Quick answer
An expansion valve is the metering device in an air conditioner or heat pump that lowers the pressure of liquid refrigerant before it enters the evaporator coil and controls how much refrigerant flows in.
A thermostatic expansion valve (TXV) adjusts itself with a temperature bulb. An electronic expansion valve (EEV) is driven by a control board. A fixed orifice does the same job with no moving parts.
Quick facts
The key facts about Expansion Valve, with sources:
- Location
- At the inlet of the indoor coil (cooling); heat pumps add one outdoors or use a two-way design
- Main types
- TXV, EEV, fixed orifice (piston) 1
- What it controls
- Superheat at the coil outlet
- Required for
- Long line set installs, in one manufacturer's guideline 2
- Charging method with a TXV
- Usually by subcooling 2
- Who services it
- EPA 608-certified technician 4
Key takeaways
- The expansion valve is where the refrigerant's pressure drops, so it can boil cold inside the indoor coil.
- A TXV or EEV adjusts flow to the load; a fixed orifice does not.
- Long line sets and many high-efficiency systems call for a TXV 2.
- A stuck valve looks a lot like a refrigerant leak. Proper diagnosis matters.
- Valve work means opening the refrigerant circuit, which requires EPA 608 certification 4.
What the expansion valve does
Liquid refrigerant leaves the outdoor unit warm and at high pressure. If it reached the indoor coil like that, it would not get cold. The expansion valve squeezes it through a tiny opening. On the far side the pressure drops sharply, and the refrigerant starts to boil at a temperature well below room air.
The valve also decides how much refrigerant enters the coil. Too little and part of the coil runs dry and warm. Too much and liquid can reach the compressor, which is built to pump vapor, not liquid. A good valve keeps the coil full but not flooded.
It does this by watching superheat: how many degrees the vapor has warmed past its boiling point by the time it leaves the coil. A few degrees means the coil is well fed and the compressor is safe.
Pressure drop in one line
- High-pressure warm liquid from the condenser → liquid line → expansion valve
- Valve opening → pressure drops → refrigerant flashes to a cold liquid-vapor mix
- Cold mix → evaporator coil → boils fully while absorbing heat
- Sensing bulb or sensor reads coil-outlet temperature → valve opens or closes
How a TXV adjusts itself
- A sensing bulb clamped to the suction line feels the vapor temperature leaving the coil.
- The bulb holds a charge that expands as it warms and pushes on a diaphragm.
- If the coil outlet runs warm (high superheat), the bulb pushes the valve wider and more refrigerant flows.
- If it runs cold (low superheat), the valve closes down.
- A spring sets the balance point. An EEV does the same with an electronic sensor and a stepper motor.
Types of metering devices
| Type | How it works | Strengths | Weaknesses |
|---|---|---|---|
| Fixed orifice (piston) | A brass piston with a sized hole | Simple, cheap, no moving parts | Does not adapt to load; sensitive to exact charge |
| Thermostatic expansion valve (TXV / TEV) | Bulb and diaphragm open and close a needle | Adapts to load; holds superheat steady | Can stick or lose its bulb charge |
| Electronic expansion valve (EEV) | Board-driven stepper motor | Precise; suits variable-speed systems | Needs working sensors and board |
| Capillary tube | Long thin tube | Very simple | Mostly window units and refrigerators |
Example: how a tech reads a TXV system
Example only. With a TXV, the valve holds superheat, so the tech charges by subcooling instead. Goodman's long line set guideline asks for 7°F to 9°F subcooling measured at the indoor unit 2.
| Reading | Value | Meaning |
|---|---|---|
| Liquid line saturation temperature (from pressure) | Say 105°F | The temperature at which the refrigerant would condense at that pressure |
| Actual liquid line temperature at the indoor unit | Say 97°F | Measured with a clamp sensor |
| Subcooling | 105 − 97 = 8°F | Inside the 7–9°F target in Goodman's guide 2 |
| If subcooling is high but superheat is also high | Pattern | Can point to a restricted or stuck valve, not low charge |
Values here are illustrative. Each maker publishes its own targets, and the readings change with indoor and outdoor conditions. The subcooling page under related terms explains the idea in more depth.
Where you will see the term
- On coil and air handler model sheets: "factory TXV," "piston," or "field-installed TXV kit."
- On replacement quotes, where a TXV may be listed as part of matching the indoor coil.
- On repair invoices: "TXV replaced," "TXV bulb re-secured," or "EEV sensor replaced."
- In installation manuals, as the required metering device for a given outdoor unit.
Why a TXV or EEV helps, and what can go wrong
Benefits
- Keeps the coil fed as indoor heat and humidity change.
- Protects the compressor from liquid flood-back.
- More forgiving of small charge differences than a fixed orifice.
- Needed for long refrigerant runs in some makers' rules 2.
Risks
- Debris or moisture can make it stick.
- A loose sensing bulb gives false readings.
- EEVs depend on sensors and boards.
- Misdiagnosis is common because a stuck valve mimics low charge.
What the valve cannot do
A TXV cannot make up for a system that is badly short of refrigerant, or one moving too little air. It can only meter what it is given. DOE's review found charge outside the maker's range in a large share of systems tested, and low airflow in more than half 1.
DOE also found that repairs combining static pressure, charge and TXV fixes delivered a 24% efficiency improvement in one study 1. The gain came from the whole set of fixes, not the valve alone.
Sources: [1]
Cost drivers
HyreHVAC lists no repair prices without enough dated quotes. Cost drivers for valve work: valve type (TXV or EEV), access inside the coil cabinet, refrigerant recovery and recharge, a new filter-drier, brazing time, and warranty coverage on the part.
Some "TXV" calls turn out to be a loose bulb or a sensor, which is a smaller fix. Others reveal moisture or debris in the system, which adds evacuation work. Use the repair cost calculator to see what moves the number.
How a valve problem is diagnosed and fixed
Refrigerant-side work by an EPA 608-certified technician 4.
- Confirm airflow and a clean coil first.
- Measure suction and liquid pressures, superheat and subcooling.
- Check that the TXV bulb is clamped tight and insulated, or that EEV sensors read correctly.
- If the valve is at fault, recover refrigerant, replace the valve and filter-drier, and braze with a nitrogen purge.
- Pressure test, evacuate and recharge to the maker's spec, then recheck readings.
Maintenance
There is nothing for a homeowner to adjust on an expansion valve. What you can do is protect it: change filters, keep the coils clean, and get the refrigerant level checked during a spring tune-up, which ENERGY STAR lists as part of a cooling check-up 3.
The valve's worst enemies are moisture and debris inside the refrigerant lines. Those come from poor installation practice, such as skipping a deep vacuum or brazing without nitrogen. That is why install quality matters long after the crew leaves.
Sources: [3]
Warning signs and when to call a pro
- Weak cooling with frost on the coil or the small liquid line.
- Hissing at the indoor unit.
- Cooling that comes and goes on hot days.
- A tech says "it's low on refrigerant" but finds no leak. Ask whether the valve was checked.
- Ice on the outdoor unit in cooling mode.
Rules that apply
Section 608. Clean Air Act Section 608 (40 CFR Part 82, Subpart F) bans venting refrigerant during service and limits refrigerant work to certified technicians 4. See our EPA 608 certification research.
New refrigerants. New residential systems must use a refrigerant under a 700 GWP limit where components were made after January 1, 2025 5. Valves are built for specific refrigerants, so a replacement must match the system's refrigerant.
Installation instructions. Many outdoor units specify the required metering device. Following the manual keeps the warranty and the rating valid.
Expansion valve vs related terms
| Term | Relationship |
|---|---|
| Expansion valve | Meters refrigerant and drops pressure |
| Superheat | What a TXV or EEV controls |
| Subcooling | How a tech checks charge on a TXV system |
| Compressor | The other pressure-change point; raises pressure |
| Refrigeration cycle | The full loop the valve sits in |
Common misconceptions
- Myth Low suction pressure always means low refrigerant.
- Reality A restricted or stuck valve can cause the same reading.
- Myth A TXV makes charge accuracy unimportant.
- Reality It is more forgiving, but wrong charge still hurts efficiency and the compressor.
- Myth Any TXV fits any coil.
- Reality Valves are sized by capacity and built for a specific refrigerant.
- Myth EEVs never fail.
- Reality They have fewer mechanical issues, but sensors, wiring and boards can fail.
Where climate comes in
The valve itself is the same everywhere, but climate changes its workload. In humid Southeast states the indoor coil sees large moisture loads, and in the Southwest it sees high outdoor temperatures that raise liquid pressure. Cold-climate heat pumps lean on EEVs to manage wide operating ranges. Check your state page for climate notes.
When it matters to you
It matters when your installer chooses an indoor coil (ask whether it has a factory TXV or EEV, and whether the outdoor unit requires one) and when a repair quote mentions "low refrigerant" or "TXV."
Next step: if the diagnosis is a leak, read about leak repair. If it is the valve, ask for the superheat and subcooling readings in writing, and compare quotes with the quote comparison tool.
Questions about Expansion Valve
What does a TXV do in an air conditioner?
A TXV, or thermostatic expansion valve, lowers refrigerant pressure before the indoor coil and controls how much refrigerant flows in. It uses a sensing bulb on the suction line to hold a steady superheat, so the coil stays well fed without sending liquid to the compressor.
What is the difference between a TXV and a fixed orifice?
A fixed orifice is a piston with a set hole size. It does not adjust as conditions change, so it relies on an exact charge. A TXV opens and closes to match the load. TXVs are common on high-efficiency systems and required in some long line set installs.
What are the symptoms of a bad expansion valve?
Common signs are weak cooling, frost on the indoor coil or liquid line, unusual pressure readings, and cooling that varies on hot days. These overlap with low refrigerant and low airflow, so a technician needs to measure superheat and subcooling to tell them apart.
What is an EEV?
An electronic expansion valve is a metering device moved by a small stepper motor under control of the system's board. It reads sensors and adjusts flow precisely. EEVs are common in variable-speed heat pumps and mini-splits, where operating conditions change widely.
Can a TXV be adjusted?
Many TXVs have an adjustable stem, but adjustment is a technician's job and is rarely the right first step. Most problems are a loose bulb, debris, moisture, or a wrong charge. Turning the stem without diagnosing the cause can make things worse.
Why did my installer add a TXV kit?
Some outdoor units require a specific metering device on the indoor coil to reach their rating, and some long refrigerant runs require a TXV. A matched TXV kit lets the installer meet the manufacturer's instructions and keep the warranty valid.
Is replacing an expansion valve a big repair?
It means recovering refrigerant, brazing out the old valve, installing a new valve and filter-drier, pulling a vacuum and recharging. The labor is significant even though the part is small. Price depends on access, refrigerant type and warranty.
Do heat pumps have two expansion valves?
Many do, one for each coil, because each coil takes a turn as the evaporator. Others use a single bidirectional valve or a combination of valve and check valves. The design depends on the manufacturer.
Sources
- U.S. DOE Building Technologies Office, Residential HVAC Installation Practices: A Review of Research Findings (June 2018), retrieved .
- Goodman, TP-107 Long Line Set Application Guideline (R-410A), retrieved .
- ENERGY STAR, Maintenance Checklist (heating and cooling), retrieved .
- U.S. EPA, Stationary Refrigeration and Air Conditioning (Section 608), retrieved .
- U.S. EPA, Technology Transitions HFC Restrictions by Sector, retrieved .
Expert review
Written by the HyreHVAC Research team. Not yet reviewed by an outside expert. We say so rather than imply a review that has not happened; see our editorial policy.
How the numbers were checked: The TXV requirement and the 7–9°F subcooling target are quoted from Goodman's TP-107 guideline; the 24% combined-repair figure from DOE's 2018 review. The subcooling example uses illustrative temperatures, labelled as such.
Suggest a correction. We fix errors and say what changed.