Hey thanks for watching this video. Sometimes, when I talk about complicated subjects like the refrigerant circuit, diagnosis or testing, a system without connecting gauges, otherwise known as non invasive testing, which, by the way, is something that you want to learn more about, because the idea that we're always connecting gauges to a system Is probably not the best practice because of contamination and refrigerant loss. But when I talk about these big topics sometimes I think I lose people because I go through the whole process and it starts to feel a little discombobulated. So, let's focus on one thing at a time this time I want to focus on liquid line temperature now, first off, where do we typically measure liquid line temperature? We typically are gon na measure it with a temperature clamp, either thermistor type or a thermocouple type or in the case of the temperature clamp I'm going to be using here.

It's the new field, piece, rapid rail clamp that works really nice. It's in there JobLink probes, family and we measure it outside at the condenser. That's generally, where we measure and the reason we measure it. There is because that's where we generally connect our gauges and we like to test our sub cooling temperature, our temperature of our liquid line, once it leaves the condenser directly after the condenser coil, where we're measuring our pressures.

But once we no longer are going to be connecting gauges every single time we go to a system. You can start to think a little bit more freely about this. So the question about the liquid line is: does the temperature of the liquid line change significantly and the answer? Is it shouldn't change much if you have a really long line set a long rise, or maybe it's going through a super hot attic or a crawlspace? That's a very different temperature from the rest of the structure. Then it's possible that it could change a few degrees, but you're generally not gon na see more than maybe a two three degree difference in your liquid line.

Your suction line will change a little bit more from inside to outside, but we'll talk about that later, your liquid line should stay pretty stable. So when you measure across a liquid line dryer, you shouldn't see much of a difference when you measure, inside or outside you, shouldn't, see much of a difference so now you're kind of free to check that liquid line temperature wherever you like. But there are still some advantages to measure and get outside now. Why do we measure the liquid line temperature? Well, liquid line temperature actually tells us quite a bit first off.

We know that our liquid line temperature cannot be any colder than the medium to which the condenser is rejecting its heat. To just to say this very simply, if you think about a regular residential air, conditioner you've got the condenser on the outside and with that condenser on the outside. It's generally air that we're rejecting our heat to outdoor air, and so you have air going into that. Condenser coil and then blowing out the top, and so it's rejecting heat to the outdoor air.

So what that means in the case of a residential condenser is that the liquid line cannot be cooler than the outdoor air. If it is first place to look as your temperature probes, either one either your outdoor air temperature measurement probe or the probe that you're using on your liquid line, but there's a common mistake, that's made when measuring temperature, no matter where you measure it, and I just Want to you know, put a quick warning, a caution up here: anytime you're measuring temperature. It's very easy for your temperature probe to be affected by radiant heat radiant heat. The worst case would be.

Your probe is right in the Sun, and so, whenever you're measuring, you can't have your probe in the Sun, and it doesn't matter even if you're measuring on a line. If that Sun is beating right down on your probe, it's going to impact the measurement that your line, temperature probe is making, and so sometimes that's a little tricky. Sometimes you may need to kind of block the Sun a little ways, a way to try to keep it off of your instruments to help prevent that, but also anytime, your probe is even exposed to a hot surface. So even the surface of the condenser coil.

For example, that can radiantly affect the sensor in the probe. Now different sensor technologies work a little differently. Something like rapid rail is going to be much less impacted because of the way that it measures through the copper than say a thermistor. That has a lot of mass and we won't go into all that.

But that's a big cause of a lot of miss diagnosis is just getting your probe in sunlight and having it affect the temperature. So there's some prerequisites there, but once you have established that if your liquid line temperature is colder lower temperature than the outdoor air, then that's an indication of a restriction and between the condenser coil, where it's fully become liquid and wherever you're measuring. So, even if you measure a temperature difference across a liquid line, dryer, for example, that's an indication of a restriction inside the outside. If there's a significant change in temperature.

Generally, I say more than three degrees under typical applications. That's a sign of a restriction from inside to outside on that liquid line. So those are some things to look at. You don't want to see a liquid line temperature, that's a lower temperature than your outdoor temperature.

So that's one reason why you would measure it so before I go any further. I want to be really clear that when I give you rules of thumb, these are generally four basic, modern residential pieces of equipment that are air to air. If you have a water source system, if you have a geothermal system, if you're working on a machine or something these, these rules can be different. But what I'm gon na give you the ability to do is the ability to make up rules for yourself based on the equipment and the conditions that you know it operates in and past measurements that you've taken.

So, from a very practical standpoint, we know that an air-cooled condenser you're rejecting heat from the refrigerant into the air, and that's where the air coming out the top of the condenser is higher temperature right. Okay, so edges are. We know that your liquid line temperature cannot be lower than the outdoor temperature. If it is, you got to start looking for a restriction before the point that you're measuring, even sometimes in your liquid line service valve somebody might not have opened it all the way whether it may be a line dryer inside that condenser inside the condensing unit that You have to look for a restriction there.

The next thing is is if your liquid line temperature is elevated, meaning it's higher than usual and for a typical residential system that would generally be more than about 15 degrees warmer than the outdoor temperature. So the term that we use for that is approach, so you measure outdoor temperature in the shade. You measure your line, temperature. If your line temperature is more than about 15 degrees warmer than your outdoor temperature on a modern system, then that's an indication of potentially an overcharge or a restricted, condenser coil.

Now a dirty condenser coil, something that's impeding the airflow, so it could also be. You know your your fans not moving enough air over that condenser. Somebody put the wrong blade and it's somebody put the wrong motor in it, but for some reason, you're not rejecting enough heat. So those are some things to look for whenever you're gon na take a measurement, you want to kind of know what the answer is gon na be before you even measure so walk up to a typical unit, and if you've read any of my articles, like the Five pillars of refrigerant circuit diagnosis: you know that there's different ranges of condensing temperature over ambient, so what your condenser temperature is when it's changing state when it's actually changing from vapor to a liquid versus the outdoor temperature on really old systems.

That was about 30 degrees. On modern systems, that's 15 to 20 degrees. If you're dealing with like a 14 seer system, it's going to be closer to 20 and if you're dealing with a really high efficiency system, it's generally gon na be closer to about 15 degrees, meaning that the temperature of that refrigerant, as it's changing state from vapor To liquid, it's going to be about 15 degrees warmer than the outdoor temperature, and we would normally measure that on a gauge. But in this case we're not going to do that.

We're just going to walk up to the system measure the liquid line temperature and say: does that seem right to me or not? The liquid leaves the condenser travels down the liquid line. We have a liquid line dryer right here. Let me check the temperature of the liquid line and compare it with the temperature of the outdoor air. This is called the approach method and we can use it as a quick method of testing system performance, along with some other non-invasive best practices.

Another thing to note: when checking some people will ask whether you check on the inlet or the outlet of liquid line. The truth is, there should be no difference and part of the process is to check the inlet and the outlet of the liquid line and make sure that there is no temperature difference. You should really see nothing more than a degree using the same, accurate probe, although some specifications will say up to 3 degrees, but I just want to see that there's no mess first off. We know it can't be colder than the upper temperature, but it's going to be warmer.

So first we have to look at our sub cooling. What is our target sub cooling on the system? So imagine that you start out with condensing temperature and you're condensing temperatures the highest temperature that were that we're going to be measuring here. And, let's imagine that our condensing temperature is we'll say 20 degrees over ambient and our ambient temperature is 95 degrees, which pretty closely represents what we have today here in Florida, and so, if you take 95 10 degrees over, that is 105 20 degrees over. That is 115, so we're gon na say we have a condensing temperature of 115 that marks the top of the scale right.

Well now we have to drop below that 115, so we're starting with our bottom temperature, which is our outdoor temperature, that's the least lowest temperature. The liquid line could be we've established that that's 95 we've established that our top temperature, which is the condensing temperature. We anticipate, in this case 20 degrees over ambient that's 115, and so our liquid temperature is gon na be somewhere in there and where it's gon na be, is going to be sub cooling, sub cooling from the condensing temperature. In the case of this condenser that I was just working on outside my office, that target sub cooling was 13 degrees Fahrenheit.

So we take this 115. We subtract 13 from it and then 102 is the number that I would expect my liquid line temperature to be with a 95 degree outdoor day. Now, there's a lot of guesstimates in there. The one thing we know is, we know the target.

Sub cooling is 13 degrees, so we know that the condensing temperature - we don't we're kind of guessing at that - we're guessing at what the ideal condensing temperature is, because the manufacturer doesn't necessarily tell us that some will give you some charts for target head pressure with ambient Conditions but the manufacturer doesn't tell us that exactly so, we're guessing at that at least a little bit, so we can't use this liquid line temperature as an end-all be-all. We have to use it in conjunction with some other temperature measurements. We're going to talk about in some other videos, but it gives you an idea that if I walk up to this system - and I connect to my liquid line and I'm not being affected by radiant temperature, I would expect it to be again starting with my outdoor Temperature, which I measured at 95, I went up 20 degrees because it's a 14 seer system that gives me 115. I dropped 13 degrees off of that because that's the target sub cooling that the manufacturer specified, and so then that gives me what my liquid line temperature.

I would anticipate that to be so again, just as simple as 115 minus 13. 102 is what I would expect to see. My liquid line temperature be now could be a little cooler than that again can't be cooler than the outdoor temperature. The system is running a little more efficiently if it's significantly high than that.

That's something I'm definitely gon na want to be concerned. If I'm gon na want to consider, you know, is the condenser coil dirty? What's going on it's driving up that liquid line temperature and at that point I'm gon na want to go ahead and connect gauges. So, it's not to say you never connect gauges, but we want to eliminate all these times that were connecting gauges where we really don't need to, or we could really just use temperatures and some common sense in order to calculate that now again, you got to know The equipment that you're working on you can't just go on my rules of thumb, modern systems, 15 to 20 degree, condensing temperature over ambient for residential, pretty common. You may still run the systems that have up to 30 degrees, condensing temperature or ambient refrigeration systems often will have higher condensing temperatures over ambient versus what we see in air conditioning, but generally we're kind of and modern systems we're trying to drive down that head pressure.

Drive down that condensing temperature in order to get lower compression ratios, which means better efficiency. So that's it! That's how you know what your liquid line temperature should be. Like I said you can measure it in multiple different locations. Recognize the generally accepted place to measure.

It is right at the outlet of the condenser which is outside, but you can check down the line and see if you have significant changes in temperature, which can be indications primarily of restrictions if that temperature drops, especially over things like line dryers, that kind of thing, Hopefully, you found that helpful. Hopefully, that gives you the tools to walk up to a system measure your outdoor outdoor air temperature, first connect to your liquid line, get that temperature and have a pretty good indication of whether or not it's correct or not. Thanks for watching we'll catch you in the next one.