We get a lot of questions about target superheat, which all targets superheat means is what superheat should I set this system to, or set the charge to now? First off, when we're talking about target superheat, at least in the way that I'm talking about it today, it has everything to do with fixed metering device air conditioning systems. This chart doesn't really apply to refrigeration. It's fixed metering device air conditioning system, keeping in mind that most modern r410a equipment has t -- xv s or electronic expansion valves, in which case this shirt doesn't apply anyway, so primarily we're talking about r22 older, fixed metering device, air conditioning equipment, but it's still a Good thing to understand how to do because it teaches you a little bit about how superheat functions within a system and what it really means in a fixed orifice. So we're gon na talk about how to find your target superheat, how to charge to it.

And then what it means so, to start with, how do you find your target super? Well, you use a chart, and you can see here. This is a this is when the true tech tools has on their website, I'm sort of a universal chart, and you can see that you have to have both the outdoor temperature in the indoor wet bulb temperature. The reason why you need the indoor wet bulb temperature and not the indoor dry bulb temperature is because wet bulb. Temperature is a much better indication of total coil load and dry bulb.

Temperature is now again, it's still not gon na be perfect. There actually is a little bit of variance here. They can occur system the system - it's always better - to use manufacturer charging charts than it is to use Universal charts, but these target super heat charts are widely accepted. I also have it built into the HVAC school app which functions in essentially the same way.

So in this case of the chart that we're talking about here, 95 degrees is 67 degree. Indoor wet-bulb, 95 degree dry bulb outside gives you a 12 degree superheat. Let's talk a little bit about what that means. That means that if you're measuring your total superheat or what we often call compressor superheat outside on the suction line, you want to charge the system until you get to about 12 degrees of superheat.

Now some people may dispute that and say well that should be at the evaporator coil. Really we use evaporator superheat, more so on TXV systems or electronic expansion valve systems, because in those cases the valve is setting superheat at the evaporator outlet. But for the purpose of these charts, we're really talking about total system superheat, which is measured on the suction line outside near the compressor. So you take your pressure and you take your temperature outside near the condenser on the suction line on this particular thermometer.

When you close it, it's now measuring the temperature of land, rather than the furrow put this right at the. This is how we measure superheat. We have our actual suction line temperature in this case 53 three point, and then we have saturated section temperature reading in the the paint scale, because it's r410a 24 degrees. We are running that nine degrees.

This is what SST saturated suction temperature, it's suction, or some people will call it the evaporator. As far as charging goes once you have a system, that's stabilized, you use your chart and you find that target superheat and you charge to superheat. So if your superheat number that you're actually measuring is too high say in this case we had a 20 degree superheat after about 20 minutes of runtime and stable operation, 95 degrees dry, bulb outside 67 degrees, dry bulb inside we're. Looking for 12, we have 20.

That means we should add a small amount of refrigerant until we approach that 12 number, a quick tip would be that generally, you want to stop charging several degrees before you hit the target because it takes a while for the system to acclimate. So, as you add a few ounces at a time, you want to let it sit and acclimate also recognize that target superheat is a moving target and so using target superheat with apps like the JobLink app from field piece or measure. Quick is going to be much more accurate because you're going to be constantly measuring that returned, wet ball before it enters out of a protocol and again measuring wet bulb requires that you use a hygrometer or psychrometer in order to measure that and the use of an Old sling psychrometer is just impractical because the amount of time it takes to take that measurement by the time you've taken it. It's already changed, and so using a high-quality digital psychrometer, either right at the return grill or in the duct is gon na.

Be your best bet, in fact, as close as you can get to the evaporator coil without being line-of-sight to that coil on the return side is gon na, be your best bet once again, whenever you're taking a temperature measurement, you want to take it in a place That the air is well mixed and that it's not exposed to any cold or hot surfaces that can affect it via radiant. So you got to take that wet bulb. You take the outdoor dry bulb in the shade not affected by radiant. You find that and then you charge to it.

If your superheat is higher than your target, you add charge slowly and carefully. If your superheat is lower than your target, then you recover charge slowly and carefully. I'm a lot of people overdo it with this. So now, let's talk about what causes this to change, and so, if you look at this chart once again and you see you're, just gon na keep the 95 degree out for temperature static here for a second and you see that the wet bulb temperature increases.

You'll. Also notice that the target superheat increases and the reason that is, is because, with a fixed metering device, you don't have any play in the metering device in order to try to set in that super heat and superheat, quite simply is an indication of how much vapor Is in that of a protocol, in other words, how Pharr is boiling refrigerant going through that evaporator coil, effectively cooling, the air and how much of it is vapor. That's picked up sensible heat. So if you remember my example of the horizon, the horizon is the saturation line.

Vapor is above the ocean, and sub cool is below it or liquid is below it. Vapor is your superheat and so the higher the superheat the further you are from that horizon, which, in this case of the evaporator coil, is boiling. So we want to. We want to try to keep as low of a super heat as we effectively can without running the risk of flooding that compressor while it's running and so zero superheat is flooded operation.

So if you notice, if you go to 68, your target super hits higher 69 and even higher so on and so forth. Now, if you look at this chart again and let's say your wet bulb, temperature goes to 63 you're gon na notice. A dash a dash indicates that you will have no superheat or your target superheat at that point would be zero, which is a dangerous condition. It was not designed to be operated below a 64 degree.

Indoor wet bulb at a 95 degree outdoor temperature. If you track upwards, you can see that at 91 degrees now you do have six degrees of superheat, even all the way down to 63. On this chart again, you can do the same thing in the app, but when you get to those conditions where you have zero superheat, that means that a properly charged system under those conditions would be at zero superheat, which is a bad condition for the system, which Means that the system wasn't designed to operate in those conditions which, again you know, we can't train our customers exactly how to operate their air conditioners, which is why a lot of fixed metering device systems, especially heat pumps, came with accumulators to help protect the compressor against That liquid coming down the suction line, but either way we have to use this chart to set our superheat the best we can you're gon na notice that, as the temperature outdoors goes up, you'll also notice that your target superheat changes. So if we stay in that 67 degree kind of plane, there 67 degree wet-bulb indoor plane and we go from 95 to 96 you'll notice, your shipper, he drops and that's because more refrigerant moves through your metering device when you have higher outdoor temperatures, because that also Increases your condensing temperature, in other words your head pressure.

So when you have higher head pressure, you're gon na feed more through that metering device and again we're just trying to charge it to this happy medium. Using these charts again, superheat tells us how well we're feeding refrigerant through this evaporator coil. Our goal is to run low super heats. If we can lower super heats are going to lead to more efficient operation of the equipment, but we can't get to zero.

Our real anything below six, we get into this danger zone where we could be potentially under changing load conditions running liquid refrigerant into that compressor, which we call flooding, that's zero superheat. That means that you're still at that horizon line in our image here, the horizon line means that there's a mixture of liquid and vapor there's an interface between liquid and vapor and the suction line. We need to keep it up in the air where it's fully vapor. That's our superheat, and so that's what our target superheat calculators, unlike the HVAC school app or our charts, which are available all over the place.

Ac service tech is another great channel. He has a lots of these charts available on his site. Ac service tech comm, but you do want to make sure that you're only doing this using these types of charts and these types of calculators. If you have a fixed metering device, if you have a t, -- xv, then your target superheat is based on how that txahe is set.

And again, that's gon na remain fairly consistent, there'll be some play in it, but on a TXV system at the outlet of the evaporator coil, depending on the type of equipment you could run, you know 6 to 14 degrees. Superheats are gon na be typical at the evaporator outlet, which is what we call evaporator superheat. These charts really apply to the way we practice it in the field and air conditioning which is total superheat right before it goes into that compressor. So there's some additional nuance here: some additional things that we'll cover in other videos, but that tells you about target superheat.

If you are above your target, you add a little charge very, very slowly, very carefully. If you're below your target, then you recover very slowly very carefully with the equipment running and just recognize that target superheat is a moving target as that outdoor temperature changes and has that indoor wet bulb changes generally, as that system is operating, you're gon na watch that Wet bulb drop, so you need to compensate for that and that's we're using something like field piece, JobLink, probes or measure. Quick are really gon na make it a lot easier to hit that number dead-on. Hopefully, that's helpful thanks for watching we'll catch you in the next one.