something like that, it's not going to run as efficiently, meaning you're not going to get as many btus per watt of input, meaning you're going to spend more money to cool your house. But the good news is: when you have a nice cold, evaporator coil, you know you're not going to die of black mold. So that's good! No sorry! That was a little extreme. The point is, is that you want to in florida we need to have cold evaporative coils, it doesn't matter if they're less efficient, we need to have cold evaporate coils or at least the ability to have a cold evaporator coil, because if we don't we're not going To dehumidify the way we need to, we need to know the evaporator temperature.
In this case we said it was 45 degrees, but then we also need to know how full is the evaporator coil of boiling temperature, because when we say a 45 degree, evaporator temperature, that's only if the refrigerant is boiling. So, if we're feeding refrigerant into these area into the bottom of the evaporator coil, that is how a vapor coil is fed. The condenser is fed from the top, and it makes liquid down at the bottom makes sense right and evaporates fed in the bottom, and it boils and vapor comes out the top. It's pretty pretty cool, pretty obvious.
If you never pay attention to it, you won't notice. It but that's how it's done so we feed liquid into the metering device. The metering device is our pressure dropper. It creates a pressure drop when it comes out of that metering device.
It immediately starts boiling it switches from about 100 or should be 100 liquid to somewhere around 70 percent liquid 30 vapor. We say that it isn't always that, but that's about right, but then, as it moves through the evaporator coil, it begins boiling and as long as it's still boiling just like the boiling pot of water, it stays at that same temperature, 45 degrees right. So it's 45 degrees here here here here here here here, but then, when it gets to 100 vapor now it stops being 45 degrees. Now it picks up additional heat and that's what we call superheat and if it's a low number, that means that it picked up very little heat by the time you measured it outside. If it's a high number, then that means they picked up a lot of heat which what does that tell us that it's not good enough? How full is this evaporative coil with boiling refrigerant? Now again, it's not all liquid, so we say we're not filling our evaporator coil with liquid in the same way that we're dropping liquid down in our condenser. But what we are doing is we're filling it with boiling refrigerant. So, what's more efficient, an evaporator coil, that's only boiling this much and the rest here is picking up super heat or an evaporator coil. That's boiling this much, and only this much is picking up superheat.
What's more efficient, the second yeah, the one we're more full right. More boiling refrigerant is better, so what does that mean? That means lower superheat equals more efficient. Evaporator coil stands to reason right. Well, then, the question is well.
Why don't we just get our super heats as low as we possibly can. Why don't we get them to one? What's wrong with a one degree superheat, we could get liquid back to the condenser right, because our metering device in this case is the txv or has what's called a minimum stable superheat, which means that, in order for it to balance out that balance of pressures and We're not talking about how txvs work today, but in order for it to do that, it has to have a little bit of a range there. It's got to have a little buffer, so they set these things and residential. What we'll most commonly see at the outlet of the evaporator coil, something around 12 13 14., but sometimes you'll, see it tighter than that i've seen like this unit here runs more like six or seven.
Is that more or less efficient than 12 or 14.? It's more efficient, lower superheat equals more efficient, but it also equals more risky. Has anybody ever seen those those engine tuners you can get where you can like get more horsepower out of your engine, but you can also blow it up. You know that's kind of a thing that people do and it's because you're running your engine, tighter you're running it more efficient, you're getting more out of it, but you're running it more risky right, more more chance of knocking. That kind of thing same thing is true.
Here the lower the super heat, the riskier we are emerson, says the the company that manufactures copeland compressors, which are the most common compressors. They want 20 degrees of superheat out at the compressor. That's what they want now, whatever they can. You know you know, wish in one hand and whatever my grandpa used to say, but it doesn't.
You know what sorry i forget, how the saying goes so forget it, and then i realize it's also a bad one. So we're not going to say it right now. The point is is that they can wish that we get 20 degrees out of the compressor, but we don't have control over that because where are we setting the superheat we're setting it inside and what's setting the superheat the metering device right now? There was a day where we were shipping all these piston systems systems with fixed orifices and on a fixed orifice system. We set the superheat by how we set the charge you want to talk about. If you think charges are hard to set now, they were far more difficult to set when you had to set superheat, because in order to set superheat on a fixed metering device system, you have to know the inside wet, bulb temperature and you have to use a Superheat calculator, so you take outside dry, bulb inside wet, bulb, use a superheat calculator. You hit that number, but it's a moving target because, as you're charging it what's happening to the indoor temperature, it's dropping right, so you're trying to hit a number and it's changing while you're trying to hit it next thing. You know you're a zero superheat and then you got ta, get your recovery tank out and pull some out. It's a real pain in the butt, whereas with us all we're doing is we're hitting a sub cool number we're watching our evaporator temperature to make sure okay.
It's good our ctoa is in range for what we would expect and we're watching our super heat to make sure our valve's doing its job, which is what back to what matthew is saying. We watch our super superheat on most of the systems we work on. Just to make sure this metering device is doing what it's supposed to do. If our superheat drops too low, that's an indication that the valve is over feeding if our superheat's too high that's an indication that the valve is under feeding.
If everything else is how it's supposed to be - and that's the key thing valves get misdiagnosed all the time, because we don't check all the other stuff we're supposed to check first, do we have a temperature drop across our liquid line? Dryer? Do we have an airflow problem? Are we providing the valve with the proper sub cooling proper, liquid refrigerant, in order for it to do its job? So we do all those things first, but we still monitor superheat, and this is what frustrates me when i ask people: how do you check the refrigerant charge on a txv system and they'll say sub cool? It's always sub cool right. Well, you could have a sub cool that you're supposed to have, but your evaporator temperature could be off. Your superheat could be off your ctoa could be off and you hit the sub cool number and they say the charge is good. If everything else is right, everything else is functioning.
How it's supposed to airflow is right, condenser airflow is correct, valve is operating properly. Then. Yes, we set the charge by sub cool and, as an installer, most cases most cases other than airflow those other. Those things are all going to be in line because it's a brand new piece of equipment right, so you know, generally speaking, you get the sub cool right, then you're you're in good shape. That's also why you follow the weigh-in guidelines and you look at what needs to be weighed in first you're, not just guessing so you're, using a lot of different data points to say this is good. This is in line. This is in line. This is in line.
I weighed the charge in sub cool is good now. This is all good. If i go here here here here, this is saying that i don't need to weight charge in, but now my sub cool is way off. Question mark right, what's going on here, why? Why is this happening? Is it that my line temperature clamp, isn't measuring properly? Is it that maybe my gauge isn't pushing the schrader in the way it's supposed to? So i'm not reading the right pressure.
Those are the sorts of things that happen all the time. You always have to second guess: when you see one of those data points, everything else looks beautiful and one data point's way off. Most often it's something wrong with the measurement itself, but sometimes it could be something wrong with the equipment, usually when it's something with the equipment you're going to have more than one data point. This is high.
This is low. You know you're going to have a couple things that are showing off in terms of what you should get on a txv system, 10 degrees, fahrenheit plus or minus 5 degrees. But again, that's measured inside if you've got a short line set your suction temperature inside and your suction temperature outside will be pretty close to each other. There's not going to be a big change.
We've insulated! It all the way out it's running underneath the house, where it's not hot, but if you have a 50 60 70 80 foot line set running through an attic. It's going to be significantly higher temperature out of the condenser than it is at the evaporator coil. And that's easy to calculate just calculate your superheat outside all right. I got 25 degrees superheat.
Is that good or bad? Well? What's your suction temperature outside look at that now, look at the difference between your suction temperature outside and your suction temperature inside that difference is going to be a decrease in superheat at the evaporator coil. So if i'm measuring 25 degrees outside and inside the suction temperature is 10 degrees colder than what it is outside, then that means that i've got 15 degrees superheat. That means i'm in line right now. Is that good, no 25 degrees? Superheat is not good for the compressor.
It means the compressor is going to run hot its whole life, but sometimes we don't have control over that. We don't have control in vista k, for example, about the fact that they run these insanely long line sets and now have really high superheats outside, but they are going to have abnormally high compressor failure on average over life over their lives. Because of that now that doesn't mean they're going to fail in five years. They might fail in 12 years when they would have normally lasted 15 right. So we don't. You know a lot of this stuff. We say: well, that's bad! It's going to cause a problem! Well, it may not cause a problem for a really long time, just like pulling a good vacuum, a lot of old-timers will say. Well, i never used a micron gauge.
I never, you know pulled a deep vacuum. Well, you know i never had a problem. Well, that doesn't mean anything because it's not like you pull a bad vacuum and the system explodes 10 minutes after you leave like that's not how that works. What it means is that you've taken years off of its life, it's just like when you know if you smoke your first cigarette.
Oh, this didn't do anything to me. Well, you know you get black lung 25 years down the road. It's not something that happens to you immediately. Black lung is probably not what that is.
That's something different. So the point is we're. Looking for 10 to 15 degrees, super heat or five to 15 degrees, superheat at the evaporator coil outlet and five is pretty low. Like five would be like ooh, that's that's low.
Six seven would be kind of more what we're looking for this rule of thumb applies to a lot of different types of equipment. You know refrigeration units a lot of different types of things. So for us, it's more going to be like 6 to 14 on the inside and then you're going to accept up to 20 uh. It's going to be acceptable on the outside unless there's some other reason that we have no control over like a really long line set.
Something like that. Does that make sense, so what is superheat, how much boiling refrigerant right? How far are we putting boiling refrigerant into this evaporator coil? We compare that to evaporator temperature, and that tells us the whole story makes sense. How do we measure it quickly? We didn't even talk about that, but i assume you all know: how do we measure superheat saturation yep? It's the difference between suction saturation, otherwise known as evaporator temperature. What you're reading on the gauge compared to the actual physical line temperature? That's all it is, and it's always the physical line temperature is always going to be higher.
You can't have negative superheat. You can't have negative sub cool if you do. That means that there's a measurement problem something's wrong with the measurement that you're taking make sense awesome have a wonderful week. Thank you all so much thanks for watching our video.
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