Anyone who has ever dealt with an air conditioning system, even in the smallest of manners, has most likely heard of the TXV. It’s one of those things like Superheat and Subcool that are essential to understand when working on a unit. But what is the TXV? How does it affect the system? When did it come about? We’re going to dive in folks to all of this, answer those questions, and maybe more. Let’s take a look.
What is the TXV?
TXVs, or Thermostatic Expansion Valves, is a metering device found in most air conditioning systems around the world. The goal of this valve is to control the amount of liquid refrigerant being fed into the system’s evaporator and to also control the amount of Superheat in a system. Depending on who you are or who you are working with you may hear TXVs be called the generic name of ‘metering devices.’
Refrigerant TXVThe TXV is located on the liquid line between the condenser and the evaporator. In most cases it sits right outside the evaporator ensuring that no extra liquid gets in and potentially floods the evaporator. When working perfectly the TXV is a precise instrument that increases the overall efficiency of your system.
As I stated above TXVs were designed to improve energy efficiency on air conditioners. This is done by metering the amount of refrigerant. TXVs were NOT designed to control humidity, capacity, head pressure, air temperature, suction pressure, or anything else. Again, it is just controlling the amount of refrigerant allowed into the evaporator.
The TXV achieves this by doing a couple of things. First, it looks at how fast the refrigerant is moving through the evaporator and how fast it is boiling off back into a gas form. It does this by looking at the temperatures of the refrigerant gas as it leaves the evaporator and the pressure inside of the evaporator. These recordings are kept in a temperatures sensing bulb built into the TXV. If metering needs to occur then a pin is moved in our out automatically in the valve to control the flow of refrigerant based off of the data that the TXV received.
When this pin is applied inside the TXV a few things begin to happen to the liquid refrigerant that is now stagnat. The pressure on the refrigerant slowly begins to drop. As this drop occurs an amount of the refrigerant converts to gas. (This is the standard response during pressure drops.) This now low pressure liquid and gas mixture moves into the evaporator and then completely boils off into it’s gaseous state.
TXV Failure Causes
Like with anything on an air conditioning system Thermostatic Expansion Valves can break. The question now is when they are broken or when they are failing how can we tell and why did they break? What should we look for? Below are a few examples of failures that can occur on your TXV:
- Build up of wax on the inside of the TXV. This can happen due to the wrong oil being used in the system.
- Containment or particulates getting stuck in the TXV. This can happen due to a few reasons, one of them is your compressor failing and burning out.
- Orifice inside the TXV freezing and filling with ice due to excessive moisture within the system.
- If at one point your compressor was flooded with refrigerant than your system’s excess oil may bog down the TXV. This can also happen if you just have too much oil in your system.
- The Thermostatic Expansion Valve may be adjusted too far closed or open for it to work effectively.
- Lastly, but still very important, is that there may just be a manufacturer’s defect on the TXV.
Remember that a system with a faulty TXV is going to display the same symptoms as a faulty liquid line. This is because the TXV is in fact part of the liquid line. So, when checking for failures it is best to check every component in the liquid line including the TXV, the drier, any solenoids, and valves.
TXV Failure Symptoms
Ok folks, so we now know what a TXV is and how it can fail but the question now is what are some of the signs that a TXV is failing? What are the things to look for? First, let’s remember that a failure on a TXV is one of two things. First it is either too restricted and it is not letting refrigerant into the evaporator. Second, it is not restricting enough and you are having excess refrigerant being fed into your evaporator.
Let’s look at the first example first where not enough refrigerant is being fed into the evaporator. Symptoms of this can be the following:
- Low pressure on your evaporator.
- High evaporator and compressor Superheat temperatures.
- Low amperage from your compressor.
- Short cycling on the low pressure control.
- A higher than normal discharge temperature.
- Low condenser pressure. (Head)
- Higher than normal condenser Subcool temperatures.
Ok, now let’s look at the second example when too much refrigerant is being fed into your evaporator. When this happens the evaporator can no longer keep up and some of the liquid refrigerant may in fact work it’s way towards your compressor. If liquid refrigerant moves into your compressor the liquid will settle at the bottom of the compressor along with the oil. All of this can cause premature failure in your compressor. Trust me folks, compressors aren’t cheap. The thing to keep in mind here is that if you do have a compressor failure then there was a reason for that. It may have not been a faulty compressor but instead something further on down the line, in this case the TXV.
Conclusion
Remember folks, nowadays the Thermostatic Expansion Valve is one of the most important things for technicians to check, monitor, and review. Couple this with checking Superheat and Subcool then you will have a pretty darn good idea what is going on with your system.