Thermal expansion valve (TXV)

For homeowners

The thermal expansion valve (TXV) is a small mechanical valve that meters refrigerant into the evaporator coil at exactly the right rate. It’s a feedback-controlled device — no electricity, no electronics, just pressures and a spring keeping themselves in balance.

The bulb at the top is clamped to the suction line. Inside the bulb is refrigerant that takes on the temperature of the suction line. That temperature corresponds to a pressure (refrigerant follows fixed pressure-temperature relationships), and that pressure pushes down on a diaphragm inside the valve body. The pressure inside the suction line pushes up on the same diaphragm. A spring underneath pushes up too. The valve continuously adjusts itself based on this three-way balance, opening more when the suction line gets warm and closing when it gets cold.

This is what keeps the evaporator coil running at the right “superheat” — meaning the refrigerant fully boils before leaving the coil, but doesn’t get heated much beyond the boiling point. Too little flow starves the coil; too much floods it and sends liquid to the compressor.

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For technicians

The TXV’s job is to maintain a constant superheat at the evaporator outlet by metering exactly the right amount of liquid refrigerant into the coil. Too little flow, the coil runs starved and the leaving gas is hot — high superheat, poor capacity, returning gas hurts the compressor. Too much flow, the coil floods with liquid that doesn’t fully boil — low or zero superheat, liquid slugging back to the compressor, also bad. The TXV sits in the middle and modulates.

The mechanism is a three-force balance on the diaphragm:

Force one: the sensing bulb pressure pushing down on the top of the diaphragm. The bulb is filled with refrigerant — usually the same one the system runs, sometimes a different mixture chosen for the desired response curve. The bulb is clamped to the suction line at the evaporator outlet, makes good thermal contact, and is insulated. The bulb’s contents reach the temperature of that suction line. Refrigerant under saturation conditions has a fixed pressure for any given temperature, so the bulb pressure tracks the suction line temperature. Warmer suction = higher bulb pressure = stronger push on the diaphragm.

Force two: the equalizer line pressure pushing up on the bottom of the diaphragm. The equalizer ties to the suction line just downstream of the bulb. It reports actual refrigerant pressure at the evaporator outlet to the underside of the diaphragm. Internal-equalized TXVs skip the external line and use the valve outlet port pressure instead — fine on small systems, inaccurate on coils with significant pressure drop, which is most modern coils.

Force three: the superheat spring pushing up through the pushrod against the underside of the diaphragm. This is the setpoint. Tighter spring = higher superheat target. The adjustment screw at the bottom changes the spring tension. Most factory settings are 10°F superheat give or take.

The balance: when bulb pressure (down) equals equalizer pressure (up) plus spring force (up), the diaphragm sits in equilibrium and the pin holds whatever position it’s in. Superheat at the evaporator outlet is whatever the saturation temperature difference is between the bulb (suction temp) and the equalizer (suction pressure at saturation temp). That difference is superheat.

The feedback loop: suction gas runs warmer than designed → bulb gets warmer → bulb pressure rises → diaphragm pushes down harder → pin moves down → orifice opens wider → more refrigerant flows into evaporator → coil floods slightly cooler → suction gas comes off cooler → bulb cools → loop settles. Same thing in reverse if suction runs cold. The TXV is a continuous proportional control device, not on-off.

Common installation errors that break the temperature sensing:

• Bulb on the bottom of the suction line instead of the 4 o’clock or 8 o’clock position
• Bulb clamped over insulation rather than directly to the pipe
• Bulb sitting on a horizontal section that gathers oil
• Bulb downstream of a vertical riser

A TXV that’s hunting — opening and closing rapidly — usually has a bulb that’s lost charge, a bulb mounted on the wrong location, or insufficient thermal contact.

A TXV stuck partially open or partially closed feels like a refrigerant charge problem on the gauges. Low superheat with normal subcooling means the valve is flooding the coil. High superheat with normal subcooling means it’s starving it. Knowing it’s the valve and not the charge is the diagnostic skill.

The “field adjustable” label on the adjustment screw is misleading because most installs don’t need adjustment — factory setting is fine for almost every condition. Service techs who adjust TXVs to fix a perceived problem usually make things worse. The instinct should be: diagnose the system, fix the root cause, leave the screw alone. The exception is engineered specialty applications where the design intentionally requires a non-standard superheat.

Heat pumps use either a bi-flow TXV — internal check valve allows refrigerant to flow either direction with the metering action only happening one way — or a pair of single-direction TXVs with external check valves so each one operates only in its design direction. The bi-flow version is mechanically more complex and slightly more failure-prone. The check-valve pair is more reliable but uses more space and refrigerant.

What kills TXVs in the field:

• Bulb charge leaking out (slow, over years)
• Valve internals contaminated by debris from a previous compressor burnout that wasn’t cleaned up properly
• Freezing moisture inside the orifice from a system that’s been opened without a filter drier change

The orifice is small — a fraction of a millimeter on a residential unit. It doesn’t take much contamination to plug it. Always install a new filter drier when opening the system.