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How to Diagnose a TXV Failure
There has been much written and many jokes made about the misdiagnosis of TXV (thermostatic expansion valves), and rightly so. This article will cut straight to the point to help those of you who may still need a bit of clarification, and hopefully, we will save the lives of a few TXVs and the pocketbooks of some customers.
Q: What is a TXV?
A: A TXV (TEV) is a type of metering device. The metering device's job is to create a pressure drop from the liquid line into the evaporator, which will result in refrigerant boiling (changing from liquid to vapor) through the majority of the evaporator coil. This low-temperature “boiling” absorbs heat from the space or product being cooled.
Q: How does a TXV Function?
A: A TXV “measures” the temperature and (usually) the pressure at the end of the evaporator coil with a bulb and a tube called an external equalizer. The bulb measures temperature and provides an opening force. The equalizer measures pressure and provides a closing force. There is also a spring that may have an adjustable tension that provides additional closing force. When working properly, these forces achieve a balance and maintain the evaporator superheat to the designed set levels at the end of the evaporator. The TXV's job is to maintain superheat within certain operational ranges and conditions.
Q: How do they fail?
A: A TXV may fail either too far open or too far closed. Too far open is also called “overfeeding,” meaning that boiling refrigerant is being fed too far through the evaporator coil; this would show up in low superheat. If the TXV fails closed, it can be said to be “underfeeding,” which means that not enough boiling refrigerant is fed through the evaporator coil; superheat will be too high at the evaporator outlet.
These failures can and do occur, but they are usually caused by contaminants or moisture in the system that have worked their way to the valve and caused it to stick or become restricted. Other causes of valve failure include rub-outs on the bulb tube or an external equalizer without a core depressor installed on a port that has a Schrader core in place.
When a valve is overfeeding, the first thing to check is bulb insulation, placement, and strapping. If the bulb isn't properly sensing the suction line, it can lead to the valve remaining too far open.
Q: Why are they misdiagnosed so often?
A: TXVs are often incorrectly condemned in cases of low evaporator airflow or load. This happens because techs will find a system with low suction pressure and assume that means it is low on refrigerant. They will then start to add refrigerant, and the TXV will respond by closing further the more refrigerant is added. The tech will see that the suction isn't increasing, and they will conclude that the TXV is failed.
This occurs because the tech is paying too much attention to suction pressure without considering the other readings.
Q: What is the correct way to diagnose a TXV?
A: First, take all of your refrigerant readings and your liquid line and suction temperature at both ends (on a split system). This means superheat, subcooling, suction saturation (evaporator coil temp), and liquid saturation (condensing temp). For a TXV to do what it is supposed to, you need a full line of liquid before the TXV. That means you need at least 1° of subcooling in theory, but in reality, you will want to make sure that you have the factory-specified subcooling, which is usually around 10°. In refrigeration, we do this same thing by looking for a clear sight glass. On a split system, checking the subcool outside and then confirming there is no big temperature difference inside to out is a great way to ensure that kinked lines or plugged line driers aren't an issue.
The next thing that a TXV needs is enough liquid pressure to have the required pressure differential. This amount of required pressure differential will vary a bit based on the valve. Usually, we want to see a 100 PSI minimum difference between the liquid line pressure and the desired evaporator pressure. If the head pressure drops too low due to low ambient conditions, this can come into play and impact the ability of the valve to do its job.
Once this is all confirmed, then it is simply a matter of checking the superheat at the end of the evaporator. Most A/C systems will maintain 6-14° of superheat at the evaporator outlet. If it is in that range, then the valve isn't bad; it's doing its job.
If it is lower than 6° of superheat at the evap outlet, then it could be overfeeding (double-check your thermometer and gauges). If the superheat is well above 14° at the evaporator outlet, with the proper subcool and liquid pressure entering, then you have a failed closed (underfeeding valve). Keep in mind that some valves will have a screen right before the valve, and this can be the cause of the restriction rather than the valve. You can intentionally freeze the coil and try to see the freezing point or use thermal imaging to help spot if it's the valve or the screen. When you find the point of temperature, you find the point of pressure drop. Just remember that the TXV is DESIGNED to provide pressure to maintain a fairly fixed superheat.
Q: Do TXVs Ever Fail?
A: They can fail internally, but most often, they fail because of a blocked inlet screen (if they have one), contaminants entering the valve, loss of charge from the powerhead, bulb location and positioning issues, and overheating of the valve. In commercial and refrigeration applications, you can often replace or clean the screen and replace the powerhead rather than replacing the entire valve.
As I have said many times, before diagnosing a problem, make sure your tools are well-calibrated and working and that you are ACTUALLY reading the pressure correctly. I've seen many misdiagnoses just because a Schrader wasn't pushing in or a multi-position valve wasn't cracked properly.