Bryan gives a masterclass to Kalos Techs on why TXV/TEV valves are misdiagnosed, how to properly diagnose a failed TXV, and how thermostatic expansion valves work.
TXVs are designed to maintain constant superheat, and many people suspect TXV issues when the suction pressure is low. When technicians see low suction pressure, many of them may add charge and then realize that the TXV throttles down to maintain constant superheat. So, they condemn the TXV even though it's working as it should because they simply don't understand how the TXV works.
TXVs generally have an external equalizer, which supplies a closing force to the TXV. The spring also supplies a closing force, but the sensing bulb applies an opening force. The size of the TXV's orifice depends on the combination of the opening and closing forces. There is a fixed amount of refrigerant inside the sensing bulb, so the hotter the bulb, the more the refrigerant expands. The more the refrigerant expands, the more opening force it will exert on the TXV. When the bulb doesn't have enough (or any) refrigerant, it can't exert enough force on the valve to open it. However, you may be able to replace the powerhead without condemning the entire TXV.
The TXV needs a full line of liquid coming into the valve for it to work properly, so we need to check the subcooling or look at a sight glass (in the case of refrigeration). We can get vapor in the liquid line from restrictions or excessively long lines.
TXVs may also have an inlet screen, which can become restricted and prevent the proper amount of refrigerant from reaching the valve. The TXV also needs a proper pressure drop (about 100 PSI); without that pressure drop, the TXV can't do its job properly.
When the suction pressure is low, the technician should take the subcooling and superheat inside and outside. The goal is to look for the temperature differences between the inside and outside measurements. If the differences are drastic, then there could be a restriction somewhere (often in the line drier or an airflow restriction).
If suction pressures are low and the temperatures are close to freezing, the next step is to take the remaining measurements of your 5 Pillars (superheat, subcooling, delta T, suction pressure, and head pressure). Use those to diagnose the problem; the superheat at the evaporator coil should be 6-14 degrees, and it's usually not productive to condemn the TXV over a couple of degrees due to the possible inaccuracy of our tools. Instead, it's more likely that you have an airflow problem.
However, if the outdoor superheat is 27 degrees and the indoor superheat is 23 degrees, then you'll have to check for restrictions. If you don't have any restrictions (which may cause low head pressure) and have sufficient subcooling and pressure drop, then it would be safe to condemn the TXV.
When taking measurements, make sure your test instruments are calibrated and work properly. Otherwise, you could misdiagnose a TXV problem.
Moisture, carbon, and other forms of contamination can lead to inlet screen restrictions and possibly even premature TXV failure. If the superheat is high, you can temporarily open the valve more by increasing the suction pressure by placing the bulb somewhere else. NEVER open the valve more if you have low superheat! If you have low superheat, try to see how well the bulb is connected to the suction line; for the TXV to work properly, the bulb needs to be strapped in a way that makes it have good contact with the suction line. In cases where the bulb needs to be insulated, make sure it is insulated.
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