One of the best measurements a technician takes without even knowing that it's a great measurement to take is they grab the suction line and they feel for the infamous spear can cold, which is you know. I've already shown that beer can cold is too variable because a lot of people like their beers, different temperatures, so you can't rely on beer, can cold. You can't rely on swept-back as a way of knowing whether or not your systems running properly but suction lying temperature is a really good indication of system operation. It's actually gon na be quite a quite an excellent indication of system operation.

But first let's talk about some of the things you need to know mentioned it before, but depending on, where you put your temperature probes that can greatly affect the temperatures that you measure, especially if they're being affected by radiant heat, so the Sun other heat sources. That can really affect the temperatures that you measure and for the purposes of anticipating what your suction line temperatures should be. There's a couple big factors, one is: where are you measuring your suction line? Temperature generally we're measuring it out by the condensing unit? And that's because that's where we generally measure superheat, but for the purpose of this we're saying what happens if you don't, can I guess what happens if you're just measuring section line temperature and you want to know what it ought to be well, then you need to Know the indoor return temperature entering the evaporator coil because, as a sort of universal rule for air-conditioning comfort, cooling in most of the United States, your evaporator temperature, the actual temperature that your refrigerant is evaporating at inside the evaporator before it starts to superheat. You can also call it the boiling temperature or the saturated suction temperature.

Some people call it SST that temperature is generally going to be about thirty five degrees below the return temperature below the air entering that of a protocol. So don't get confused. That doesn't mean the temperature at the thermostat. It means what it is right before it goes over the evaporator, so the most common one that's used is 75 degrees, indoor temperature, because that's a common comfortable temperature, especially for residential.

You subtract 35 from that that equals a forty degree of Aperta cool. So if you've heard people say you should have a forty degree of a protocol on most systems, especially systems that don't have variable frequency drives in them, or anything like that. We're not talking about commercial Vav systems or anything like that, but just a typical system. That's just you turn on and off it's going to be about 35 degree, evaporator temperature below indoor air, temp dry-bulb.

If it's working properly, that is now again, if you're running slightly lower air flow for dehumidification, then that difference will be greater, meaning you'll run slightly colder, evaporator coil and in marketplaces that are very dry and arid they'll often run higher air flows. In order to get a slightly higher evaporator coil temperature, but regardless 35 degrees below that indoor ambient dry bulb, is about what we often look for, and so you have that air coming down the return going over that of a protocol and that's where we would then Measure something like our delta T, which we're not going to talk about in this video, we'll talk about it in another video, but just recognize. The evaporator coil is absorbing heat from that return, air and so that evaporator coil temperature has to be significantly lower temperature than the return air temperature. So that's rule number one, obviously, that section saturation temperature, if you will has to be lower than the indoor returned air temperature going over it, and that number is usually about 35 degrees.

So, in order to calculate what our suction line temperature ought to be, we first have to know what our indoor temperature is at our return right before it goes in the evaporator coil, but without being affected by the evaporator coil. You want to put it in the return somewhere close to the unit, but not directly impacted by an evaporator coil on a fan coil, or something like that. Once you get that number and we're gon na just say that it's 75 now that becomes the top number in the range. So if you're looking at a scale here, you start with 75.

That's the high number! Now we subtract 35 from it. So now the low number is 40, so that's our evaporator temperature and now to get our suction line temperature. We have to add in superheat, adding in super each tricky, because a lot of compressor manufacturers will say that they want the total superheat the superheat right before the compressor to be about 20 degrees. But they also want a maximum suction line.

Temperature of about 65 degrees - and so these are actually some pretty good little numbers where you kind of know, without even taking into account the indoor temperature is that manufacturers, if the system's been running under normal conditions, so it's not in a hot pool down, is what We call it hot pull down is when you know the space was really hot. The air conditioner is off and he just got it running when it's in those conditions, the section line temperature will often be higher than that on a hot day, but when it's under normal operating conditions, they do not want to see a suction line temperature above 65. We know automatically that if we see a section line, temperature above 65 at the compressor outside total superheat is what they call that or compressor superheat. Depending on who, you ask if it's above 65, then compressor manufacturers aren't happy because that compressor relies on suction for it to operate, and that means that that compressors are going to run hot if that suction line temperatures above 65.

So, to start with, we know we want it to be below 65 during normal operating conditions. We know that our evaporator temperature in this situation should be about 40 degrees. Now again, if you know more about the system - and you know the evaporator temperature runs a different PD is the term. That's used temperature difference between the return air and the evaporator temperature.

If you know that number is different, then you use the number you know. But if you don't know, the number 35 degrees is generally acceptable, plus or minus 3 degrees, so we're talking to 32 to 38 degrees differential is typical. So anyway, we take that indoor temperature 75 degrees, that's the top of the scale. We subtract 35 from that, and now we have to add in our super heat or super heat at the outlet of the evaporator coil on air conditioning.

We would generally figure to be about 10 degrees, 10 to 12 degrees and at the condenser we would generally figure that it's going to be more like 15 to 20 degrees. Now again, compressor manufacturers will tell you that it needs to be 20 degrees outside and in cases like refrigeration, that's generally what they set in because they just don't. They just want to make sure that they're not gon na flood back in air conditioning. We will often see lower super heats outside then 20 degrees and again this is assuming that you have some sort of an expansion valve.

If you have a fixed metering device, all bets are off with a fixed metering device. You pretty much have to operate within its operational zone, otherwise you could very easily have higher or lower suction line temperatures. So what I'm talking to you about now specifically applies to modern air conditioning equipment that have TX vs or electronic expansion valves. If you have a 50 meter and device and you're in that normal operating range, where it's 75 degrees inside anywhere from 80 to 95 degrees outside you're, still generally going to be sort of in this within the range of this rule.

That I'm about to tell you here, but at that point, when you're using a fixed metering device, you have to use a super heat calculator. So you have to figure out your target superheat, which we're gon na do in another. Video show you how to do that, but for the sake of this once again, you're gon na take that 75 degree. Number minus 35 is 40, then add in superheat.

So if you're measuring it inside, you would expect a section line temperature of about 50, if you're measuring it outside. You would expect a section line: temperature of about 55 to 60 degrees outside during normal operating conditions. Because again, if your evaporator temperature 10 degrees superheat at the outlet of the evaporator coil, but then that suction line makes a journey and that journey the section line makes we gon na pick up more heat. It's gon na run through a crawl space or an attic or through a chase or something, and it's gon na pick up some more heat, and so by the time it gets outside.

That's we're, 15 to 20 degrees of superheat on a t. -- xv are gon na, be pretty common, might see a little bit more than that, but what I would expect to see on this system that I'm working on today is about a 55 degree section line temperature outside in about a 50 degree section line temperature inside Now that's based on a combination of manufacturer specs and just experience of what I'm used to seeing on this equipment that I work on every day. Right if I'm measuring, when I connect gauges - and I know that on a TXV 10 degree, super heat inside 15 degrees, super heat outside - is pretty typical. That's gon na give me that 40 degree of africa, coil plus 10 degrees, is 50 inside and now, if you know that your TXV is set it six degrees of superheat on the inside well now you would expect to see a 46 degree temperature on your section Line inside and all of this without needing to connect a gauge, that's the point now connecting a suction engages in the end of the world, especially if you use a probe like the field piece JobLink probes, because you're gon na have very little loss.

So if you want to do that or especially if you expect or suspect that there is an issue with the system, then go ahead and connect your probes, that's perfectly fine! So now we're gon na just look at section line temperature. Now I've got this here. Just measuring outside temperature for reference, but we've got enough: Jael 3rh field, piece, psychrometer, xin, side, the return of this system, and so we measuring the actual suction line temperature out here be comparing it to the indoor temperature. Now keep in mind.

This suction line has travelled a little distance. This isn't that very far line said. The air handle is just on the other side of this wall, so it's only about 15 feet long, so we don't expect there to be a big difference from inside to outside. Now, if we were measuring something abnormal, we would then measure on the inside to check the differential on our suction line temperature, but we're gon na compare this sectional line temperature to the indoor temperature.

Generally speaking, we like to see a 35 degree decrease from the evaporator temperature. Then we have to add in our superheat, because our superheat is our increase in the vapor temperature after it's left the evaporator coil and is fully vapor. So it's our saturation temperature plus our superheat, plus our additional superheat gained in our suction line and inside the outside. That's going to be our total line, temperature what it ends up being outside, so we can calculate what we anticipated to be, which I show here and then we compare to what we actually have a lot of cases.

It's gon na be really helpful for you to walk through and quickly take some measurements and know whether or not this systems working said before. If you're working on a facility that has a bunch of identical equipment and a lot of its running, you can compare them to each other. You have a system that you know is working right. You've checked the charge on it.

You can compare other pieces of equipment to that in order to get a really good idea, based on your current conditions and that type of equipment. So a lot of this trade is about using solid common sense rather than sticking to rigid rules and I'm just giving you the tools necessary to help you kind of make your own rules but again you're section line temperature is indoor dry bulb temperature. Your TD equals your coil temperature, plus your super heat equals. What you expect your suction line to be, and you have to keep in mind that that section line temperature difference it's gon na vary inside the outside.

So you're, generally gon na have a cooler section line inside a lower section line temperature inside at the evaporator coil. Your evaporator superheat than you will at the outside another way of saying that is that evaporator superheat is generally lower than compressor superheater total superheating, which is measured at the outside, so hopefully that helps so that gives you something to go on again. This is not a business for people who aren't thinkers, you have to think through the process, but when you're dealing with suction temperature, you start with indoor dry bulb temperature and then you work down from there. Hopefully that was helpful, we'll catch you next time.