Servicing HVAC Expansion Valves
The thermostatic expansion valve, or TXV/TEV, is an essential component of any HVAC system. Paying attention to how it is behaving, and servicing any issues, will ensure efficient functioning of your HVAC system.
When to Service the TXV
For the most part the thermostatic expansion valve will not need any maintenance if it was installed properly. The only real time it will need servicing is when something is wrong. And when something is wrong, it is a great tool to help diagnose what is wrong with the system, when looking at it symptomatically.
There are three main symptoms that the TXV will manifest when something is amiss: starving, flooding, and hunting.
Starving is when the TXV is not feeding enough refrigerant into the evaporator to equal the rate at which the refrigerant is being evaporated off. Symptoms of starving will include load temperature and superheat being too high. Superheat measurement is the more accurate way to verify this. The causes of starving include moisture being frozen in the valve, or foreign material in the lines which became stuck in the valve, blocking the flow of refrigerant. Installing a filter-drier ahead of the TXV will prevent this.
The next symptom that can manifest in the TXV is flooding. This occurs when the TXV releases too much refrigerant and the evaporator is unable to evaporate all of it off. When this happens, liquid is sent to the compressor which it is unable to compress and will make a lot of noise. The superheat will also be too low when flooding is a factor. As with starving, installing a filter-drier will help to prevent this.
Hunting is a combination of both flooding and starving. It will starve first, then flood to overcompensate, then starve, overcompensating again. The remedy for this is to ensure the installation was correct, check all seals and the bulb placement.
Many modern systems will have safety features that will not allow the system to operate if there is an issue with the TXV and alert you that way. A solid installation and a filter-drier will prevent nearly all issues with a TXV.
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