What it is

Both the TXV and the EEV are metering devices: they drop high-pressure liquid to low-pressure liquid entering the evaporator and control how much flows, targeting a stable superheat so the evaporator stays full but the compressor stays protected from liquid. They do the same job two different ways:

  • TXV (thermostatic expansion valve) — mechanical. A sensing bulb, a diaphragm, and a spring balance against each other to modulate a pin. No electronics, no power needed. The standard on most residential and light-commercial gear for decades.
  • EEV (electronic expansion valve) — electronic. A small stepper motor opens and closes the port in tiny steps, commanded by a controller reading temperature and pressure sensors. Found on inverter mini-splits, modern high-efficiency systems, and most commercial/refrigeration controls. Faster, more precise, and adjustable in software.

This article is about telling them apart, what each needs, sizing a replacement, and their failure modes. (For deep TXV superheat operation, see the TXV-specific topics.)

How it works

The TXV balances three forces on a diaphragm:

  • Bulb pressure (opening force) — the sensing bulb on the suction line is charged with a fluid; warmer suction line = higher bulb pressure pushing the valve open.
  • Spring pressure (closing force) — the superheat-adjustment spring pushes the valve closed; this sets the static superheat.
  • Evaporator/equalizer pressure (closing force) — the pressure under the diaphragm (from an external equalizer line on most modern valves) pushes closed.

When superheat rises, the bulb warms, bulb pressure overcomes spring + equalizer, and the valve opens to feed more refrigerant. It's a self-contained feedback loop with no power.

The EEV replaces all that with a controller and a stepper motor. The controller reads suction-line temperature and suction pressure (or two temperatures), calculates actual superheat, compares it to the target, and steps the motor open or closed in small increments. It can hold tighter superheat, react faster, change targets on the fly, and shut fully closed when off. The trade-off is it needs power, a controller, and good sensors.

In the field

Identify which you have. A TXV has a sensing bulb strapped to the suction line, usually a thin equalizer tube, and a spring-adjustment cap — no wires. An EEV has electrical leads/a connector going to a stepper-motor head and is driven by a board; no sensing bulb (it uses electronic sensors instead). Wires = EEV; bulb = TXV.

For a TXV replacement, match these: refrigerant type (R-410A, R-22, R-454B — the valve is charged/calibrated for a specific refrigerant), capacity (tonnage), the superheat setting/range, the equalizer type (external is standard now), and whether it has an MOP (max operating pressure) feature. The bulb charge type matters too. Getting the refrigerant and tonnage right is the big one.

Mount the TXV bulb correctly — clean suction line, firm strap, on the side of the line (roughly the 4 or 8 o'clock position on lines above about 7/8", on top for smaller lines per the valve's instructions), insulated. A poorly mounted bulb reads the wrong temperature and the valve hunts or floods. Equalizer connects downstream of the bulb.

For an EEV, the valve is part of a control system. You verify the controller is commanding it (it steps audibly/measurably on startup), the sensors read correctly, and the connector is solid. Many EEVs do a full open-close "homing" cycle on power-up — that clicking/buzzing is normal. Don't condemn an EEV without checking the controller and sensors first.

Normal values & targets

  • Target superheat: both aim to hold the system's correct superheat — on a TXV, commonly factory-set around 8–12°F and adjustable; an EEV holds a programmed target, often tighter.
  • TXV superheat adjustment: the spring screw changes static superheat; a common rule is roughly 1–4°F change per full turn (valve-dependent) — adjust a turn, wait several minutes for it to settle, re-read.
  • TXV must be matched to refrigerant and tonnage: a 3-ton R-410A valve is not interchangeable with a 3-ton R-22 valve — the charge and port are refrigerant-specific.
  • EEV step range: stepper motors move in fine increments (often a few hundred steps fully closed to fully open), giving precise modulation.
  • MOP feature (if present): limits how high the valve lets suction pressure rise on a hot startup to protect the compressor — built into the bulb charge on MOP TXVs.

Common faults & what they mean

  • TXV hunting (superheat swinging up and down) → bulb mounting/insulation problem, oversized valve, or low load — the valve overshoots correcting.
  • TXV stuck closed / starving (high superheat, low suction, low amps, warm evap) → lost bulb charge, debris in the valve, or moisture/ice at the pin. Warm the bulb in your hand: if suction doesn't rise, the valve isn't responding.
  • TXV stuck open / flooding (low superheat, high suction, possible liquid to compressor) → debris holding it open or a failed power element.
  • EEV not stepping on startup → controller not powered/commanding, bad connector, or a failed stepper coil; check the control side first.
  • EEV erratic superheat → bad temperature or pressure sensor feeding the controller, or a sticking valve; sensors are the usual culprit.
  • Either valve: restriction symptoms with frost/sweat at the valve inlet → moisture freezing or debris at the seat; both point to a contamination/filter-drier problem upstream.

Tech tips & gotchas

The single most common TXV mistake is a bad bulb mount. If the bulb isn't clean-strapped, on the right clock position, and insulated, the valve reads false suction temperature and either hunts or floods — and you'll chase it as a "bad valve" when it's an installation problem.

Match the TXV to the refrigerant. A valve charged for R-22 will not meter R-410A correctly — the pressures and the bulb charge are different. Always confirm the refrigerant stamp.

Warming the bulb in your hand is a quick TXV test: a healthy valve responds within a minute (suction pressure rises as it opens). No response = the power element is dead or the valve is stuck.

Don't condemn an EEV from the outside. It's a valve plus a controller plus sensors. A "stuck EEV" is very often a sensor or controller fault. Listen for the homing cycle on power-up and verify the controller is sending steps before blaming the valve.

A new metering-device failure right after a compressor burnout is almost always contamination — acid and debris fouling the port. Install the right filter-drier and clean the system, or the new valve fouls too.

On inverter mini-splits, the EEV and the inverter board work together to hold superheat across a huge capacity range — that's why those systems use EEVs. Fault codes often point you to the EEV or its sensors directly.

Safety / code notes

Recover refrigerant properly before opening the system to replace either valve — venting is prohibited under EPA Section 608. After replacing a metering device, evacuate to a proper vacuum (a 500-micron decay test is the standard) and install the correct filter-drier; moisture left in the system freezes at the valve and mimics a restriction. Match the valve to the system's refrigerant and capacity to maintain rated performance and the equipment listing. On A2L refrigerants, follow the additional handling and leak-check requirements for mildly flammable refrigerants.