What might cause high superheat readings in a thermal expansion valve (TXV) system?

High superheat readings in a thermal expansion valve (TXV) system typically indicate that the evaporator is not receiving enough refrigerant to properly absorb heat. When the refrigerant charge is low, there is an insufficient volume of refrigerant entering the evaporator. This leads to the refrigerant turning into vapor too quickly, resulting in a higher superheat value because the vapor has a higher temperature before it returns to the compressor.

In contrast, if the refrigerant charge were excessive, superheat readings would generally be low or even negative, as there would be too much refrigerant in the evaporator, preventing complete vaporization. A functional metering device like the TXV normally helps to regulate refrigerant flow according to the cooling load; therefore, it should not cause high superheat on its own. Additionally, a clean evaporator coil allows for efficient heat exchange, further reducing the likelihood of high superheat.

Thus, the low refrigerant charge is the primary factor contributing to high superheat readings in this situation.