The Thermostatic Expansion Valve — TXV
The Thermostatic Expansion Valve — TXV
The thermostatic expansion valve is the precision metering device in TXV-equipped AC systems. Its job is to control exactly how much refrigerant enters the evaporator at any moment — enough to absorb maximum heat without flooding the evaporator with liquid refrigerant that would then flow back to the compressor.
How it works
The TXV has two sides. A high-pressure liquid refrigerant inlet from the receiver-drier. And a low-pressure refrigerant outlet feeding the evaporator. A spring-loaded needle valve sits between them. When the valve opens, high-pressure liquid refrigerant flows through the small orifice, pressure drops dramatically, and the refrigerant becomes very cold as it enters the evaporator.
A sensing bulb — a small sealed tube filled with temperature-sensitive gas — is clamped to the evaporator outlet line. This bulb senses how warm the refrigerant is when it leaves the evaporator. If the refrigerant is too warm when it leaves, the TXV opens wider to allow more refrigerant flow and more cooling. If it is cool enough, the valve closes down slightly to restrict flow. This continuous adjustment maintains the proper refrigerant level in the evaporator.
The concept of superheat
Superheat is how much above its boiling point the refrigerant is at the evaporator outlet. A properly functioning TXV maintains a specific superheat value. Too little superheat means liquid refrigerant is leaving the evaporator and heading back to the compressor — dangerous. Too much superheat means the evaporator is starving for refrigerant — inadequate cooling.
TXV failure symptoms
A TXV stuck open floods the evaporator with refrigerant. Too much refrigerant absorbs too much heat, the evaporator freezes over, airflow through the evaporator stops, and you get no airflow from the vents even with the blower on. Evaporator icing is the classic stuck-open TXV symptom.
A TXV stuck closed starves the evaporator of refrigerant. Low side pressure drops abnormally low. Little to no cooling. The evaporator may frost around the TXV inlet only. Low side pressure near zero with high side normal or high points to a TXV stuck closed or a restriction upstream of the TXV.