In this video i want to talk about the ever elusive, hard shut off txv and some of the confusion that goes around that we've talked about it before how the valve goes completely open under vacuum. But let's talk about why the valve closes and under what circumstances. You're going to see it close so first off the valve is going to close it's going to go shut. The seat is going to close when the spring force and the external equalizer force that comes from the outlet of the evaporator coil.

Assuming that your particular valve is an externally equalized valve, which the majority of them are nowadays when that force overcomes, that bulb force, the the bulb, that's strapped to your suction line at the outlet of the evaporator coil and is measuring its temperature. When those two forces overcome your bulb force, then the valve will go shut and eventually, if there's no balancing, then it will go completely closed and that will happen when the system goes off and let's talk about why that happens so, first off when the system goes Off what happens the compressor shuts down and the pressure begins to equalize and very quickly as that pressure equalizes, that pressure in your evaporator coil rises significantly higher than the corresponding suction saturation temperature that's being affected on the bulb, so just in layman's terms that suction line Stays cold within just a little bit. This is going to go shut because you have more pressure in that evaporator coil. Then you have force to open it back up, but then people will say well what happens when that suction line does warm up, and now it starts to exert more force which is going to push back against this.

Well, here's what happens! There's the easiest way to think about this when the system is off, what is the superheat? This isn't a trick question sometime, do this? Take your temperature clamp, put it on your suction line. Take your suction, saturated pressure, off of your suction line. Compare them and you'll see that your superheat is zero. So it's not a trick question your superheat goes to zero and what does the txv do when the superheat is zero? Well, the txv's job is to control superheat, so it's gon na throttle down and when superheat's zero.

That valve is going to go completely closed in order to try to maintain superheat. So, from a very practical standpoint, if your txv is a constant superheat valve and the superheat is zero, when the system is off, then the valve is going to close and prevent refrigerant from flowing into the evaporator coil when the system is off the other time. It's going to go closed is when you are pressurizing with nitrogen if you've worked with a txv system and you've been pressurizing with nitrogen before, especially when you're trying to pressurize to pretty good pressure, you'll notice that it'll get to a point where you're pressurizing, if you, If you're putting pressure in the liquid line and you're watching it rise on the suction line, you're going to keep adding into the liquid line and you're going to notice your liquid line pressure going up. But your suction line pressure that you're measuring out at your condensing unit on the suction line doesn't continue to go up.

And that's because you hit a point where the pressure in that evaporator coil exerts enough force to fully close the valve. Because, again, that external equalizer is the closing force, the bulb temperature is not changing when the system's off and you're pressurizing with nitrogen. It's just sitting there strapped to the suction line, exerting the amount of force it exerts, based on the temperature of that suction line. But at some point, as that pressure on the external equalizer increases that closing force enough.

Eventually, that valve is going to go completely shut and then you're going to watch your liquid line. Pressure continue to rise and your suction line pressure is not going to go up anymore, and so, when you're, trying to pressurize a system that can be real irritating. You end up having to pressurize from both sides in order to do your pressure test. But that's why that happens, because eventually, that evaporator external equalizer force overcomes the bulb force and the thing closes completely.

So that's how a hard shut off txv works. It's desirable in some ways because it helps prevent liquid refrigerant from migrating into the crankcase or really just refrigerant in general, because the oil in your compressor has an affinity for refrigerant and will actually hold refrigerant in it. We want to prevent that from happening so by that valve, shutting off. It prevents a lot of that refrigerant from making it all the way through and back to the compressor, because, as you know, when the system's running, the majority of your refrigerant is contained in the condenser condenser and liquid line are where most of the refrigerants contained.

And so by allowing that valve to close when the system goes off, it prevents a lot of that refrigerant migration back to the compressor crankcase. So that's a good thing. The downside is, is that the compressor starts under a little bit more load, because it's a little bit more imbalanced once it first starts up, so that suction drops pretty quick, and then it takes a little bit for that valve to open you've. Seen this many times when you start up a piece of equipment initially, when you start it up your suction drops, and then it slowly rises again and that's because initially that valve is shut and then, as that evaporator pressure drops well now that valve goes open.

So it requires that evaporative pressure to drop down before that valve opens and starts to modulate all right. Hopefully, you found that helpful, we'll catch you on the next video thanks for watching our video, if you enjoyed it and got something out of it. If you wouldn't mind hitting the thumbs up button to like the video subscribe to the channel and click, the notifications bell to be notified, when new videos come out, hvac school is far more than a youtube channel. You can find out more by going to hvacrschool.com, which is our website and hub for all of our content, including tech tips, videos, podcasts and so much more.

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