When troubleshooting a system with a thermostatic expansion valve (TXV), it is a good practice to measure the refrigerant’s superheat value leaving the evaporator. This is one of several good indicators of how a system is performing.

While taking this measurement, you might discover a cyclical fluctuation in the refrigerant’s superheat value. This is caused by the TXV excessively opening and closing in an attempt to maintain a constant operating condition. This is commonly referred to as “hunting” and can be observed as rapid changes in suction temperature and/or suction pressure. Excessive hunting can reduce the capacity and efficiency of the system, resulting in poor system performance.

There are several common conditions that can cause a TXV to hunt:

• An undercharged system. This can occur if a saturated refrigerant instead of a subcooled refrigerant is entering the orifice of the TXV, causing low flow into the evaporator.
• Poor contact of a TXV’s sensing bulb. The loss or delay of the temperature signal to the bulb can cause erratic and unpredictable operation.
• TXV is oversized. Because the capacity of the valve is larger than necessary, it attempts to adjust to system load and continues to open and close regularly.
• An incorrect charge selection. The charge selected does not have the necessary control characteristics to allow stable operation.

On a multi-circuit evaporator, another possible cause of hunting that is sometimes overlooked is an imbalanced heat load on the coil. This imbalance creates a false temperature signal at the expansion valve’s sensing bulb and results in erratic operation. If one or more circuits have a lighter heat load, some refrigerant from that circuit can remain unevaporated as it exits its circuit, causing the suction temperature where the bulb is mounted to be lower than the average of the other circuits. This can cause the valve to move to a more closed position prematurely. The other circuits operating at a higher superheat value then quickly heat up the temperature at the sensing bulb, causing a rapid rise in temperature and quickly move the valve to a more open position.

To identify this possible cause, measure the exit temperature of each circuit upstream of the suction manifold. Average the temperatures of all the circuits and compare this average temperature to the actual temperature of the suction manifold close to where the bulb is mounted. If the average value of the circuit exit temperatures exceeds the actual suction temperature value by more than 2°F, then there is likely an imbalance in one or more circuit(s). A closer examination of the individual circuit temperatures should reveal which circuit(s) is causing the problem.

There are several reasons why circuits can become unevenly loaded. This problem can occur due to the incorrect choice of a distributor or feeder tubes; partially blocked passageways of feeder tubes; unequal feeder tube lengths; and/or kinked feeder tubes. Uneven airflow across the evaporator from dirty coils or damaged coil fins can also cause this issue.

If TXV hunting is observed when troubleshooting a refrigeration system, its cause should be identified and repaired to allow for a more stable operating system.