A TXV, or thermostatic expansion valve, is a precision flow-control device in your air conditioning or refrigeration system that regulates how much liquid refrigerant enters the evaporator coil. It does this automatically, adjusting in real time based on cooling demand. Most residential AC systems with a TXV come factory set to maintain 8 to 12 degrees Fahrenheit of superheat at the evaporator outlet, which keeps the system running efficiently across a wide range of conditions.
What a TXV Actually Does
Your AC system works by cycling refrigerant between a high-pressure liquid state and a low-pressure vapor state. The TXV sits right before the evaporator coil and acts as a gateway. It throttles high-pressure liquid refrigerant through a small opening called an orifice, which causes the refrigerant to rapidly expand into a low-pressure mix of liquid and vapor. That cold, expanding refrigerant then flows through the evaporator coil, absorbing heat from your indoor air.
The key job of the TXV is making sure the evaporator gets exactly the right amount of refrigerant at any given moment. Too much, and liquid refrigerant could flood back into the compressor and damage it. Too little, and the evaporator can’t absorb enough heat, leaving your home warm. The TXV constantly balances between these extremes, opening wider when cooling demand increases and closing down when demand drops.
How the Valve Works
A TXV balances three pressures against each other to decide how far open or closed the valve should be. Understanding these three forces explains why the valve is self-regulating.
Bulb pressure is the opening force. A small sensing bulb, clamped to the suction line at the evaporator outlet, is filled with refrigerant and connected to one side of a flexible diaphragm inside the valve. When the suction line gets warmer (meaning the evaporator needs more refrigerant), the fluid inside the bulb expands and pushes the diaphragm down, opening the valve wider.
Evaporator pressure is a closing force. It pushes up against the diaphragm from the other side. When suction pressure rises, it opposes the bulb pressure and tends to close the valve.
Spring pressure is the other closing force. A factory-calibrated spring pushes constantly against the diaphragm, setting the baseline superheat the valve will maintain. This spring is what keeps the valve from opening too far too fast.
The valve position at any moment is simply the result of these three pressures reaching equilibrium. If the evaporator warms up, bulb pressure increases, the diaphragm shifts, and the valve opens to let in more refrigerant. As the evaporator cools back down, the forces rebalance and the valve closes slightly. This happens continuously and without any electronics or external power.
Parts Inside a TXV
Despite being a critical component, a TXV is mechanically simple. Every TXV contains four core parts:
- Diaphragm: A thin, flexible membrane that moves in response to the three competing pressures. One side connects to the sensing bulb, the other faces the evaporator pressure and spring.
- Sensing bulb and capillary tube: The bulb is a small metal capsule filled with the same type of refrigerant used in the system. It clamps onto the suction line and connects to the valve body through a thin capillary tube. It’s the valve’s only “sensor.”
- Orifice with needle or ball valve: The diaphragm is mechanically linked to a tapered needle or ball that sits in the orifice. As the diaphragm moves, the needle lifts or drops, changing the size of the opening.
- Superheat spring: A spring beneath the valve pin that provides constant upward pressure against the diaphragm. Adjusting this spring (on models that allow it) changes the superheat setpoint.
Why It Matters: TXV vs. Fixed Orifice
The simpler alternative to a TXV is a fixed orifice (sometimes called a piston metering device). It’s literally a small hole of a set diameter that never changes. Fixed orifices work fine when the system is perfectly charged and operating in ideal conditions, but they can’t adapt. If the refrigerant charge drifts even slightly from spec, performance drops.
Field measurements from the American Council for an Energy-Efficient Economy found that when systems had improper refrigerant charge amounts, TXV-equipped air conditioners showed an average efficiency gain of about 21 percent compared to their performance before charge correction, while non-TXV systems gained about 17 percent. The practical takeaway: TXV systems are more forgiving of real-world imperfections. When charge levels are perfect, the two perform about the same. But in the field, charge is rarely perfect, and a TXV compensates for that automatically.
Internally vs. Externally Equalized TXVs
TXVs come in two configurations based on how they sense evaporator pressure. An internally equalized valve reads pressure from just inside the valve body, near the inlet of the evaporator. An externally equalized valve reads pressure from a separate line tapped into the suction line near the evaporator outlet.
The distinction matters when the evaporator coil has a significant pressure drop from inlet to outlet, which is common in systems with distributors or large coils. In those cases, an internally equalized valve would “think” pressure is higher than it actually is at the outlet, and it would underfeed the evaporator. An externally equalized valve gets a more accurate reading and adjusts correctly. Most modern residential systems use externally equalized valves as a default because there’s no downside to using one on a system with low pressure drop, but there’s a real penalty for using an internally equalized valve where a pressure drop exists.
Sensing Bulb Placement
The sensing bulb is the most installation-sensitive part of a TXV. If it’s mounted in the wrong spot, the valve gets bad temperature readings and the whole system suffers.
On suction lines 7/8-inch in diameter or smaller, the bulb should sit at the 12 o’clock position (top of the pipe). On larger lines, it should go at either the 4 or 8 o’clock position. It should never be placed at the bottom (6 o’clock), because oil pooling in the bottom of the suction line would insulate the bulb from the actual refrigerant temperature and give a false reading. The bulb should always be insulated after installation to prevent ambient air from influencing its reading.
Signs of a Failing TXV
A TXV can fail in two ways: stuck open or stuck closed (restricted). Each produces a distinct set of symptoms.
Stuck Closed or Restricted
A restricted TXV starves the evaporator of refrigerant. The symptoms mimic a low-charge system: low suction pressure, high superheat readings, reduced cooling, and the system may short-cycle on its low-pressure safety switch. Head pressure also drops because the condenser is starved along with everything else in the loop. Compressor amp draw will be noticeably low. One clue that it’s the TXV rather than a genuine low-charge situation is that subcooling at the condenser will be normal or slightly high, since the refrigerant is actually present in the system but just not flowing through the valve.
Stuck Open
A TXV stuck open floods the evaporator with too much refrigerant. Superheat drops toward zero, suction pressure rises, and liquid refrigerant can make it back to the compressor. This is called liquid slugging and can cause serious compressor damage. You might notice the suction line feeling unusually cold or even frosting over.
Electronic Expansion Valves
Newer variable-speed and inverter-driven systems often use electronic expansion valves (EEVs) instead of TXVs. An EEV replaces the mechanical sensing bulb and diaphragm with a stepper motor controlled by a circuit board and electronic temperature sensors. This gives it a faster response speed, a wider flow adjustment range, and much finer control over superheat. EEVs are standard in variable refrigerant flow (VRF) systems, inverter mini-splits, and inverter heat pumps, where the compressor speed changes constantly and the metering device needs to keep up.
The tradeoff is cost and complexity. TXVs are cheaper, require no electrical connection, and have a long track record of reliability in single-speed systems. They’re still the standard metering device in most conventional residential air conditioners and heat pumps. EEVs make the most sense when the system’s capacity is constantly changing and the mechanical response of a TXV can’t keep pace.