Many refrigeration systems use the thermostatic expansion valve, a combination of valve and sensor. The TXV is designed to sense the pressure and temperature of a refrigerant gas in liquid form. The amount of pressure the liquid is exerting on the sensors determines how much liquid the valve lets through. This type of device, already common on industrial coolant systems, now is showing up on many household-commercial versions.
Signs of Expansion Valve Trouble
You can usually depend on your expansion valves, but if you suspect that one might be causing problems, you should look for a loose TXV. Valves that are attached improperly to the suction line can cause leaks and misinterpret pressure readings. Also, when TXVs break, they tend to develop faulty bulb readings. The bulb senses temperature in the line and uses the readings to tell the valve when to open. A bulb that is not attached properly to the suction line (as close to the coolant as it can be) produces inaccurate readings. You can also use the easy hand test. Detach the temperature bulb, and hold it in your hand. The heat of your hand should cause the valve to open. If it does not, the bulb itself is malfunctioning.
Spotting Other Problems
In one common symptom of a faulty TXV, the coolant system goes through a series of short cycles that don't seem to accomplish very much. However, other things can cause this problem, and diagnosing a faulty TXV simply from this symptom can be dangerous. The valve only opens with enough pressure or temperature change in the liquid. If the system is low on coolant, or if more vapor is coming out than liquid, the valve does not operate correctly, even though the problem lies elsewhere in the system. Look for coolant leaks, and check any timers or default safety settings before assuming that the TXV has broken.
Expansion Valve Solutions
Usually, the only thing you can do with a malfunctioning TXV is to replace it. But you should always keep in mind that coolant leaks are more likely. Before you try to replace the expansion valve, seal any leaks, and replace the refrigerant. Unless you have experience with highly pressure-sensitive valves operating with a refrigerant gas, you should probably let a professional replace the valve itself.
You may also try replacing the system's filter, which usually sits directly before or near the TXV. If the filter becomes clogged with dirt or grime, it may cause problems. Unfortunately, a bad filter also lets dirt into the TXV, possibly tearing its delicate diaphragm and causing permanent damage to the valve, so you may have to replace both.