TXV Explained: The Thermostatic Expansion Valve and What It Does

A TXV — Thermostatic Expansion Valve — is the metering device that controls how fast liquid refrigerant flows into the evaporator coil. It's the brain between the high-pressure liquid line and the low-pressure suction side, and when it works right, you forget it exists. When it doesn't, the whole system runs wrong.

What it actually does

The TXV's job is to maintain a constant superheat at the evaporator outlet — usually somewhere between 8°F and 14°F depending on the system. Superheat is the temperature of the suction gas above its boiling point at the current pressure. If superheat drops too low, the TXV throttles back (less refrigerant in). If it rises too high, the TXV opens up (more refrigerant in).

That constant-superheat behavior is why TXVs handle varying conditions — different outdoor temps, different indoor loads — without losing capacity. A fixed-orifice piston, by contrast, is sized for one condition and gets worse the further you get from it.

How it works mechanically

Inside the valve are three forces pushing against each other:

• Bulb pressure — a sensing bulb clamped to the suction line is filled with refrigerant. Suction line warms up, bulb pressure rises, valve opens.
• Evaporator (equalizer) pressure — pushes the valve closed.
• Spring pressure — the superheat setpoint, factory-set, usually adjustable on commercial valves.

The valve sits at whatever opening balances those three forces. As load on the evaporator changes, the balance shifts and the orifice repositions itself.

Internal vs external equalizer

On a short evaporator with low pressure drop, the valve can read its own outlet pressure directly — “internally equalized.” Most modern multi-circuit evaporators have meaningful pressure drop across the coil, so they use an external equalizer — a small tube tapped into the suction line downstream of the sensing bulb — to read the actual pressure the bulb is sensing. Hook one up backwards and the valve loses its mind.

The four common failure modes

1. Stuck closed. Restrictor effect, starved evaporator, high superheat, low suction pressure, no cooling capacity. Often caused by moisture freezing inside the valve or solid debris (filter-drier failure).
2. Stuck open. Flooded evaporator, low superheat, possible liquid slugging back to the compressor. Often caused by lost bulb charge or contamination.
3. Hunting. Valve oscillates open and closed, superheat swings, suction pressure swings. Usually a sign the bulb isn't properly mounted — loose, on top of the line instead of at 4 or 8 o'clock, or insulated wrong.
4. Wrong size. Undersized valve gives high superheat even fully open; oversized valve hunts at low loads. Sizing is a manufacturer match, not a guess.

How to diagnose

Two readings tell you almost everything:

• Superheat too high (above 20°F sustained, with normal subcooling on the liquid side): valve is starving the coil. Suspect a stuck-closed TXV, a clogged inlet screen, or a refrigerant restriction upstream.
• Superheat too low (under 5°F, especially approaching 0°F): valve is flooding the coil. Suspect a stuck-open TXV or lost bulb charge.

Before condemning the valve, check the bulb mount. A loose or improperly insulated bulb causes more “bad TXV” call-backs than actual valve failures.

TXV vs EEV

An EEV — Electronic Expansion Valve — does the same job with a stepper motor and a temperature sensor instead of a mechanical bulb. EEVs are faster to respond, easier to network into a control system, and almost universal in inverter-driven and variable-speed equipment now. The underlying physics is the same; the actuation is electronic.