XV’s are often misdiagnosed, but this article will help clarify why that happens as well as how to properly diagnose a TXV failure.
A TXV can be incorrectly diagnosed if a technician finds a system with low suction pressure and assumes it means low refrigerant. When they add refrigerant, they will see that suction isn’t increasing and will conclude that the TXV has failed. However, this places too much emphasis on the suction pressure and not enough on the other readings.
To correctly diagnose a TXV, follow these steps:
- Take all your refrigerant readings as well as the liquid line and suction temperature at both ends.
- Suction Saturation (evaporator coil temperature)
- Liquid Saturation (condensing temperature)
- For a TXV to operate properly, you need a full line of liquid before the TXV. Make sure that you have the factory specified subcooling.
- Make sure the TXV has enough liquid pressure to have the required pressure differential.
- This means you should have a difference of 100 PSI between the liquid line pressure and desired evaporator pressure.
- Check the superheat at the end of the evaporator. If it is maintaining 6-14° of superheat, then the TXV is doing its job and hasn’t gone bad.
- Superheat lower than 6° could be due to overfeeding.
- Superheat above 14° (with the proper subcooling and liquid pressure) means you have a failed closed valve.